Chapter 39 - Plant Responses to Internal/External Signals
2. Once a shoot reaches the sunlight, changes occur in its morphology and biochemistry. Sunlight acts as a signal. Signals, whether internal or external, are first detected by receptors, proteins that change shape in response to a specific stimulus. In this case, when the light signal is received and transduced, it produces a response called de-etiolation, or greening. This process involves stem elongation slows, leaf expansion, root elongation, and the shoot production of chlorophyll.
Phytochrome, a light-absorbing pigment that is attached to a specific protein, acts as a receptor for de-etiolation in plants. It differs from other receptors, however, because it is in the cytoplasm instead of the plasma membrane. In de-etiolation, each activated phytochrome can give rise to 100s of molecules of a second messenger, each of which can lead to the activation of 100s of molecules of a specific enzyme.
Light causes phytochrome to undergo a conformational change that leads to increases in levels of the second messengers’ cyclic GMP (cGMP) and Ca2+. Changes in cGMP levels can lead to ionic changes within the cell by influencing properties of ion channels. Cyclic GMP also activates specific protein kinases, enzymes that phosphorylate and activate other proteins. One protein kinase can phosphorylate other protein kinases and create a kinase cascade that leads to the phosphorylation of transcription factors that impact gene expression. Changes in cytosolic Ca2+ levels also play an important role in phytochrome signal transduction. The concentration of Ca2+ is generally very low in the cytoplasm. Phytochrome activation can open Ca2+ channels and lead to transient 100-fold increases in cytosolic Ca2+.
In the case of phytochrome-induced de-etiolation, several transcription factors are activated by phosphorylation, some through the cyclic GMP pathway, while activation of others requires Ca2+.
During the de-etiolation response, a variety of proteins are either synthesized or activated. These include enzymes that function in photosynthesis directly or that supply the chemical precursors for chlorophyll production. Others affect the levels of plant hormones that regulate growth. The levels of Auxin and brassinosteroids—two hormones that enhance stem elongation-- decrease following phytochrome activation.
** Red light is the most effective color in interrupting the nighttime portion of the photoperiod. Action spectra and photoreversibility experiments show that phytochrome is the active pigment. If a flash of red light during the dark period is followed immediately by a flash of far-red light, then the plant detects no interruption of night length, demonstrating red/far-red photoreversibility.
3. Because receptors must be sensitive to weak environmental and chemical signals, these signals are amplified by second messengers. Second messengers are small, internally produced chemicals that transfer and amplify the signal from the receptor to the proteins that cause the specific response. Thus, these messengers intensify environmental and chemical signals so that even small changes are sensed by receptors. In the de-etiolation response, each activated phytochrome may give rise to hundreds of molecules of a second messenger, each of which may lead to the activation of hundreds of molecules of a specific enzyme. This response enables dark-grown oak seedling to slow stem elongation after just a few seconds of moonlight.
4. Ultimately, a signal transduction pathway leads to the regulation of one or more cellular activities. In most cases, these responses to stimulation involve the increased activity of certain enzymes. This occurs through two mechanisms: by stimulating transcription of mRNA for the enzyme or by activating existing enzyme molecules (post-translational modification). In transcriptional regulation, transcription factors bind directly to specific regions of DNA and control the transcription of specific genes. The mechanism by which a signal promotes a new developmental course may depend on the activation of positive transcription factors (proteins that increase transcription of specific genes) or negative transcription factors (proteins that decrease transcription). During post-translational modifications of proteins, the activities of existing proteins are modified. In most cases, these modifications involve phosphorylation, which is the addition of a phosphate group onto a protein by a protein kinase. Many second messengers, such as cyclic GMP, and some receptors, including some phytochromes, activate protein kinases directly. One protein kinase can phosphorylate other protein kinases and create a kinase cascade that leads to the phosphorylation of transcription factors that impact gene expression. Thus, the synthesis of new proteins if usually regulated by turning specific genes on and off.
Signal pathways must also have a means for turning off once the initial signal is no longer present. Protein phosphatases, enzymes that dephosphorylate specific proteins, are involved in these “switch off” processes. At any given moment, the activities of a cell depend on the balance of activity of many types of protein kinases and protein phosphatases.
5. Through the study of mutant plants, researchers have learned about the activity of plant hormones.
Valuable insights have been provided about the roles played by various molecules in the three stages of cell-signal processing: reception, transduction, and response.
- After investigating a tomato mutant that greens less when exposed to light, the importance of phytochrome was confirmed. In experiments, when additional phytochrome was injected into aurea leaf cells and normal light exposure occurred, the standard de-etiolation response took place.
- Experiments with Arabidopsis and tobacco mutants have demonstrated the importance of “falling statoliths” in root gravitropism, but these have also indicated that other factors or organelles may be involved. Mutants lacking statoliths have a slower response than wild-type plants. One possibility is that the entire cell helps the root sense gravity by mechanically pulling on proteins that tether the protoplast to the cell wall, stretching proteins on the “up” side and compressing proteins on the “down side.” Other dense organelles may also contribute to gravitropism by distorting the cytoskeleton.
- Arabidopsis mutants with abnormal triple responses have been used to investigate the signal transduction pathways leading to this response. Normal seedlings growing free of all physical impediments will undergo the triple response if ethylene is applied. Ethylene-insensitive (ein) mutants fail to undergo the triple response after exposure to ethylene. Some lack a functional ethylene receptor. Other mutants undergo the triple response in the absence of physical obstacles. Some mutants (eto) produce ethylene at 20 times the normal rate. Other mutants, called constitutive triple-response (ctr) mutants, undergo the triple response in air but do not respond to inhibitors of ethylene synthesis. Ethylene signal transduction is permanently turned on even though there is no ethylene present. The affected gene in ctr mutants codes for a protein kinase. Because this mutation activates the ethylene response, this suggests that the normal kinase product of the wild-type allele is a negative regulator of ethylene signal transduction. One hypothesis proposes that binding of the hormone ethylene to a receptor leads to inactivation of the kinase and inactivation of this negative regulator allows synthesis of the proteins required for the triple response.
6. Scientists, their hypothesis, experiments, and conclusions on the mechanisms of phototropism:
- Charles and Francis Darwin - In the late 19th century, Charles Darwin and his son Francis observed that a grass seedling bent toward light only if the tip of the coleoptile was present. This response stopped if the tip was removed or covered with an opaque cap (but not a transparent cap). While the tip was responsible for sensing light, the actual growth response occurred some distance below the tip, leading the Darwin’s to postulate that some signal was transmitted from the tip downward.
- Peter Boysen-Jensen – demonstrated that the signal was a mobile chemical substance. He separated the tip from the remainder of the coleoptile by a block of gelatin, preventing cellular contact, but allowing chemicals to pass. However, if the tip was segregated from the lower coleoptile by an impermeable barrier, no phototropic response occurred.
- Frits Went - extracted the chemical messenger for phototropism, naming it auxin. Modifying the Boysen-Jensen experiment, he placed excised tips on agar blocks, collecting the hormone. If an agar block with this substance was centered on a coleoptile without a tip, the plant grew straight upward. If the block was placed on one side, the plant began to bend away from the agar block.
7. Six classes f plants hormones, their functions and production
- Auxin – Stimulates cell elongation, root growth, cell differentiation, and branching; regulates development of fruit; enhances apical dominance’ functions in phototropism and gravitropism; promotes xylem differentiation; retards leaf abscission – embryo of seed, meristems of apical buds, young leaves
- Cytokinins – affect root growth and differentiation; stimulate cell division and growth; stimulate germination; delay senescence – synthesized in roots and transported to other organs
- Gibberellins – Promote seed and bud germination, stem elongation, and leaf growth; stimulate flowering and development of fruit; affect root growth and differentiation – meristems of apical buds and roots, young leaves, and floral buds
- Brassinosteroids – inhibit root growth; retard leaf abscission; promote xylem differentiation – seeds, fruits, shoots, leaves, and floral buds
- Abscisic Acid (ABA)– inhibits growth; closes stomata during water stress; promotes seed dormancy – leaves, stems, roots, green fruits
- Ethylene – Promotes fruit ripening, opposes some auxin effects; promotes or inhibits growth and development of roots, leaves, and glowers, depending on species
8. In general, plant hormones control plant growth and development by affecting the division, elongation, and differentiation of cells. Some hormones also mediate shorter-term physiological responses of plants to environmental stimuli. Each hormone has multiple effects, depending on its site of action, its concentration, and the developmental stage of the plant.
9. In growing shoots, auxin is transported unidirectionally, from the shoot apex down to the base. Auxin seems to be transported directly through parenchyma tissue, from one cell to the next. This unidirectional transport of auxin is called polar transport, and has nothing to do with gravity. The polarity of auxin transport is due to the polar distribution of auxin transport protein in the cells. Concentrated at the basal end of the cells, auxin transporters move the hormone out of the cell and into the apical end of the neighboring cell.
10. According to the acid growth hypothesis, in a shoot’s region of elongation, auxin stimulates plasma membrane proton pumps, increasing the voltage across the membrane and lowering the pH in the cell wall. Lowering the pH activates expansin enzymes that break the cross-links between cellulose microfibrils and other cell wall constituents, loosening the wall. Increasing the membrane potential enhances ion uptake into the cell, which causes the osmotic uptake of water. Uptake of water increases turgor and elongates the loose-walled cell.
11. Synthetic auxins, such as 2, 4-dinitrophenol (2, 4-D), are widely used as selective herbicides. Monocots, such as maize or turfgrass, can rapidly inactivate these synthetic auxins. However, dicots cannot and die from a hormonal overdose. Spraying cereal fields or turf with 2, 4-D eliminates dicot (broadleaf) weeds such as dandelions.
12. In the presence of cytokinins and auxins, the cells divide, while cytokinins alone have no effect. If the ratio of cytokinins and auxins is at a specific level, then the mass of growing cells, called a callus, remains undifferentiated. If cytokinin levels are raised, shoot buds form from the callus. If auxin levels are raised, roots form.
13. Cytokinins, auxins, and other factors interact in the control of apical dominance, the ability of the terminal bud to suppress the development of axillary buds. Until recently, the leading hypothesis for the role of hormones in apical dominance—the direct inhibition hypothesis—proposed that auxin and cytokinin act antagonistically in regulating axillary bud growth. Auxin levels would inhibit axillary bud growth, while cytokinins would stimulate growth. Many observations are consistent with the direct inhibition hypothesis. If the terminal bud, the primary source of auxin, is removed, the inhibition of axillary buds is removed and the plant becomes bushier. This can be inhibited by adding auxins to the cut surface. The direct inhibition hypothesis predicts that removing the primary source of auxin should lead to a decrease in auxin levels in the axillary buds. However, experimental removal of the terminal shoot (decapitation) has not demonstrated this. In fact, auxin levels actually increase in the axillary buds of decapitated plants.
14. In stems, gibberellins stimulate cell elongation and cell division. One hypothesis proposes that gibberellins stimulate cell wall–loosening enzymes that facilitate the penetration of expansin proteins into the cell well. Thus, in a growing stem, auxin, by acidifying the cell wall and activating expansins, and gibberellins, by facilitating the penetration of expansins, act in concert to promote elongation. In many plants, both auxin and gibberellins must be present for fruit to set. Spraying of gibberellin during fruit development is used to make the individual grapes grow larger and to make the internodes of the grape bunch elongate.
15. The embryo of a seed is a rich source of gibberellins. After a seed has hydrated, gibberellins are released from the embryo, and the seed receives a signal to break dormancy and germinate. Gibberellins probably trigger seed germination by signal transduction pathways. Some seeds that require special environmental conditions to germinate, such as exposure to light or cold temperatures, will break dormancy if they are treated with gibberellins. Gibberellins support the growth of cereal seedlings by stimulating the synthesis of digestive enzymes that mobilize stored nutrients.
17. Abscisic acid (ABA) helps prepare plants for winter by slowing down growth. ABA (based on ratios and concentration levels) antagonizes the actions of the growth hormones—auxins, cytokinins, and gibberellins.
18. One major affect of ABA on plants is seed dormancy. The levels of ABA may increase 100-fold during seed maturation, leading to inhibition of germination and the production of special proteins that help seeds withstand the extreme dehydration that accompanies maturation. Seed dormancy has great survival value because it ensures that the seed will germinate only when there are optimal conditions of light, temperature, and moisture. Many types of dormant seeds will germinate when ABA is removed or inactivated (i.e. through certain environmental conditions). ABA is also the primary internal signal that enables plants to withstand drought. When a plant begins to wilt, ABA accumulates in leaves and causes stomata to close rapidly, reducing transpiration and preventing further water loss. ABA causes an increase in the opening of outwardly directed potassium channels in the plasma membrane of guard cells, leading to a massive loss of potassium. The accompanying osmotic loss of water leads to a reduction in guard cell turgor, and the stomata close. In some cases, water shortages in the root system can lead to the transport of ABA from roots to leaves, functioning as an “early warning system.”
19. Plants produce ethylene in response to stresses such as drought, flooding, mechanical pressure, injury, and infection. Ethylene production also occurs in response to high concentrations of externally applied auxins or during fruit ripening or programmed cell death.
Ethylene instigates a seedling to perform a growth maneuver called the triple response that enables a seedling to circumvent an obstacle as it grows through soil. Ethylene production is induced by mechanical stress on the stem tip. Then, stem elongation slows, the stem thickens, and curvature causes the stem to start growing horizontally. As the stem continues to grow horizontally, its tip touches upward intermittently. If the probes continue to detect a solid object above, then another pulse of ethylene is generated, and the stem continues its horizontal progress. If upward probes detect no solid object, then ethylene production decreases, and the stem resumes its normal upward growth.
During programmed death, which is called apoptosis, a bust of ethylene is produced. There is active expression of new genes, which produce enzymes that break down many chemical components, including chlorophyll, DNA, RNA, proteins, and membrane lipids.
The gas ethylene is also the active factor that causes leaves to drop from trees. The loss of leaves each autumn is an adaptation that keeps deciduous trees from desiccating during winter when roots cannot absorb water from the frozen ground. Near the base of the petiole, there is a breaking point called the abscission layer. Before leaves abscise, many essential elements are salvaged from the dying leaves and stored in stem parenchyma cells. A change in the balance of ethylene and auxin controls abscission. An aged leaf produces less auxin, and thus is more sensitive to ethylene. Cells in the abscission layer produce enzymes that digest the cellulose and other components of cell walls.
The process of fruit ripening is also controlled by ethylene, and is important because it helps disperse the seeds of flowering plants. Immature fruits are tart, hard, and green but become edible at the time of seed maturation, triggered by a burst of ethylene production. A chain reaction occurs: ethylene triggers ripening and then ripening triggers even more ethylene production. Furthermore, because ethylene is a gas, the signal to ripen even spreads from fruit to fruit. This positive feedback on physiology. Enzymatic breakdown of cell wall components softens the fruit, and conversion of starches and acids to sugars makes the fruit sweet. The production of new scents and colors helps advertise fruits’ ripeness to animals, which eat the fruits and disperse the seeds.
20. Photomorphogenesis is the effects of light on plant morphology. Plants detect the presence, direction, intensity, and wavelength of light. Action spectra can be useful in the study of any process that depends on light. It revealed that red and blue light are the most important colors regulating a plant’s photomorphogenesis. These observations led researchers to two major classes of light receptors: a heterogeneous group of blue-light photoreceptors and a family of photoreceptors called phytochromes that absorb mostly red light.
· Light is an especially important factor in the lives of plants.
° In addition to being required for photosynthesis, light also cues many key events in plant growth and development.
° These effects of light on plant morphology are what plant biologists call photomorphogenesis.
Light reception is also important in allowing plants to measure the passage of days and seasons.
· Plants detect the presence, direction, intensity, and wavelength of light.
° For example, the measure of the action spectrum of photosynthesis has two peaks, one in the red and one in the blue.
§ These match the absorption peaks of chlorophyll.
· Action spectra can be useful in the study of any process that depends on light.
° A close correspondence between an action spectrum of a plant response and the absorption spectrum of a purified pigment suggests that the pigment may be the photoreceptor involved in mediating the response.
· Action spectra reveal that red and blue light are the most important colors regulating a plant’s photomorphogenesis.
° These observations led researchers to two major classes of light receptors: a heterogeneous group of blue-light photoreceptors and a family of photoreceptors called phytochromes that absorb mostly red light.
21. Blue-light photoreceptors are a heterogeneous group of pigments. The action spectra of many plant processes demonstrate that blue light is effective in initiating diverse responses. However, there are three completely different types of pigments that detect blue light. These are cryptochromes (for the inhibition of hypocotyl elongation), phototropin (for phototropism), and a carotenoid-based photoreceptor called zeaxanthin (for stomatal opening).
Phytochromes function as photoreceptors in many plant responses to light. Phytochromes were discovered from studies of seed germination. Because of their limited food resources, successful sprouting of many types of small seeds, such as lettuce, requires that they germinate only when conditions, especially light conditions, are near optimal. Such seeds often remain dormant for many years until light conditions change. The photoreceptor responsible for opposing effects of red and far-red light is a phytochrome. It consists of a protein covalently bonded to a nonprotein part that functions as a chromophore, the light-absorbing part of the molecule. The chromophore is photoreversible and reverts back and forth between two isomeric forms with one (Pr) absorbing red light and becoming (Pfr), and the other (Pfr) absorbing far-red light and becoming (Pr). This interconversion between isomers acts as a switching mechanism that controls various light-induced events in the life of the plant. The Pfr form triggers many of the plant’s developmental responses to light. Exposure to far-red light inhibits the germination response. Plants synthesize phytochrome as Pr, and if seeds are kept in the dark, the pigment remains almost entirely in the Pr form. If the seeds are illuminated with sunlight, the phytochrome is exposed to red light (along with other wavelengths), and much of the Pr is converted to Pfr, triggering germination. The phytochrome system also provides plants with information about the quality of light. During the day, with the mix of both red and far-red radiation, the Pr <=>Pfr photoreversion reaches a dynamic equilibrium. Plants can use the ratio of these two forms to monitor and adapt to changes in light conditions.
23. Such physiological cycles with a frequency of about 24 hours that are not directly paced by any known environmental variable are called circadian rhythms. If an organism is kept in a constant environment, its circadian rhythms deviate from a 24-hour period to free-running periods ranging from 21 to 27 hours. Deviations of the free-running period from 24 hours does not mean that the biological clocks drift erratically, but that they are not synchronized with the outside world. Biological clocks control circadian rhythms in plants and other eukaryotes.
· Many plant processes, such as transpiration and synthesis of certain enzymes, oscillate during the day.
° This is often in response to changes in light levels, temperature, and relative humidity that accompany the 24-hour cycle of day and night.
° Even under constant conditions in a growth chamber, many physiological processes in plants, such as opening and closing of stomata and the production of photosynthetic enzymes, continue to oscillate with a frequency of about 24 hours.
· For example, many legumes lower their leaves in the evening and raise them in the morning.
° These movements continue even if plants are kept in constant light or constant darkness.
° Such physiological cycles with a frequency of about 24 hours that are not directly paced by any known environmental variable are called circadian rhythms.
° These rhythms are ubiquitous features of eukaryotic life.
· Because organisms continue their rhythms even when placed in the deepest mine shafts or when orbited in satellites, they do not appear to be triggered by some subtle but pervasive environmental signal.
° All research thus far indicates that the oscillator for circadian rhythms is endogenous (internal).
° This internal clock, however, is entrained (set) to a period of precisely 24 hours by daily signals from the environment.
· If an organism is kept in a constant environment, its circadian rhythms deviate from a 24-hour period to free-running periods ranging from 21 to 27 hours.
° Deviations of the free-running period from 24 hours does not mean that the biological clocks drift erratically, but that they are not synchronized with the outside world.
· In considering biological clocks, we need to distinguish between the oscillator (clock) and the rhythmic processes it controls.
° For example, if we were to restrain the leaves of a bean plant so they cannot move, they will rush to the appropriate position for that time of day when we release them.
° We can interfere with a biological rhythm, but the clockwork goes right on ticking off the time.
· A leading hypothesis for the molecular mechanisms underlying biological timekeeping is that it depends on synthesis of a protein that regulates its own production through feedback control.
° This protein may be a transcription factor that inhibits transcription of the gene that encodes for the transcription factor itself.
° The concentration of this transcription factor may accumulate during the first half of the circadian cycle and decline during the second half due to self-inhibition of its own production.
24. Light is a common factor that entrains the biological clock. Because the free running period of many circadian rhythms is greater than or less than the 24-hour daily cycle, they eventually become desynchronized with the natural environment when denied environmental cues. Plants are capable of reestablishing (entraining) their circadian synchronization though. Both phytochrome and blue-light photoreceptors can also entrain circadian rhythms of plants. The phytochrome system involves turning cellular responses off and on by means of the Pr <=> Pfr switch. In darkness, the phytochrome ratio shifts gradually in favor of the Pr form, in part from synthesis of new Pr molecules and, in some species, by slow biochemical conversion of Pfr to Pr. When the sun rises, the Pfr level suddenly increases by rapid photoconversion of Pr. This sudden increase in Pfr each day at dawn resets the biological clock.
25. Photoperiodism is a physiological response to photoperiod. It synchronizes many plant responses, such as flowering, to changes of season. The appropriate appearance of seasonal events is of critical importance in the life cycles of most plants. These seasonal events include seed germination, flowering, and the onset and breaking of bud dormancy. The environmental stimulus that plants use most often to detect the time of year is the photoperiod, the relative lengths of night and day.
26. Short-day plant are so named because they require a light period shorter than a critical length to flower. Examples include chrysanthemums, poinsettias, and some soybean varieties. Long-day plants will only flower when the light period is longer than a critical number of hours. Examples include spinach, iris, and many cereals. Day-neutral plants will flower when they reach a certain stage of maturity, regardless of day length. Examples include tomatoes, rice, and dandelions.
These names are misleading, however, because it is actually night length, not day length, that controls flowering and other responses to photoperiod. For example, short-day plants will flower if their daytime period is broken by brief exposures to darkness, but not if their nighttime period is broken by a few minutes of dim light. Thus, short-day plants are actually long-night plants, requiring a minimum length of uninterrupted darkness. Similarly, long-day plans are actually short-night plants. Long-day and short-day plants are distinguished not by an absolute night length but by whether the critical night lengths sets a maximum (long-day plants) or minimum (short-day plants) number of hours of darkness required for flowering. In both cases, the actual number of hours in the critical night length is specific to each species of plant. Plants measure night length very accurately. Some short-day plants will not flower if night is even one minute shorter than the critical length. Some plants species always flower on the same day each year.
27. Factors besides night length may control flowering. 1 Certain plants respond to photoperiod only if pretreated by another environmental stimulus. For example, winter wheat will not flower unless it has been exposed to several weeks of temperatures below 10oC (called vernalization) before exposure to the appropriate photoperiod. 2 Photoperiods are detected by plant leaves, and thus, plants lacking leaves will not flower, even if exposed to the correct photoperiod. The flowering signal, not yet chemically identified, is called florigen, and it may be a hormone or some change in the relative concentrations of two or more hormones. 3 Because flowering involves the transition of a bud’s meristem from a vegetative state to a flowering state, meristem-identity genes that induce the bud to form a flower must be switched on. Then organ-identity genes that specify the spatial organization of floral organs—sepals, petals, stamens, and carpels—are activated in the appropriate regions of the meristem. Signal transduction pathways link external cues to the gene changes required for flowering.
28. Gravitropism is the response of plant roots and shoots to gravity. Roots are positively gravitropic and shoots are negatively gravitropic, which ensures that roots grow down into the soil and shoots grow upwards toward the sun, regardless of seed orientation upon landing on the ground. Auxin plays a major role in gravitropic responses. Plants may tell up from down by statoliths—specialized plastids containing dense starch grains— which settle to the lower portions of cells. It is hypothesized that the aggregation of statoliths at low points in cells of the root cap triggers the redistribution of calcium, which in turn causes lateral transport of auxin within the root. This increased concentration of auxin on the lower side of the zone of elongation inhibits cell elongation, slowing growth on that side and curving the root downward.
29. In response to mechanical perturbations, plants can change form using a process called thigmomorphogenesis. For example, if two plants of the same species were taken and planted in a sheltered location and on a windy cliff, respectively, the former would be taller and more slender, while the later would be shorter and stockier. Plants are very sensitive to mechanical stress. Mechanical stimulation activates a signal transduction pathway that increases cytoplasmic calcium, which mediates the activity of specific genes, including some that encode for proteins that affect cell wall properties.
Thus, plants can respond by quickly by altering their growth. Thigmotropism is when contact stimulates some sort of response. Caused by differential growth of cells on opposite sides, this response is shown by vines and other climbing plants that have tendrils. The tendrils grow straight until they touch something, and then they begin to coil. This allows the plants to take use various objects as mechanical support as it climbs upwards. Some plants can rapidly respond to mechanical stimulation through leaf movements. For example, when the compound leaf of a Mimosa plant is touched, it collapses and leaflets fold together. Pulvini, motor organs at the joints of leaves, become flaccid from a loss of potassium and water is lost by osmosis. It takes about ten minutes for the cells to regain their turgor and restore the “unstimulated” form of the leaf. This folding of leaves may help reduce surface area in response to strong winds, thus prevent dehydration or water loss. Collapse of the leaves also exposes thorns on the stem, which may serve to deter herbivory.
Friday, February 22, 2008
AP Biology Ch38 Objectives
Chapter 38 - Angiosperm Reproduction and Biotechnology
1. Plant and angiosperm life cycles are characterized by an alternation of generations. Haploid (n) and diploid (2n) generations take turns producing each other. Through meiosis, the diploid sporophyte, produces haploid spores, which divide by mitosis, giving rise to multicellular male and female haploid plants—the gametophytes. The gametophytes produce gametes—sperm and eggs. Fertilization results in diploid zygotes, which divide by mitosis to form new sporophytes.
In angiosperms, the dominant generation is the conspicuous sporophyte plant, which produces the flower. Flowers are specialized shoots that function as unique reproductive structures bearing the reproductive organs of the angiosperm sporophyte. Male and female gametophytes develop within the anthers and ovules, respectively, of a sporophyte flower. Gametophytes became reduced in seed plants over the course of time, and evolved to become dependent upon their sporophyte parents. Consisting of only a few cells, angiosperm gametophytes are the most reduced of all plants. A male gametophyte (pollen grain) is brought to a female gametophyte (contained in an ovule embedded in the ovary of a flower) through pollination by wind, water, or animals. A union of gametes (fertilization) takes place inside the ovary. Ovules develop into seeds, while the ovary itself develops into the fruit around the seed.
2. Floral parts in order from outside in: sepals, petals, stamen, carpels
3. Sepals, petals, stamens, and carpels are all floral organs. They attach to the stem at the receptacle.
Sepals enclose/protect the floral bud before it opens. They are generally green and more leaflike in appearance than the other floral organs. Petals are brightly colored organs that attract insects/pollinators. Sepals and petals are sterile. Stamens are male reproductive organs that consist of a stalk (filament) and a terminal anther containing chambers called pollen sacs, which produce pollen. Carpels are the female reproductive organs of a flower. Flowers can have more than one carpel. At the base of a carpel is an ovary, inside of which one or more ovules can be found. Carpels also have a slender neck called the style. A sticky structure called the stigma exists are the top of the style, and serves as a landing platform for pollen. The anthers and the ovules bear sporangia, where spores are produced by meiosis and gametophytes later develop. The male gametophytes are sperm-producing structures called pollen grains, which form within the pollen sacs of anthers. The female gametophytes are egg-producing structures called embryo sacs, which form within the ovules in ovaries.
5. Complete flowers have all four organs and has both male and female reproductive organs), while incomplete flowers lack one or more of the four floral parts.
A bisexual flower is equipped with both stamens and carpels. All complete and many incomplete flowers are bisexual. A unisexual flower is missing either stamens (making it a carpellate flower) or carpels (making it a staminate flower).
A monoecious plant has staminate and carpellate flowers at separate locations on the same individual plant. For example, maize and other corn varieties have ears derived from clusters of carpellate flowers, while the tassels consist of staminate flowers. Meanwhile, dioecious species have staminate flowers and carpellate flowers on separate plants. For example, date palms have carpellate individuals that produce dates and staminate individuals that produce pollen.
6. Gametes are produced in the haploid generation by gametophytes. Male and female gametophytes develop within the anthers and ovules, respectively, of a sporophyte flower. They are produced by the process of mitosis.
7. Male gametophytes= pollen grains & female gametophytes= embryo sacs
8. The female gametophytes are egg-producing structures called embryo sacs, which form within the ovules in ovaries. One cell in the sporangium of each ovule, the megasporocyte, grows and then goes through meiosis, producing four haploid megaspores. In many angiosperms, only one megaspore survives. This megaspore divides by mitosis three times without cytokinesis, forming in one cell with eight haploid nuclei. Membranes partition this mass into a multicellular female gametophyte—the embryo sac. Three cells sit at one end of the embryo sac: two synergid cells flanking the egg cell. The synergids function in the attraction and guidance of the pollen tube. At the other end of the egg sac are three antipodal cells of unknown function. The other two nuclei, the polar nuclei, share the cytoplasm of the large central cell of the embryo sac.
9. Pollination, which brings male and female gametophytes together, is the first step in the chain of events that leads to fertilization. Some plants, such as grasses and many trees, release large quantities of pollen on the wind to compensate for the randomness of this dispersal mechanism. Some aquatic plants rely on water to disperse pollen. Most angiosperms interact with insects or other animals that transfer pollen directly between flowers.
10. Pollination by wind, water, or animals brings a male gametophyte (pollen grain) to a female gametophyte contained in an ovule embedded in the ovary of a flower. Fertilization is the union of the gametes.
11. The various barriers that prevent self-fertilization contribute to genetic variety by ensuring that sperm and eggs come from different parents. Dioecious plants cannot self-fertilize because they are unisexual. In plants with bisexual flowers, a variety of mechanisms may prevent self-fertilization. For example, in some species stamens and carpels mature at different times. Alternatively, they may be arranged in such a way that it is mechanically unlikely that an animal pollinator could transfer pollen from the anthers to the stigma of the same flower. The most common anti-self fertilizing mechanism is self-incompatibility, the ability of a plant to reject its own pollen and that of closely related individuals. If a pollen grain from an anther happens to land on a stigma of a flower on the same plant, a biochemical block prevents the pollen from completing its development and fertilizing an egg.
12. Double fertilization gives rise to the zygote and endosperm. The process begins after a pollen grain lands on a plant’s stigma. The grain absorbs moisture and then germinates, producing a pollen tube that extends down the style toward the ovary. The tip of the pollen tube enters the ovary directed by a chemical attractant (possibly calcium), and probes through the micropyle (a gap in the integuments of the ovule). The germinated pollen grain contains the mature male gametophyte. The nucleus of the generative cell divides by mitosis to produce 2 sperm (male gametes), which are discharged within the embryo sac. Both sperm fuse with nuclei in the embryo sac. One fertilizes the egg to form the zygote. The other combines with the two polar nuclei to form a triploid nucleus in the central cell, which will give rise to the endosperm, a food-storing tissue of the seed. The endosperm provides nutrients to the developing embryo. In most monocots and some dicots, the endosperm also stores nutrients that can be used by the seedling after germination.
Double fertilization ensures that the endosperm will develop only in ovules where the egg has been fertilized, thus preventing angiosperms from squandering nutrients.
13. The self-incompatibility systems in plant are analogous to the immune response of animals. Both are based on the ability of organisms to distinguish “self” from “nonself.” The key difference is that the animal immune system rejects nonself, but self-incompatibility in plants is a rejection of self.
14. The union of two sperm cells with different nuclei of the embryo sac is termed double fertilization. After double fertilization, the ovule develops into a seed, and the ovary develops into a fruit enclosing the seed(s). As the embryo develops, the seed stockpiles proteins, oils, and starch. Initially, these nutrients are stored in the endosperm. Later in seed development in many species, the storage function is taken over by the swelling storage leaves (cotyledons) of the embryo itself.
15. Endosperm development usually precedes embryo development. After double fertilization, the triploid nucleus of the ovule’s central cell divides, forming a multinucleate “supercell” having a milky consistency. It becomes multicellular when cytokinesis partitions the cytoplasm between nuclei. Cell walls form, and the endosperm becomes solid. Coconut “milk” is an example of liquid endosperm and coconut “meat” is an example of solid endosperm.
16. The first mitotic division of the zygote is transverse, splitting the fertilized egg into a basal cell and a terminal cell. The terminal cell gives rise to most of the embryo. The basal cell continues to divide transversely, producing a thread of cells, the suspensor, which anchors the embryo to its parent. The suspensor functions in the transfer of nutrients to the embryo from the parent. The terminal cell divides several times and forms a spherical proembryo attached to the suspensor. Cotyledons begin to form as bumps on the proembryo. After the cotyledons appear, the embryo elongates. Cradled between cotyledons is the embryonic shoot apex with the apical meristem of the embryonic shoot. At the opposite end of the embryo axis is the apex of the embryonic root, also with a meristem. After the seed germinates, the apical meristems at the tips of the shoot and root sustain primary growth as long as the plant lives. During the last stages of maturation, a seed dehydrates until its water content is only about 5–15% of its weight. The embryo stops growing and becomes dormant until the seed germinates. The embryo and its food supply are enclosed by a protective seed coat formed by the integuments of the ovule.
17. Describe the development of a plant embryo from the first mitotic division to the embryonic plant with rudimentary organs.
After double fertilization occurs, the 1st mitotic division of the zygote (transverse) splits the fertilized egg into a basal cell and a terminal cell. The terminal cell gives rise to most of the embryo. The basal cell continues to divide (transversely), and produces a thread of cells, the suspensor, which anchor the embryo to its parent and functions in the transfer of nutrients to the embryo from the parent. The terminal cell divides several times and forms a spherical proembryo attached to the suspensor. Bumps on the proembryo turn into cotyledons (dicots have two and look heart-shaped at this stage, monocots only have one). Then, the embryo elongates. The shoot apex with the apical meristem grows between the cotyledons. At the opposite end of the embryo axis is the apex of the embryonic root, with its own meristem. After the seed germinates, the apical meristems at the root and shoot tips sustain primary growth as long as the plant lives.
During the last stages of maturation, a seed will dehydrate itself until its water content is only 55-15% of its weight. Growth stops and the seed becomes dormant. The embryo and its food supply are enclosed by a protective seed coat formed by the integuments of the ovule.
18. The seed coat, a protective layer of integument formed by the ovule, encloses the embryo and its food supply. The radicle is the embryonic root. The endosperm is a food-storing tissue of the seed, rich in nutrients to feed to developing embryo. The embryo is a structure in seeds that generally consists of an elongate structure (the embryonic axis), which is attached to fleshy cotyledons. Cotyledons are storage leaves of the embryo. They absorb nutrients from the endosperm and transfer them to the embryo when the seed germinates. Members of the grass family (i.e. maize & wheat), have a specialized cotyledons called scutellums, which are very thin, with a large surface area pressed against the endosperm. Below the point at which the fleshy cotyledons are attached, the embryonic axis is called the hypocotyl. The hypocotyl terminates in the radicle, or embryonic root. Above it is the epicotyl. At the tip of the epicotyl is the plumule, which consists of the shoot tip with a pair of miniature leaves. Cradled between cotyledons is the embryonic shoot apex with the apical meristem of the embryonic shoot.
19. Monocots and dicots seeds differ in many ways. To begin with, their endosperms are not alike. Generally, in monocots (and in some dicots), the endosperm can be used to stored nutrients even after the seed has germinated. Most dicots, however, completely export the food reserves of the endosperm to their cotyledons before seed development has ended. Thus, mature seeds lack endosperms. Secondly, monocot embryos have a single cotyledon, while dicots have two.
20. Fruits are plant ovaries adapted for seed disperal. Generally, however, if no pollination occurs, a fruit will not develop. Instead, the flower will wither and fall off.While seeds develop from ovules, the flowering plant’s ovary develops into a fruit, to protect the enclosed seeds, and aid in their dispersal by wind or animals. The transformation is triggered by hormonal changes after fertilization. The ovary’s walls become the pericarp, the thickened wall of the fruit. Normally, other parts of the flower wither and are shed, but in certain angiosperms, floral parts may contribute to the fruit. For example, in apples, the fleshy part of the fruit is mostly derived from the swollen receptacle, while the core of the apple fruit develops from the ovary. Depending on developmental origin, there are several types of fruits.
- simple = typical fruit derived from a single carpel or several fused carpels... can be fleshy (i.e. peach) or dry (i.e. pea pod)
- aggregate = results from single flower that has more than one carpelà each forms a small fruit à fruitlets are clustered together on a single receptacle (ex: raspberry)
- multiple = develops from a group of flowers tightly clustered together (inflorescence) à when walls of ovaries thicken, they fuse together and form one fruit (ex: pineapple)
Fruits usually ripen about the same time that their seeds are completing development. For dry fruit (i.e. soybean pods), the ripening occurs so that the fruit will open and release the seeds. In fleshy fruits, however, ripening is controlled by complex hormone interactions. The fruit becomes edible and enticing to animals, to help seed dispersal. The fruit’s “pulp” becomes soften due to enzymes that begin digesting components of the cell walls. Color changes will also occur, from green to red, orange, or yellow. Lastly, fruits becomes sweeter, as organic acids or starch molecules are converted to sugar.
22. As a seed matures, it dehydrates and enters a dormancy phase, a condition of extremely low metabolic rate and a suspension of growth and development. Seed dormancy increases the chances that germination will occur at a time and place advantageous to the seedling. Conditions required to break dormancy and resume growth/development vary between species. Some seeds germinate as soon as they are in a suitable environment. Others remain dormant until some specific environmental cue causes them to break dormancy. Where natural fires are common, many seeds require intense heat to break dormancy, allowing them to take advantage of new opportunities and open space. In the desert, many plants germinate only after a substantial rainfall, ensuring enough water to complete development. Small seeds require light for germination, and break dormancy only if they are buried near the soil’s surface. Other seeds require a chemical attack or physical abrasion as they pass through an animal’s digestive tract before they can germinate. The length of time that a dormant seed remains viable and capable of germinating varies from a few days to decades or longer. Most seeds are durable enough to last for a year or two until conditions are favorable for germination. Nongerminated seeds can accumulated for several years. Germination of seeds depends on imbibition, the uptake of water due to the low water potential of the dry seed. This causes the expanding seed to rupture its seed coat and triggers metabolic changes in the embryo that enable it to resume growth. Enzymes begin digesting the storage materials of endosperm or cotyledons, and the nutrients are transferred to the growing regions of the embryo.
23. Variation occur in germination as they occur in breaking dormancy. Different conditions and environments effect the plant’s growth and development process. Generally, however, the first organ to emerge from the germinating seed is the radicle, the embryonic root. Next, the shoot tip must break through the soil surface. In many dicots, a hook forms in the hypocotyl, and growth pushes it aboveground. Stimulated by light, the hypocotyl straightens, raising the cotyledons and epicotyl. As it rises into the air, the epicotyl spreads its first foliage [true] leaves. These expand, become green, and begin making food by photosynthesis. After the cotyledons have transferred all their nutrients to the developing plant, they shrivel and fall off the seedling. Monocots may use use a different method for breaking ground when they germinate. First, the coleoptile (the sheath enclosing/protecting the embryonic shoot) pushes upward through the soil and into the air. The shoot tip then grows straight up through the tunnel provided by the tubular coleoptile. The tough seed gives rise to a fragile seedling. Because this plant is exposed to both animals and the elements, its survival rate is not high. Thus, as a mature parent, it must produce enormous numbers of seeds to compensate for low individual survival. Ample genetic variation is provided for natural selection to screen.
25. Plants can reproduce sexually or asexually. Asexual reproduction is an extension of the capacity of plants for indeterminate growth. Meristematic tissues with dividing undifferentiated cells can sustain or renew growth indefinitely. Parenchyma cells throughout the plant can divide and differentiate into various types of specialized cells. Detached fragments of some plants can develop into whole offspring. In fragmentation, a parent plant separates into parts that re-form into whole plants. *A variation of this process occurs in some dicots, in which the root system of a single parent gives rise to many adventitious shoots that become separate root systems, forming a clone.* A different method of asexual reproduction, called apomixis, is found in dandelions and some other plants. These produce seed without their flowers being fertilized. A diploid cell in the ovule gives rise to the embryo, and the ovules mature into seeds, which are dispersed by the wind. This process combines asexual reproduction and seed dispersal.
1. Plant and angiosperm life cycles are characterized by an alternation of generations. Haploid (n) and diploid (2n) generations take turns producing each other. Through meiosis, the diploid sporophyte, produces haploid spores, which divide by mitosis, giving rise to multicellular male and female haploid plants—the gametophytes. The gametophytes produce gametes—sperm and eggs. Fertilization results in diploid zygotes, which divide by mitosis to form new sporophytes.
In angiosperms, the dominant generation is the conspicuous sporophyte plant, which produces the flower. Flowers are specialized shoots that function as unique reproductive structures bearing the reproductive organs of the angiosperm sporophyte. Male and female gametophytes develop within the anthers and ovules, respectively, of a sporophyte flower. Gametophytes became reduced in seed plants over the course of time, and evolved to become dependent upon their sporophyte parents. Consisting of only a few cells, angiosperm gametophytes are the most reduced of all plants. A male gametophyte (pollen grain) is brought to a female gametophyte (contained in an ovule embedded in the ovary of a flower) through pollination by wind, water, or animals. A union of gametes (fertilization) takes place inside the ovary. Ovules develop into seeds, while the ovary itself develops into the fruit around the seed.
2. Floral parts in order from outside in: sepals, petals, stamen, carpels
3. Sepals, petals, stamens, and carpels are all floral organs. They attach to the stem at the receptacle.
Sepals enclose/protect the floral bud before it opens. They are generally green and more leaflike in appearance than the other floral organs. Petals are brightly colored organs that attract insects/pollinators. Sepals and petals are sterile. Stamens are male reproductive organs that consist of a stalk (filament) and a terminal anther containing chambers called pollen sacs, which produce pollen. Carpels are the female reproductive organs of a flower. Flowers can have more than one carpel. At the base of a carpel is an ovary, inside of which one or more ovules can be found. Carpels also have a slender neck called the style. A sticky structure called the stigma exists are the top of the style, and serves as a landing platform for pollen. The anthers and the ovules bear sporangia, where spores are produced by meiosis and gametophytes later develop. The male gametophytes are sperm-producing structures called pollen grains, which form within the pollen sacs of anthers. The female gametophytes are egg-producing structures called embryo sacs, which form within the ovules in ovaries.
5. Complete flowers have all four organs and has both male and female reproductive organs), while incomplete flowers lack one or more of the four floral parts.
A bisexual flower is equipped with both stamens and carpels. All complete and many incomplete flowers are bisexual. A unisexual flower is missing either stamens (making it a carpellate flower) or carpels (making it a staminate flower).
A monoecious plant has staminate and carpellate flowers at separate locations on the same individual plant. For example, maize and other corn varieties have ears derived from clusters of carpellate flowers, while the tassels consist of staminate flowers. Meanwhile, dioecious species have staminate flowers and carpellate flowers on separate plants. For example, date palms have carpellate individuals that produce dates and staminate individuals that produce pollen.
6. Gametes are produced in the haploid generation by gametophytes. Male and female gametophytes develop within the anthers and ovules, respectively, of a sporophyte flower. They are produced by the process of mitosis.
7. Male gametophytes= pollen grains & female gametophytes= embryo sacs
8. The female gametophytes are egg-producing structures called embryo sacs, which form within the ovules in ovaries. One cell in the sporangium of each ovule, the megasporocyte, grows and then goes through meiosis, producing four haploid megaspores. In many angiosperms, only one megaspore survives. This megaspore divides by mitosis three times without cytokinesis, forming in one cell with eight haploid nuclei. Membranes partition this mass into a multicellular female gametophyte—the embryo sac. Three cells sit at one end of the embryo sac: two synergid cells flanking the egg cell. The synergids function in the attraction and guidance of the pollen tube. At the other end of the egg sac are three antipodal cells of unknown function. The other two nuclei, the polar nuclei, share the cytoplasm of the large central cell of the embryo sac.
9. Pollination, which brings male and female gametophytes together, is the first step in the chain of events that leads to fertilization. Some plants, such as grasses and many trees, release large quantities of pollen on the wind to compensate for the randomness of this dispersal mechanism. Some aquatic plants rely on water to disperse pollen. Most angiosperms interact with insects or other animals that transfer pollen directly between flowers.
10. Pollination by wind, water, or animals brings a male gametophyte (pollen grain) to a female gametophyte contained in an ovule embedded in the ovary of a flower. Fertilization is the union of the gametes.
11. The various barriers that prevent self-fertilization contribute to genetic variety by ensuring that sperm and eggs come from different parents. Dioecious plants cannot self-fertilize because they are unisexual. In plants with bisexual flowers, a variety of mechanisms may prevent self-fertilization. For example, in some species stamens and carpels mature at different times. Alternatively, they may be arranged in such a way that it is mechanically unlikely that an animal pollinator could transfer pollen from the anthers to the stigma of the same flower. The most common anti-self fertilizing mechanism is self-incompatibility, the ability of a plant to reject its own pollen and that of closely related individuals. If a pollen grain from an anther happens to land on a stigma of a flower on the same plant, a biochemical block prevents the pollen from completing its development and fertilizing an egg.
12. Double fertilization gives rise to the zygote and endosperm. The process begins after a pollen grain lands on a plant’s stigma. The grain absorbs moisture and then germinates, producing a pollen tube that extends down the style toward the ovary. The tip of the pollen tube enters the ovary directed by a chemical attractant (possibly calcium), and probes through the micropyle (a gap in the integuments of the ovule). The germinated pollen grain contains the mature male gametophyte. The nucleus of the generative cell divides by mitosis to produce 2 sperm (male gametes), which are discharged within the embryo sac. Both sperm fuse with nuclei in the embryo sac. One fertilizes the egg to form the zygote. The other combines with the two polar nuclei to form a triploid nucleus in the central cell, which will give rise to the endosperm, a food-storing tissue of the seed. The endosperm provides nutrients to the developing embryo. In most monocots and some dicots, the endosperm also stores nutrients that can be used by the seedling after germination.
Double fertilization ensures that the endosperm will develop only in ovules where the egg has been fertilized, thus preventing angiosperms from squandering nutrients.
13. The self-incompatibility systems in plant are analogous to the immune response of animals. Both are based on the ability of organisms to distinguish “self” from “nonself.” The key difference is that the animal immune system rejects nonself, but self-incompatibility in plants is a rejection of self.
14. The union of two sperm cells with different nuclei of the embryo sac is termed double fertilization. After double fertilization, the ovule develops into a seed, and the ovary develops into a fruit enclosing the seed(s). As the embryo develops, the seed stockpiles proteins, oils, and starch. Initially, these nutrients are stored in the endosperm. Later in seed development in many species, the storage function is taken over by the swelling storage leaves (cotyledons) of the embryo itself.
15. Endosperm development usually precedes embryo development. After double fertilization, the triploid nucleus of the ovule’s central cell divides, forming a multinucleate “supercell” having a milky consistency. It becomes multicellular when cytokinesis partitions the cytoplasm between nuclei. Cell walls form, and the endosperm becomes solid. Coconut “milk” is an example of liquid endosperm and coconut “meat” is an example of solid endosperm.
16. The first mitotic division of the zygote is transverse, splitting the fertilized egg into a basal cell and a terminal cell. The terminal cell gives rise to most of the embryo. The basal cell continues to divide transversely, producing a thread of cells, the suspensor, which anchors the embryo to its parent. The suspensor functions in the transfer of nutrients to the embryo from the parent. The terminal cell divides several times and forms a spherical proembryo attached to the suspensor. Cotyledons begin to form as bumps on the proembryo. After the cotyledons appear, the embryo elongates. Cradled between cotyledons is the embryonic shoot apex with the apical meristem of the embryonic shoot. At the opposite end of the embryo axis is the apex of the embryonic root, also with a meristem. After the seed germinates, the apical meristems at the tips of the shoot and root sustain primary growth as long as the plant lives. During the last stages of maturation, a seed dehydrates until its water content is only about 5–15% of its weight. The embryo stops growing and becomes dormant until the seed germinates. The embryo and its food supply are enclosed by a protective seed coat formed by the integuments of the ovule.
17. Describe the development of a plant embryo from the first mitotic division to the embryonic plant with rudimentary organs.
After double fertilization occurs, the 1st mitotic division of the zygote (transverse) splits the fertilized egg into a basal cell and a terminal cell. The terminal cell gives rise to most of the embryo. The basal cell continues to divide (transversely), and produces a thread of cells, the suspensor, which anchor the embryo to its parent and functions in the transfer of nutrients to the embryo from the parent. The terminal cell divides several times and forms a spherical proembryo attached to the suspensor. Bumps on the proembryo turn into cotyledons (dicots have two and look heart-shaped at this stage, monocots only have one). Then, the embryo elongates. The shoot apex with the apical meristem grows between the cotyledons. At the opposite end of the embryo axis is the apex of the embryonic root, with its own meristem. After the seed germinates, the apical meristems at the root and shoot tips sustain primary growth as long as the plant lives.
During the last stages of maturation, a seed will dehydrate itself until its water content is only 55-15% of its weight. Growth stops and the seed becomes dormant. The embryo and its food supply are enclosed by a protective seed coat formed by the integuments of the ovule.
18. The seed coat, a protective layer of integument formed by the ovule, encloses the embryo and its food supply. The radicle is the embryonic root. The endosperm is a food-storing tissue of the seed, rich in nutrients to feed to developing embryo. The embryo is a structure in seeds that generally consists of an elongate structure (the embryonic axis), which is attached to fleshy cotyledons. Cotyledons are storage leaves of the embryo. They absorb nutrients from the endosperm and transfer them to the embryo when the seed germinates. Members of the grass family (i.e. maize & wheat), have a specialized cotyledons called scutellums, which are very thin, with a large surface area pressed against the endosperm. Below the point at which the fleshy cotyledons are attached, the embryonic axis is called the hypocotyl. The hypocotyl terminates in the radicle, or embryonic root. Above it is the epicotyl. At the tip of the epicotyl is the plumule, which consists of the shoot tip with a pair of miniature leaves. Cradled between cotyledons is the embryonic shoot apex with the apical meristem of the embryonic shoot.
19. Monocots and dicots seeds differ in many ways. To begin with, their endosperms are not alike. Generally, in monocots (and in some dicots), the endosperm can be used to stored nutrients even after the seed has germinated. Most dicots, however, completely export the food reserves of the endosperm to their cotyledons before seed development has ended. Thus, mature seeds lack endosperms. Secondly, monocot embryos have a single cotyledon, while dicots have two.
20. Fruits are plant ovaries adapted for seed disperal. Generally, however, if no pollination occurs, a fruit will not develop. Instead, the flower will wither and fall off.While seeds develop from ovules, the flowering plant’s ovary develops into a fruit, to protect the enclosed seeds, and aid in their dispersal by wind or animals. The transformation is triggered by hormonal changes after fertilization. The ovary’s walls become the pericarp, the thickened wall of the fruit. Normally, other parts of the flower wither and are shed, but in certain angiosperms, floral parts may contribute to the fruit. For example, in apples, the fleshy part of the fruit is mostly derived from the swollen receptacle, while the core of the apple fruit develops from the ovary. Depending on developmental origin, there are several types of fruits.
- simple = typical fruit derived from a single carpel or several fused carpels... can be fleshy (i.e. peach) or dry (i.e. pea pod)
- aggregate = results from single flower that has more than one carpelà each forms a small fruit à fruitlets are clustered together on a single receptacle (ex: raspberry)
- multiple = develops from a group of flowers tightly clustered together (inflorescence) à when walls of ovaries thicken, they fuse together and form one fruit (ex: pineapple)
Fruits usually ripen about the same time that their seeds are completing development. For dry fruit (i.e. soybean pods), the ripening occurs so that the fruit will open and release the seeds. In fleshy fruits, however, ripening is controlled by complex hormone interactions. The fruit becomes edible and enticing to animals, to help seed dispersal. The fruit’s “pulp” becomes soften due to enzymes that begin digesting components of the cell walls. Color changes will also occur, from green to red, orange, or yellow. Lastly, fruits becomes sweeter, as organic acids or starch molecules are converted to sugar.
22. As a seed matures, it dehydrates and enters a dormancy phase, a condition of extremely low metabolic rate and a suspension of growth and development. Seed dormancy increases the chances that germination will occur at a time and place advantageous to the seedling. Conditions required to break dormancy and resume growth/development vary between species. Some seeds germinate as soon as they are in a suitable environment. Others remain dormant until some specific environmental cue causes them to break dormancy. Where natural fires are common, many seeds require intense heat to break dormancy, allowing them to take advantage of new opportunities and open space. In the desert, many plants germinate only after a substantial rainfall, ensuring enough water to complete development. Small seeds require light for germination, and break dormancy only if they are buried near the soil’s surface. Other seeds require a chemical attack or physical abrasion as they pass through an animal’s digestive tract before they can germinate. The length of time that a dormant seed remains viable and capable of germinating varies from a few days to decades or longer. Most seeds are durable enough to last for a year or two until conditions are favorable for germination. Nongerminated seeds can accumulated for several years. Germination of seeds depends on imbibition, the uptake of water due to the low water potential of the dry seed. This causes the expanding seed to rupture its seed coat and triggers metabolic changes in the embryo that enable it to resume growth. Enzymes begin digesting the storage materials of endosperm or cotyledons, and the nutrients are transferred to the growing regions of the embryo.
23. Variation occur in germination as they occur in breaking dormancy. Different conditions and environments effect the plant’s growth and development process. Generally, however, the first organ to emerge from the germinating seed is the radicle, the embryonic root. Next, the shoot tip must break through the soil surface. In many dicots, a hook forms in the hypocotyl, and growth pushes it aboveground. Stimulated by light, the hypocotyl straightens, raising the cotyledons and epicotyl. As it rises into the air, the epicotyl spreads its first foliage [true] leaves. These expand, become green, and begin making food by photosynthesis. After the cotyledons have transferred all their nutrients to the developing plant, they shrivel and fall off the seedling. Monocots may use use a different method for breaking ground when they germinate. First, the coleoptile (the sheath enclosing/protecting the embryonic shoot) pushes upward through the soil and into the air. The shoot tip then grows straight up through the tunnel provided by the tubular coleoptile. The tough seed gives rise to a fragile seedling. Because this plant is exposed to both animals and the elements, its survival rate is not high. Thus, as a mature parent, it must produce enormous numbers of seeds to compensate for low individual survival. Ample genetic variation is provided for natural selection to screen.
25. Plants can reproduce sexually or asexually. Asexual reproduction is an extension of the capacity of plants for indeterminate growth. Meristematic tissues with dividing undifferentiated cells can sustain or renew growth indefinitely. Parenchyma cells throughout the plant can divide and differentiate into various types of specialized cells. Detached fragments of some plants can develop into whole offspring. In fragmentation, a parent plant separates into parts that re-form into whole plants. *A variation of this process occurs in some dicots, in which the root system of a single parent gives rise to many adventitious shoots that become separate root systems, forming a clone.* A different method of asexual reproduction, called apomixis, is found in dandelions and some other plants. These produce seed without their flowers being fertilized. A diploid cell in the ovule gives rise to the embryo, and the ovules mature into seeds, which are dispersed by the wind. This process combines asexual reproduction and seed dispersal.
AP Biology Ch36 Objectives
Chapter 36 – Transport in Vascular Plants
1. The most important active transport protein in the plasma membrane of plant cells is the proton pump. It hydrolyzes ATP and uses the released energy to pump hydrogen ions (H+) out of the cell. This creates a proton gradient because the H+ concentration is higher outside the cell than inside. It also creates a membrane potential or voltage, a separation of opposite charges across a membrane. Both the concentration gradient and the membrane potential are forms of potential (stored) energy that can be harnessed to perform cellular work. The proton gradient also functions in cotransport, in which the downhill passage of one solute (H+) is coupled with the uphill passage of another, such as NO3- or sucrose. The role of proton pumps in transport is a specific application of the general mechanism called chemiosmosis, a unifying process in cellular energetics. In chemiosmosis, a transmembrane proton gradient links energy-releasing processes to energy-consuming processes.
2. The net uptake or loss of water by a cell occurs by osmosis, the passive transport of water across a membrane. In the case of a plant cell, the direction of water movement depends on solute concentration and physical pressure. The combined effects of solute concentration and pressure are called water potential, represented by the Greek letter “psi.” Plant biologists measure psi in units called megapascals (MPa), where one MPa is equal to about 10 atmospheres of pressure. An atmosphere is the pressure exerted at sea level by an imaginary column of air—about 1 kg of pressure per square centimeter.
3. Both pressure and solute concentration affect water potential. The combined effects of pressure and solute concentrations on water potential are incorporated into psi = psip + psis, where psip is the pressure potential and psis is the solute potential (or osmotic potential). The addition of solutes lowers the water potential because the solutes bind water molecules, which have less freedom to move than they do in pure water.
4. In a flaccid cell, where the pressure potential is 0, the cell is limp. If this cell is placed in a solution with a higher solute concentration (and, therefore, a lower psi), water will leave the cell by osmosis. Eventually, the cell will plasmolyze by shrinking and pulling away from its wall. If a flaccid cell is placed in pure water (psi = 0), the cell will have lower water potential than pure water due to the presence of solutes, and water will enter the cell by osmosis. As the cell begins to swell, it will push against the cell wall, producing turgor pressure. The partially elastic wall will push back until this pressure is great enough to offset the tendency for water to enter the cell because of solutes. A walled cell with a greater solute concentration than its surroundings will be turgid, or firm.
5. In a flaccid cell, psip = 0 and the cell is limp. The cell will plasmolyze by shrinking and pulling away from its wall when water is leaving the cell by osmosis. Water in living cells is usually under positive pressure. The cell contents press the plasma membrane against the cell wall, producing turgor pressure. A walled cell with a greater solute concentration than its surroundings will be turgid, or firm.
6. Both plant and animal membranes have specific transport proteins, aquaporins, which facilitate the passive movement of water across a membrane. Aquaporins do not affect the water potential gradient or the direction of water flow, but rather increase the rate at which water diffuses down its water potential gradient. Evidence is accumulating that the rate of water movement through aquaporins is regulated by changes in second messengers such as calcium ions (Ca2+). This raises the possibility that the cell can regulate its rate of water uptake or loss when its water potential is different from that of its environment.
7. The three major compartments in vacuolated plants cells are: the cell wall, cytosol, and vacuole.
8. In most plant tissues, two of the three cellular compartments are continuous from cell to cell. Plasmodesmata connect the cytosolic compartments of neighboring cells. This cytoplasmic continuum, the symplast, forms a continuous pathway for transport of certain molecules between cells. The walls of adjacent plant cells are also in contact, forming a second continuous compartment, the apoplast.
9. Three routes are available for lateral transport. In one route, substances move out of one cell, across the cell wall, and into the neighboring cell, which may then pass the substances along to the next cell by the same mechanism (this transmembrane route requires repeated crossings of plasma membranes). The second route, via the symplast, requires only one crossing of a plasma membrane (after entering one cell, solutes and water move from cell to cell via plasmodesmata). The third route is along the apoplast, the extracellular pathway consisting of cell wall and extracellular spaces (water and solutes can move from one location to another within a root or other organ through the continuum of cell walls without ever entering a cell).
10. Diffusion is much too slow for long-distance transport within a plant, such as the movement of water and minerals from roots to leaves. Water and solutes move through xylem vessels and sieve tubes by bulk flow, the movement of a fluid driven by pressure. In phloem, hydrostatic pressure generated at one end of a sieve tube forces sap to the opposite end of the tube. In xylem, it is actually tension (negative pressure) that drives long-distance transport. Transpiration, the evaporation of water from a leaf, reduces pressure in the leaf xylem. This creates a tension that pulls xylem sap upward from the roots.
11. To maximize bulk flow, the sieve-tube members are almost entirely devoid of internal organelles. Vessel elements and tracheids are dead at maturity. The porous plates that connect contiguous sieve-tube members and the perforated end walls of xylem vessel elements also enhance bulk flow.
12. Water and mineral from the soil enter the plant through the epidermis of roots, cross the root cortex, pass into the vascular cylinder, and then flow up xylem vessels to the shoot system. The uptake of soil solution by the hydrophilic epidermal walls of root hairs provides access to the apoplast, and water and minerals can soak into the cortex along this route. Minerals and water that cross the plasma membranes of root hairs enter the symplast. Some water and minerals are transported into cells of the epidermis and cortex and then move inward via the symplast. Materials flowing along the apoplastic route are blocked by the waxy Casparian strip at the endodermis. Some minerals detour around the Casparian strip by crossing the plasma membrane of an endodermal cell to pass into the vascular cylinder. Endodermal and parenchyma cells within the vascular cylinder discharge water and minerals into their walls (apoplast). The water and minerals enter the dead cells of xylem vessels and are transported upward into the shoots.
13. The mycorrhizae create an enormous surface area for absorption and enable older regions of the roots to supply water and minerals to the plant.
14. The endodermis, with it’s Casparian strip, functions as a selective barrier between the root cortex and the vascular cylinder by acting as a filter. The flow of materials is blocked by the waxy Casparian strip. Only certain minerals can cross by using the plasma membrane of an endodermal cell to pass into the vascular cylinder.
15. Root cells pump mineral ions into the xylem, where they accumulate in the vascular cylinder. Consequentially, water potential is lowered [in the cylinders]. When the water potential lowers, it forces fluid up the xylem. This force is called root pressure. Root pressure helps xylem sap travel upwards in trees against gravity. However, it is not the major mechanism driving ascent of xylem sap, because root pressure can only force water up a few meters at a time. In addition, many plants do not generate root pressure.
16. Transpiration is the evaporation of water from a leaf. It is the main mechanism driving the ascent of xylem sap. Guttation is the exudation of water droplets that can be seen in the morning on the tips of grass blades or the leaf margins of some small, herbaceous dicots as a result of root pressure.
17. Transpiration, in simple terms, reduces pressure in the leaf xylem, and creates a tension that pulls xylem sap upward from the roots. The tension created by transpiration, however, only causes movement of water because of water’s properties of cohesion and adhesion (caused by hydrogen bonding), which transmit the upward pull along the entire length of the xylem to the roots.
Negative pressure or tension must be generated in order for transpiration to occur. Water’s physical properties are also an important factor. Water’s cohesive properties due to hydrogen bonding makes it possible to pull a column of sap up. Water’s adhesion to the hydrophilic walls of the xylem cells help fight the force of gravity. Also, the very small diameter of the tracheids and vessel elements exposes a large proportion of the water to the hydrophilic walls. *The upward pull on the cohesive sap creates tension within the xylem.
STEPS
- Water transpires from a leaf à Water coating the mesophyll cells replaces water lost from the air spaces
- Transpiring water evaporates à The remaining film of water is drawn back into the cell wall’s pores (due to its attraction to the hydrophilic walls)
- Water’s cohesive properties make it resist an increase in surface area of the film.
- A meniscus forms on the surface of the water due to surface tension and cohesive/adhesive forces.
- The film of water at the surface of the leaf cells has a negative pressure less than that of the atmosphere. The more concavity, the move negative pressure.
- The negative pressure pulls the water out of the leaf xylem, through the mesophyll, towards the cells and surface film bordering the air spaces.
- The tension generated by adhesion and surface tension lowers the water potential, drawing water from an area of high water potential to an area of lower water potential.
- Mesophyll cells lose water to the surface film lining the air spaces, which in turn loses water by transpiration.
- The water lost via the stomata is replaced by water pulled out of the leaf xylem. The transpirational pull on xylem sap is transmitted all the way from the leaves to the root tips and the soil solution.
18. Cavitation is the formation of water vapor pockets in the xylem vessel, and occurs when xylem sap freezes in water. Cavitation prevents the transport of water through xylem vessels because it breaks the chain of H2O. Transpirational pull can only extend down to the roots through an unbroken chain of water molecules.
19. The movement of xylem sap upwards is ultimately solar-powered bulk flow. The fluid’s ascent is basically long-distance transport, driven by a water potential difference at opposite ends of xylem vessels of chains of tracheids (called conduits, in general). No metabolic energy is used to lift xylem sap up to the leaves. Sunlight absorbed by the plants drives transpiration and evaporation of water from the walls of a plant’s mesophyll cells. The water potential differences generated by transpirational pull at the leaf end, which also lowers the water potential at the “upstream” end of the xylem (in the leaves’ air spaces lowers). This lowering of water potential increases tension. Water potential drives the osmotic movement of water from cell to cell within the root and leaf tissue. Differences in solution concentration and turgor pressure are also factors in water movement. However, in bulk flow, pressure is the only mechanism for long-distance transport up xylem vessels, and the whole solution is moved—the solvent and the solutes.
20. An extensive inner surface area of a leaf is both important and costly. Generally, leaves have have broad surface areas and high ratios of surface area to volume to enhance the absorption of light for photosynthesis. This, however, increases water loss through stomata. To make food, a plant must spread its leaves to the sun and obtain CO2 from air. While oxygen diffuses out of the leaf via the stomata, carbon dioxide diffuses into the leaf and enters a honeycomb of air spaces formed by the parenchyma cells (irregularly shaped). This internal surface can be anywhere from 10 to 30 times greater than the external leaf surface in order to increases exposure to CO2 while also increasing the surface area for evaporation.
21. A leaf’s stomatal density is affected by both its environment and its genes. In terms of environment, the heat and humidity of a region are important factors. Plants lose water through their stomata to cool them down. Desert plants have lower stomatal densities than do marsh plants. High light intensities and low carbon dioxide levels during plant development tend to increase stomatal density in many plant species. Increased CO2 levels tend to lead to decreased stomatal density. Changes in the density of the stomata prevent the leaf from reaching temperatures that could denature enzymes.
22. Flanking either side of every stomata, pairs of guard cells are suspended by epidermal cells over air chambers that lead to the internal air space. Guard cells control the stoma’s diameter by changing shape and narrowing or widening the gap between two of them. After water intake through osmosis, these cells become more turgid. Due to the orientation of their cellulose microfibrils, the guard cells buckle outward. When water is lost by the guard cells, they become flaccid and less bowed, and consequentially, the space between them closes.
Guard cells’ role in photosynthesis-transpiration?
23. Stomata open and close based on changes in turgor pressure. Primarily, these changes occur as a result of the loss and uptake of potassium ions (K+) by guard cells. When these ions are actively being accumulated, stomata are open; the water potential in guard cells decreases while their turgor increases due to the inflow of water by osmosis. When K+ ions leave the guard cells, water is lost through osmosis, and the stomata close.
The guard cells’ shrinking/swelling and the stomatal opening/closings maybe also be linked to the regulation of aquaporins, which vary the permeability of the membranes to water. The K+ fluctuations across the guard cell membranes are coupled with the generation of membrane potentials by proton pumps. When stomata are open, H+ ions are being actively transported out of guard cells. The resulting voltage/membrane potential drives K+ into the cell through specific membrane channels.
Stomata are generally open during the day and closed at night to minimize water loss when it is too dark for photosynthesis. Stomata usually open at dawn as a result of 3 cues. Blue-light receptors in the guard cells stimulate proton pumps that work in the uptake of K+. Because the pumps require ATP, light reactions begin to occur, and photosynthesis occurs to create a supply of ATP. CO2 within the mesophyll is depleted by these processes. The internal “clock” of the guard cells regulates the cyclic process and monitors the daily rhythmic opening and closing (this is an example of circadian rhythm, or a cycle with an internal of about 24 hours).
Various environmental stresses, however, can cause stomata to close during the day. If the plant is suffering a water deficiency, turgor in the guard cells may be lost. A hormone called abscisic acid will then be produced by mesophyll cells in response to water deficiency. Guard cells will be given a signal, and the stomata will close. Thus, wilting is stopped and photosynthesis is slowed.
24. Xerophytes are plants that adapted to arid climates by modifying their leaves in various ways that help reduce the rate of transpiration. To begin with, surface area to volume ratios in leaves are reduced. Consequentially, xerophytes have small, thick leaves. In the driest months, some plants shed their leaves, while others live in water stored in their fleshy stems (from the rainy season). Thick cuticles on xerophyte leaves help preventing drying, but also give some plants a leathery consistency. Stomata are located in depressions or crypts that protect the pores from dry wind. Hairs called trichomes also break up air flow and help keep humidity high in the crypt (compared to the surrounding atmosphere).
25. Some xerophytes reduce their transpiration by assimilating CO2 through an alternative photosynthetic pathway, crassulacean acid metabolism (CAM). The mesophyll cells in these CAM plants store CO2 in organic acids during the night, and release the CO2 from these organic acids during the day. Only during the day do CAM plants synthesize sugars, and when doing so, use the conventional (C3) photosynthetic pathway. This process allows stomata to stay closed during day and to prevent great transpiration.
26. Translocation is a process in which the organic products of photosynthesis are transported throughout the plant by the phloem. In angiosperms, the specialized cells of the phloem involved in this process are called the sieve-tube members, and are arranged end to end to form long tubes with porous cross-walls between the cells. Sieve tubes always carry food from a sugar source to a sugar sink. Sugar sources are plant organs (i.e. mature leaves) where sugar is being produced through photosynthesis or starch breakdown. Sugar sinks are organs (i.e. growing roots, shoots, or fruit) that consume or store sugar as well as minerals. Depending on the season, storage organs (i.e. tubers or bulbs) can be sources or sinks. In the summer, they stockpile carbs and are sugar sinks. In the spring, the same organs become sources, as their starches are broken down to sugars and are carried away in the phloem to growing buds of the shoot system. Sieve tubes in the same vascular bundle can carry sap in different directions. The direction of transport depends on the location of the source and sink connected by the tube.
27. Before being exported to sugar sinks, sugar from mesophyll cells (or other sources) must be loaded into sieve-tube members. Depending on the species of the plant, this movement can be via the symplast or by a combination of symplastic and apoplastic pathways. Sucrose can diffuse through the symplast from mesophyll cells into small veins. Much of this sugar moves out of the cells into the apoplast in the vicinity of sieve-tube members and companion cells. Companion cells pass the sugar they accumulate into the sieve-tube members via plasmodesmata. In some plants, companion cells (transfer cells) have numerous ingrowths in their walls to increase the cell’s surface area and enhance the transfer of solutes between apoplast and symplast. Because some sieve-tube members accumulate sucrose of very high concentrations, active transport is required to load the phloem. Proton pumps generate an H+ gradient, which drives sucrose across the membrane via a cotransport protein that couples sucrose transport to the diffusion of H+ back into the cell. Downstream, at the sink end of the sieve tube, phloem unloads its sucrose. The mechanism of phloem unloading is highly variable and depends on plant species and type of organ. Regardless of mechanism, because the concentration of free sugar in the sink is lower than in the phloem, sugar molecules diffuse from the phloem into the sink tissues. Water follows by osmosis.
28. In angiosperms, the mechanism of translocation is called pressure flow. It involves moving phloem sap from sugar sources to sugar sinks by bulk flow driven by positive pressure. Pressure flow moves the sap faster than diffusion or cytoplasmic streaming. Pressure flow in a sieve tube drives the bulk flow of phloem sap. Sugar is loaded into the tube at source. This reduces the water potential inside the sieve-tube members, and causes water uptake. The absorption of water then generates hydrostatic pressure, which forces the sap to flow along the tube. At the sink, pressure is relieved when the sugars are unloaded and water is lost from the tube. In leaf-to-root translocation, water is recycled by the xylem from sink to source.
1. The most important active transport protein in the plasma membrane of plant cells is the proton pump. It hydrolyzes ATP and uses the released energy to pump hydrogen ions (H+) out of the cell. This creates a proton gradient because the H+ concentration is higher outside the cell than inside. It also creates a membrane potential or voltage, a separation of opposite charges across a membrane. Both the concentration gradient and the membrane potential are forms of potential (stored) energy that can be harnessed to perform cellular work. The proton gradient also functions in cotransport, in which the downhill passage of one solute (H+) is coupled with the uphill passage of another, such as NO3- or sucrose. The role of proton pumps in transport is a specific application of the general mechanism called chemiosmosis, a unifying process in cellular energetics. In chemiosmosis, a transmembrane proton gradient links energy-releasing processes to energy-consuming processes.
2. The net uptake or loss of water by a cell occurs by osmosis, the passive transport of water across a membrane. In the case of a plant cell, the direction of water movement depends on solute concentration and physical pressure. The combined effects of solute concentration and pressure are called water potential, represented by the Greek letter “psi.” Plant biologists measure psi in units called megapascals (MPa), where one MPa is equal to about 10 atmospheres of pressure. An atmosphere is the pressure exerted at sea level by an imaginary column of air—about 1 kg of pressure per square centimeter.
3. Both pressure and solute concentration affect water potential. The combined effects of pressure and solute concentrations on water potential are incorporated into psi = psip + psis, where psip is the pressure potential and psis is the solute potential (or osmotic potential). The addition of solutes lowers the water potential because the solutes bind water molecules, which have less freedom to move than they do in pure water.
4. In a flaccid cell, where the pressure potential is 0, the cell is limp. If this cell is placed in a solution with a higher solute concentration (and, therefore, a lower psi), water will leave the cell by osmosis. Eventually, the cell will plasmolyze by shrinking and pulling away from its wall. If a flaccid cell is placed in pure water (psi = 0), the cell will have lower water potential than pure water due to the presence of solutes, and water will enter the cell by osmosis. As the cell begins to swell, it will push against the cell wall, producing turgor pressure. The partially elastic wall will push back until this pressure is great enough to offset the tendency for water to enter the cell because of solutes. A walled cell with a greater solute concentration than its surroundings will be turgid, or firm.
5. In a flaccid cell, psip = 0 and the cell is limp. The cell will plasmolyze by shrinking and pulling away from its wall when water is leaving the cell by osmosis. Water in living cells is usually under positive pressure. The cell contents press the plasma membrane against the cell wall, producing turgor pressure. A walled cell with a greater solute concentration than its surroundings will be turgid, or firm.
6. Both plant and animal membranes have specific transport proteins, aquaporins, which facilitate the passive movement of water across a membrane. Aquaporins do not affect the water potential gradient or the direction of water flow, but rather increase the rate at which water diffuses down its water potential gradient. Evidence is accumulating that the rate of water movement through aquaporins is regulated by changes in second messengers such as calcium ions (Ca2+). This raises the possibility that the cell can regulate its rate of water uptake or loss when its water potential is different from that of its environment.
7. The three major compartments in vacuolated plants cells are: the cell wall, cytosol, and vacuole.
8. In most plant tissues, two of the three cellular compartments are continuous from cell to cell. Plasmodesmata connect the cytosolic compartments of neighboring cells. This cytoplasmic continuum, the symplast, forms a continuous pathway for transport of certain molecules between cells. The walls of adjacent plant cells are also in contact, forming a second continuous compartment, the apoplast.
9. Three routes are available for lateral transport. In one route, substances move out of one cell, across the cell wall, and into the neighboring cell, which may then pass the substances along to the next cell by the same mechanism (this transmembrane route requires repeated crossings of plasma membranes). The second route, via the symplast, requires only one crossing of a plasma membrane (after entering one cell, solutes and water move from cell to cell via plasmodesmata). The third route is along the apoplast, the extracellular pathway consisting of cell wall and extracellular spaces (water and solutes can move from one location to another within a root or other organ through the continuum of cell walls without ever entering a cell).
10. Diffusion is much too slow for long-distance transport within a plant, such as the movement of water and minerals from roots to leaves. Water and solutes move through xylem vessels and sieve tubes by bulk flow, the movement of a fluid driven by pressure. In phloem, hydrostatic pressure generated at one end of a sieve tube forces sap to the opposite end of the tube. In xylem, it is actually tension (negative pressure) that drives long-distance transport. Transpiration, the evaporation of water from a leaf, reduces pressure in the leaf xylem. This creates a tension that pulls xylem sap upward from the roots.
11. To maximize bulk flow, the sieve-tube members are almost entirely devoid of internal organelles. Vessel elements and tracheids are dead at maturity. The porous plates that connect contiguous sieve-tube members and the perforated end walls of xylem vessel elements also enhance bulk flow.
12. Water and mineral from the soil enter the plant through the epidermis of roots, cross the root cortex, pass into the vascular cylinder, and then flow up xylem vessels to the shoot system. The uptake of soil solution by the hydrophilic epidermal walls of root hairs provides access to the apoplast, and water and minerals can soak into the cortex along this route. Minerals and water that cross the plasma membranes of root hairs enter the symplast. Some water and minerals are transported into cells of the epidermis and cortex and then move inward via the symplast. Materials flowing along the apoplastic route are blocked by the waxy Casparian strip at the endodermis. Some minerals detour around the Casparian strip by crossing the plasma membrane of an endodermal cell to pass into the vascular cylinder. Endodermal and parenchyma cells within the vascular cylinder discharge water and minerals into their walls (apoplast). The water and minerals enter the dead cells of xylem vessels and are transported upward into the shoots.
13. The mycorrhizae create an enormous surface area for absorption and enable older regions of the roots to supply water and minerals to the plant.
14. The endodermis, with it’s Casparian strip, functions as a selective barrier between the root cortex and the vascular cylinder by acting as a filter. The flow of materials is blocked by the waxy Casparian strip. Only certain minerals can cross by using the plasma membrane of an endodermal cell to pass into the vascular cylinder.
15. Root cells pump mineral ions into the xylem, where they accumulate in the vascular cylinder. Consequentially, water potential is lowered [in the cylinders]. When the water potential lowers, it forces fluid up the xylem. This force is called root pressure. Root pressure helps xylem sap travel upwards in trees against gravity. However, it is not the major mechanism driving ascent of xylem sap, because root pressure can only force water up a few meters at a time. In addition, many plants do not generate root pressure.
16. Transpiration is the evaporation of water from a leaf. It is the main mechanism driving the ascent of xylem sap. Guttation is the exudation of water droplets that can be seen in the morning on the tips of grass blades or the leaf margins of some small, herbaceous dicots as a result of root pressure.
17. Transpiration, in simple terms, reduces pressure in the leaf xylem, and creates a tension that pulls xylem sap upward from the roots. The tension created by transpiration, however, only causes movement of water because of water’s properties of cohesion and adhesion (caused by hydrogen bonding), which transmit the upward pull along the entire length of the xylem to the roots.
Negative pressure or tension must be generated in order for transpiration to occur. Water’s physical properties are also an important factor. Water’s cohesive properties due to hydrogen bonding makes it possible to pull a column of sap up. Water’s adhesion to the hydrophilic walls of the xylem cells help fight the force of gravity. Also, the very small diameter of the tracheids and vessel elements exposes a large proportion of the water to the hydrophilic walls. *The upward pull on the cohesive sap creates tension within the xylem.
STEPS
- Water transpires from a leaf à Water coating the mesophyll cells replaces water lost from the air spaces
- Transpiring water evaporates à The remaining film of water is drawn back into the cell wall’s pores (due to its attraction to the hydrophilic walls)
- Water’s cohesive properties make it resist an increase in surface area of the film.
- A meniscus forms on the surface of the water due to surface tension and cohesive/adhesive forces.
- The film of water at the surface of the leaf cells has a negative pressure less than that of the atmosphere. The more concavity, the move negative pressure.
- The negative pressure pulls the water out of the leaf xylem, through the mesophyll, towards the cells and surface film bordering the air spaces.
- The tension generated by adhesion and surface tension lowers the water potential, drawing water from an area of high water potential to an area of lower water potential.
- Mesophyll cells lose water to the surface film lining the air spaces, which in turn loses water by transpiration.
- The water lost via the stomata is replaced by water pulled out of the leaf xylem. The transpirational pull on xylem sap is transmitted all the way from the leaves to the root tips and the soil solution.
18. Cavitation is the formation of water vapor pockets in the xylem vessel, and occurs when xylem sap freezes in water. Cavitation prevents the transport of water through xylem vessels because it breaks the chain of H2O. Transpirational pull can only extend down to the roots through an unbroken chain of water molecules.
19. The movement of xylem sap upwards is ultimately solar-powered bulk flow. The fluid’s ascent is basically long-distance transport, driven by a water potential difference at opposite ends of xylem vessels of chains of tracheids (called conduits, in general). No metabolic energy is used to lift xylem sap up to the leaves. Sunlight absorbed by the plants drives transpiration and evaporation of water from the walls of a plant’s mesophyll cells. The water potential differences generated by transpirational pull at the leaf end, which also lowers the water potential at the “upstream” end of the xylem (in the leaves’ air spaces lowers). This lowering of water potential increases tension. Water potential drives the osmotic movement of water from cell to cell within the root and leaf tissue. Differences in solution concentration and turgor pressure are also factors in water movement. However, in bulk flow, pressure is the only mechanism for long-distance transport up xylem vessels, and the whole solution is moved—the solvent and the solutes.
20. An extensive inner surface area of a leaf is both important and costly. Generally, leaves have have broad surface areas and high ratios of surface area to volume to enhance the absorption of light for photosynthesis. This, however, increases water loss through stomata. To make food, a plant must spread its leaves to the sun and obtain CO2 from air. While oxygen diffuses out of the leaf via the stomata, carbon dioxide diffuses into the leaf and enters a honeycomb of air spaces formed by the parenchyma cells (irregularly shaped). This internal surface can be anywhere from 10 to 30 times greater than the external leaf surface in order to increases exposure to CO2 while also increasing the surface area for evaporation.
21. A leaf’s stomatal density is affected by both its environment and its genes. In terms of environment, the heat and humidity of a region are important factors. Plants lose water through their stomata to cool them down. Desert plants have lower stomatal densities than do marsh plants. High light intensities and low carbon dioxide levels during plant development tend to increase stomatal density in many plant species. Increased CO2 levels tend to lead to decreased stomatal density. Changes in the density of the stomata prevent the leaf from reaching temperatures that could denature enzymes.
22. Flanking either side of every stomata, pairs of guard cells are suspended by epidermal cells over air chambers that lead to the internal air space. Guard cells control the stoma’s diameter by changing shape and narrowing or widening the gap between two of them. After water intake through osmosis, these cells become more turgid. Due to the orientation of their cellulose microfibrils, the guard cells buckle outward. When water is lost by the guard cells, they become flaccid and less bowed, and consequentially, the space between them closes.
Guard cells’ role in photosynthesis-transpiration?
23. Stomata open and close based on changes in turgor pressure. Primarily, these changes occur as a result of the loss and uptake of potassium ions (K+) by guard cells. When these ions are actively being accumulated, stomata are open; the water potential in guard cells decreases while their turgor increases due to the inflow of water by osmosis. When K+ ions leave the guard cells, water is lost through osmosis, and the stomata close.
The guard cells’ shrinking/swelling and the stomatal opening/closings maybe also be linked to the regulation of aquaporins, which vary the permeability of the membranes to water. The K+ fluctuations across the guard cell membranes are coupled with the generation of membrane potentials by proton pumps. When stomata are open, H+ ions are being actively transported out of guard cells. The resulting voltage/membrane potential drives K+ into the cell through specific membrane channels.
Stomata are generally open during the day and closed at night to minimize water loss when it is too dark for photosynthesis. Stomata usually open at dawn as a result of 3 cues. Blue-light receptors in the guard cells stimulate proton pumps that work in the uptake of K+. Because the pumps require ATP, light reactions begin to occur, and photosynthesis occurs to create a supply of ATP. CO2 within the mesophyll is depleted by these processes. The internal “clock” of the guard cells regulates the cyclic process and monitors the daily rhythmic opening and closing (this is an example of circadian rhythm, or a cycle with an internal of about 24 hours).
Various environmental stresses, however, can cause stomata to close during the day. If the plant is suffering a water deficiency, turgor in the guard cells may be lost. A hormone called abscisic acid will then be produced by mesophyll cells in response to water deficiency. Guard cells will be given a signal, and the stomata will close. Thus, wilting is stopped and photosynthesis is slowed.
24. Xerophytes are plants that adapted to arid climates by modifying their leaves in various ways that help reduce the rate of transpiration. To begin with, surface area to volume ratios in leaves are reduced. Consequentially, xerophytes have small, thick leaves. In the driest months, some plants shed their leaves, while others live in water stored in their fleshy stems (from the rainy season). Thick cuticles on xerophyte leaves help preventing drying, but also give some plants a leathery consistency. Stomata are located in depressions or crypts that protect the pores from dry wind. Hairs called trichomes also break up air flow and help keep humidity high in the crypt (compared to the surrounding atmosphere).
25. Some xerophytes reduce their transpiration by assimilating CO2 through an alternative photosynthetic pathway, crassulacean acid metabolism (CAM). The mesophyll cells in these CAM plants store CO2 in organic acids during the night, and release the CO2 from these organic acids during the day. Only during the day do CAM plants synthesize sugars, and when doing so, use the conventional (C3) photosynthetic pathway. This process allows stomata to stay closed during day and to prevent great transpiration.
26. Translocation is a process in which the organic products of photosynthesis are transported throughout the plant by the phloem. In angiosperms, the specialized cells of the phloem involved in this process are called the sieve-tube members, and are arranged end to end to form long tubes with porous cross-walls between the cells. Sieve tubes always carry food from a sugar source to a sugar sink. Sugar sources are plant organs (i.e. mature leaves) where sugar is being produced through photosynthesis or starch breakdown. Sugar sinks are organs (i.e. growing roots, shoots, or fruit) that consume or store sugar as well as minerals. Depending on the season, storage organs (i.e. tubers or bulbs) can be sources or sinks. In the summer, they stockpile carbs and are sugar sinks. In the spring, the same organs become sources, as their starches are broken down to sugars and are carried away in the phloem to growing buds of the shoot system. Sieve tubes in the same vascular bundle can carry sap in different directions. The direction of transport depends on the location of the source and sink connected by the tube.
27. Before being exported to sugar sinks, sugar from mesophyll cells (or other sources) must be loaded into sieve-tube members. Depending on the species of the plant, this movement can be via the symplast or by a combination of symplastic and apoplastic pathways. Sucrose can diffuse through the symplast from mesophyll cells into small veins. Much of this sugar moves out of the cells into the apoplast in the vicinity of sieve-tube members and companion cells. Companion cells pass the sugar they accumulate into the sieve-tube members via plasmodesmata. In some plants, companion cells (transfer cells) have numerous ingrowths in their walls to increase the cell’s surface area and enhance the transfer of solutes between apoplast and symplast. Because some sieve-tube members accumulate sucrose of very high concentrations, active transport is required to load the phloem. Proton pumps generate an H+ gradient, which drives sucrose across the membrane via a cotransport protein that couples sucrose transport to the diffusion of H+ back into the cell. Downstream, at the sink end of the sieve tube, phloem unloads its sucrose. The mechanism of phloem unloading is highly variable and depends on plant species and type of organ. Regardless of mechanism, because the concentration of free sugar in the sink is lower than in the phloem, sugar molecules diffuse from the phloem into the sink tissues. Water follows by osmosis.
28. In angiosperms, the mechanism of translocation is called pressure flow. It involves moving phloem sap from sugar sources to sugar sinks by bulk flow driven by positive pressure. Pressure flow moves the sap faster than diffusion or cytoplasmic streaming. Pressure flow in a sieve tube drives the bulk flow of phloem sap. Sugar is loaded into the tube at source. This reduces the water potential inside the sieve-tube members, and causes water uptake. The absorption of water then generates hydrostatic pressure, which forces the sap to flow along the tube. At the sink, pressure is relieved when the sugars are unloaded and water is lost from the tube. In leaf-to-root translocation, water is recycled by the xylem from sink to source.
AP Biology Ch33 Objectives
Chapter 33 - Invertebrates
1. Parts of a Sponge
spongocoel- water is drawn through the pores into this central cavity
porocyte- cell with pores that allow water into the sponge
epidermis- layer of cells that covers the outer surface of the sponge
choanocyte- cholar cells; create a flow of water through the sponge with their flagella and trap food with their collars
mesohyl- gelanitous region separating two cell layers of body of sponge
amoebocyte- wander through mesohyl, take up food from water and from choanocytes, digest it, and carry nutrients to other cells, secrete tough skeletal fibers within the mesohyl
osculum- large opening coming from central cavity, allows water to flow out
spicules- sharp spikes (made of calcium carbonate) located in the mesohyl; form the "skeleton" of many sponges
2. Distinguishing Characteristics of Phylum Cnidaria (hydras, jellies, sea anemones, and coral animals)
- radial symmetry
- gastrovascular cavity
- cnidocytes
o have batteries that may help with defense/attack
o can inject poison, stick to, or entangle the target
- mostly marine
- relatively simple body construction
o diploblastic
o sac with central digestive compartment
o single opening functions as both mouth and anus
o two variations
§ sessile polyp
· adhere to the substratum by the aboral end
· extend their tentacles, waiting for prey
§ floating medusa
· flattened, mouth-down versions of polyps
· move by drifting passively and contracting
· bell-shaped bodies
- carnivores à capture prey
- tentacles arranged in a ring
- nuscles and nerves exist in simplest forms
- four major classes: Hydrozoa, Scyphozoa, Cubozoa, and Anthozoa
· Most hydrozoans alternate polyp and medusa forms, as in the life cycle of Obelia.
° The polyp stage, often a colony of interconnected polyps, is more conspicuous than the medusa.
· Hydras, among the few freshwater cnidarians, are unusual members of the class Hydrozoa in that they exist only in the polyp form.
° When environmental conditions are favorable, a hydra reproduces asexually by budding, the formation of outgrowths that pinch off from the parent to live independently.
° When environmental conditions deteriorate, hydras form resistant zygotes that remain dormant until conditions improve.
· The medusa generally prevails in the life cycle of class Scyphozoa.
° The medusae of most species live among the plankton as jellies.
· Most coastal scyphozoans go through small polyp stages during their life cycle.
° Jellies that live in the open ocean generally lack the sessile polyp.
· Cubozoans have a box-shaped medusa stage.
° They can be distinguished from scyphozoans in other significant ways, such as having complex eyes in the fringe of the medusae.
· Cubozoans, which generally live in tropical oceans, are often equipped with highly toxic cnidocytes.
· Sea anemones and corals belong to the class Anthozoa.
° They occur only as polyps.
° Coral animals live as solitary or colonial forms and secrete a hard external skeleton of calcium carbonate.
° Each polyp generation builds on the skeletal remains of earlier generations to form skeletons that we call coral.
· In tropical seas, coral reefs provide habitat for a great diversity of invertebrates and fishes.
3. Specialized cells found in cnidaria include cells of the epidermis and gastrodermis, which have bundles of microfilaments arranged into contractile fibers. Cnidarians also have muscles and nerves (in very simple forms). Their movements are controlled by their noncentralized nerve net and simple sensory receptors
4. The two basic body plans in Cnidaria are sessile polyps and floating medusas. The cylindrical polyps, such as hydras and sea anemones, use their aboral ends to adhere to their substratum. They wait for their prey with extended tentacles. The medusas are flattened, mouth-down versions of polyps that move by drifting passively and by contracting their bell-shaped bodies. Some cnidarians exist only as polyps, while others exist only as medusas. Some pass sequentially through both a medusa stage and a polyp stage in their life cycle.
5. Four Classes of Cnidaria
- Hydrozoa
o alternate polyp and medusa forms
o colony of interconnected polyps is more conspicuous than medusa
o favorable environmental conditions à asexual reproduction through budding
o bad conditions à hydras form resistant zygotes that remain dormant until conditions improve
- Scyphozoa
o medusa generally prevails in the life cycle
o small polyp stages
- Cubozoans
o boxed shaped medusa stage
o complex eyes embedded
o often have highly toxic cnidocytes
- Anthozoas
o occur only as polyps
o each generation builds on skeletal remains of earlier generations
o form skeletons called coral
o provide habitat for a great diversity of invertebrates and fishes
6. The term diploblastic refers to an animal with only 2 germ layers, such as the cnidarian. An animal with 3 germ layers is triploblastic.
Coeloms form from mesoderm tissue. Animals with a true coelom are known as coelomates. Animals that lack a coelom are known as acoelomates. They have a solid bodies without body cavities.
A gastrovascular cavity is a central digestive compartment while an alimentary canal is a digestive tract with a separate mouth and anus.
Protostomes undergo spiral cleavage, in which planes of cell division are diagonal to the vertical axis of the embryo. Some protostomes also show determinate cleavage, where the fate of each embryonic cell is determined early in development. Many deuterostomes undergo radial cleavage in which the cleavage planes are parallel or perpendicular to the vertical egg axis. Most deuterostomes show indeterminate cleavage, whereby each cell in the early embryo retains the capacity to develop into a complete embryo.
7. Distinguishing Characteristics of Platyhelminthes
- live in marine, freshwater, and damp terrestrial habitats
- thin bodies, ranging in size
- triploblastic
o middle embryonic tissue layer (a mesoderm)
o contributes to more complex organs/organ systems and to true muscle tissue
- gastrovascular cavity with only one opening
- lack a coelom
- flat shape
o places cells close to surrounding water
o enables gas exchange & elimination of nitrogenous wastes (ammonia) by diffusion across body surface
- no specialized organs for gas exchange/circulation
- relatively simple excretory apparatus mainly maintains osmotic balance
8. Four Classes of Platyhelminthes
- Turbellaria (planarians)
o nearly all free-living/nonparasitic
o mostly marine
o carnivores or scavengers
o move using cilia on ventral epidermis
o glide along secreted film of mucus
o muscles can be used for undulatory swimming
o pair of eyespots detect light
o lateral flaps function mainly for smell
o nervous system = complex & centralized à able to modify responses to stimuli
o reproduce asexually through regeneration (parent constricts in the middle, each half regenerates the missing end)
o able to reproduce sexually (hermaphrodites cross-fertilize)
- Monogenia
o parasites in/on host
o tough protective covering
o suckers help with attachment
o large reproductive organs take up most of body
o mostly external parasites of fishes
o simple life cycles
§ ciliated, free-living larva that infects host
- Trematoda
o parasites in/on wide range of hosts
o suckers help with attachment
o complex life cycles with alternation of sexual & asexual stages
§ many need intermediate host for larvae development
§ adult worm appears in final host (normally vertebrate)
§ large reproductive organs take up most of body
o release molecules that manipulate the host’s immune system
o tough protective covering
- Cestoidea (tapeworms)
o parasitic, mostly in vertebrates, including humans
o scolex, suckers and hooks on head, anchor worm in host’s digestive tract
o lack a gastrovascular cavity à absorb food particles
o series of proglottids, sacs of sex organs, lie posterior to the scolex
§ eggs develop into larvae à encyst hosts’s muscles
§ larvae develop into mature adults
§ mature proglottids= loaded with thousands of eggs
§ mature worms release eggs from posterior end
§ eggs leave with host’s feces
9. Trematodes live as parasites in or on other animals, and have very complicated life cycles. Most species have alternations of sexual and asexual stages. They require an intermediate host where the larvae can develop before being able to infect a final host (usually a vertebrate) where the adult worms lives. The blood fluke, Schistosoma, is an example of one such parasite. This organisms causes body pains and dysentery. It uses snails as an intermediate host before infecting humans. Thus, it must evade the immune systems of two very different hosts. These blood flukes create a partial immunological camouflage by mimicking their host’s surface proteins. They are also able to release molecules that manipulate the host’s immune system.
10. Trematodes evade detection by mimicking their host’s surface proteins and releasing molecules that manipulate the host’s immune system. Thus, they create a partial immunological camouflage
11. Tapeworms, of the class Cestoidea, are parasitic. They are mostly invertebrates and live mostly in vertebrates. They have suckers and hooks on the head, or scolex, which anchor them in the digestive tract of their host. They lack a gastrovascular cavity, but absorb food particles from their host. A long series of proglottids, sacs of sex organs, lie posterior to the scolex. Mature proglottids loaded with thousands of eggs are released from the posterior end of the tapeworm and leave with the host’s feces. The eggs are then ingested by intermediary hosts, such as pig and cattle. The eggs develop into larvae that encyst in the muscle of the host. Undercooked meat may cause the cysts to be transferred to the human where the larvae develop into mature adults inside the body.
12. Rotifers are tiny animals (ranging in size from 5 µm to 2 mm), most of which live in freshwater. They are smaller than many protists but are truly multicellular, with specialized organ systems. Rotifers have an alimentary canal, a digestive tract with a separate mouth and anus. Internal organs lie in the pseudocoelom, a body cavity that is not completely lined with mesoderm. The fluid in the pseudocoelom serves as a hydrostatic skeleton. Through the movements of nutrients and wastes dissolved in the coelomic fluid, the pseudocoelom also functions as a circulatory system. The word rotifer refers to the crown of cilia that draws a vortex of water into the mouth. Food particles drawn in by the cilia are captured by the jaws (trophi) in the pharynx and ground up. Some rotifers exist only as females that produce more females from unfertilized eggs, a type of parthenogenesis. Other species produce two types of eggs that develop by parthenogenesis. One type forms females, and the other forms degenerate males that survive just long enough to fertilize eggs. The zygote forms a resistant stage that can withstand environmental extremes until conditions improve. The zygote then begins a new female generation that reproduces by parthenogenesis until conditions become unfavorable again.
13. Parthenogenesis is a type of reproduction where the female produces offspring from unfertilized eggs. Some rotifers produce more females from unfertilized eggs; some produce two types of eggs. One forms females and the other forms degenerate males that survive just long enough to fertilize eggs. Species that reproduce asexually tend to accumulate harmful mutations in their genomes faster than sexually reproducing species. As a result, asexual species experience higher rates of extinction and lower rates of speciation. It is puzzling that so many rotifers survive without males.
14. Lophophores are horseshoes-shaped or circular fold of the body wall bearing ciliated tentacles that surround and draw water towards the mouth. Tentacles trap suspended food particles. There are three lophophore phyla: Ectoprocta, Phoronida, and Brachiopoda.
15. Nemertea have bodies much like flatworms. However, they have small fluid-filled sacs that may be a reduced version of a true coelom. The sac and fluid hydraulics operate an extensible proboscis, which the worm uses to capture prey. These organisms have an alimentary canal and a closed circulatory system in which the blood is contained in vessels. Nemerteans have no heart, and the blood is propelled by muscles squeezing the vessels. Nearly all nemerteans are marine.
16. Nemerteans and flatworms have very similar bodies. They have similar excretory, sensory, and nervous systems. However, what separates Nemerteans from flatworms is that they have an alimentary canal and a closed circuit system where blood is contained in vessels. Nemerteans have small fluid-filled sacs that may be a reduced version of a true coelom. Nemerteans also have no heart, the blood is propelled by muscles squeezing the vessels
17. Mollusca is an invertebrate phyla that includes snails and slugs, oysters and clams, and octopuses and squids. There are several characteristics that distinguishing them from other animal phyla.
- generally marine, but some may have fresh water or terrestrial habitat
- soft bodied... most protected by hard shell made of calcium carbonate (some shells have been lost/reduced in evolution)
- muscular foot typically for movement
- visceral mass with most of the internal organs
- mantle à secretes shell, drapes over visceral mass, creates water-filled chamber with gills, anus, and excretory pores
- feed using radula, a straplike rasping organ that scrapes up food
- generally have separate sexes, with gonads found in visceral mass*many snails, however, are hermaphrodites
- trochophore (ciliated larva stage) found in life cycle*also in marine annelids and certain lophotrochozoans
18. The basic molluscan body plan has evolved into distinct ways that constitute the 8 classes of the phylum. The are 4 prominent classes.
- Bivalvia (clams, oysters, mussels, and scallops)
o shells divided into 2 halves, hinged at the mid-dorsal line
o powerful adductor muscles close shell tightly for protection
o most = suspension feeders à trap fine particles in mucus that coats their gills
· cilia convey the particles to the mouth
· water flows into mantle cavity through incurrent siphon, passes over gills, exits through excurrent siphon
· generally have sedentary lives
· attach to surface, move using muscular foot or swim (flap shells, jet out water)
o no distinct head or radula
o mantle
· outer edge may have eyes/sensory tentacles
· cavity contains gills used for feeding/gas exchange
- Cephalopoda (squids, octopuses, cuttlefish, and chambered nautiluses)
o ancestors probably were shelled molluscs that took up predatory lifestyle
o only molluscs with closed circulatory system
o have well-developed sense organs and complex brain
o capture prey with long tentacles
o move quickly by contracting mantle cavity, firing stream of water out of excurrent siphon
o foot modified into muscular siphon, parts of tentacles, and head
o mantle covers visceral mass
o shell maybe be reduced/internal/missing
- Gastropoda (snails and slugs)
o make up three-quarters of all living species of molluscs
o shells formation = independent developmental process, generally conical, may be flattened (i.e. abalones)
o radulas used to graze on algae/plants
· in predators, modified to bore holes in the shells or to tear apart tough animal tissues
· teeth can form separate poison darts, which penetrate and stun prey
o have distinct heads with eyes at the tips of tentacles
o movement created by a rippling motion of foot or by cilia
o undergo torsion during embryonic development
· visceral mass is rotated up to 180 degrees
· anus and mantle cavity are above the head (in adults)
· some of the organs that were bilateral are reduced/lost on one side of the body
o aquatic species = gills
o terrestrial species = lining of the mantle cavity functions as lung
- Polyplacophora (chitons)
o oval shaped
o unsegmented bodies
o shells divided into eight dorsal plates
o muscular foot used to grips rock substrates and creep over them
o grazers à used radulas to scrape & ingest algae
19. There are two major characteristics that distinguish Annelida from other animal phyla, being that they have segmented bodies and live in moist habitats (sea, freshwater, damp soil)
20. The phylum Annelida is divided into three classes...
- Oligochaeta
o segmented worms
o named for their relatively sparse chaetae (bristles made of chitin)
o Earthworms
· eat their way through soil and digest it in their ailementary canal à undigested material is egested as castings, which enrich the tilled soil
· cross-fertilizing hermaphrodites
Ø 2 earthworms exchange sperm, separate, store sperm
Ø clitellum (special organ) secretes mucous cocoon
Ø cocoon slides along worm’s body, picking up stored eggs and sperm
· some are asexual and reproduce through fragmentation followed by regeneration
- Polychaeta
o polychaetes = “many setae”
o marine (w/ some exceptions)
o pair of paddle/ridgelike parapodia (“almost feet”)
· function in locomotion
· have several chitinous setae
· rich blood vessel inside function as gills (w/ some exceptions)
- Hirudinea (leeches)
o majority inhabit fresh water
o land leeches move through moist vegetation
o range in size from about 1 to 30 cm
o generally feed on other invertebrates
o some are blood-sucking parasites
· feed by attaching temporarily to host
· use blade-like jaws or enzymes to get through skin to blood
· anesthetic keeps host unaware
· secrete hirudin, an anticoagulant
21. Leeches are able to feed on blood due to their special adaptations, such as their feeding “technique.” To get through their hosts’ skin, leeches are equip with either blade-like jaws or enzymes that they can secrete to get to the blood source. Leeches also secrete anesthetics to keep their hosts unaware, so that they can suck longer and get more blood. Their hirudin secretions also are an adaptation, as they prevent blood from coagulating and allow the leeches to feed longer.
22. Characteristics Distinguishing Nematoda (Roundworms) from Other Wormlike Animals
- tough cuticle coating cylindrical body
o exoskeleton periodically shed
o new one is excreted
- alimentary tract
o use the fluid in their pseudocoelom to transport nutrients
o lack a circulatory system
- move by contracting longitudinal muscles (thrashing motion)
- reproduce sexually
o sexes generally separate
o fertilization = internal
o females can lay over 100,000 eggs per day
o zygote = resistant cell, able to survive harsh conditions
- major role in decomposition/nutrient recycling
- parasitize animals or attack plant roots
23. Over 50 species of nematodes—such as pinworms and hookworms—parasitize humans. For example, Trichinella spiralis is a species of nematodes that causes trichinosis and is acquired by consuming undercooked infected meat. The worms encyst in a variety of human organs, including skeletal muscle. These worms can hijack some of their hosts’ cellular function, thus effecting gene expression and protein coding. Nematodes do have a positive effect on the environment, however. Free-living nematodes help with decomposition and nutrient recycling. Caenorhabditis elegans, a soil organism, is a model organism in developmental biology.
24. Characteristics Distinguishing Anthropods from Other Animal Phyla
- seen as most successful phylum (three reasons in italics)
- diverse
- widely distributed à represented in nearly all habitats in the biosphere
- large population
- body segmentation and jointed appendages
o segments & appendages become specialized for variety of functions
o labor division is efficient among regions
- hard exoskeleton
o body completely covered by cuticle made of protein and chitin
o very strong – offers protection
o provides points of attachment for muscles that move appendages
o relatively impermeable to water – prevent desiccation, provides support
o can be thick & inflexible or thin & flexible – ce dépend
o must be molted during growth
§ new, larger on is secreted = ecdysis
§ animal temporarily vulnerable
- well-developed sense organ
o eyes (vision)
o olfactory receptors (smell)
o antennae (touch & smell)
o generally located at anterior end à shows extensive cephalization
- open circulatory system
o hemolymph fluid propelled by heart through short arteries into sinuses (hemocoel... not a coelom) surrounding tissues & organs
o hemolymph returns to heart through valved pores
- reduced true coelom
- specific organs for gas exchange
o aquatic à gills
§ thin, feathery extensions
§ extensive surface area in contact with water
o terrestrial à specialized internal surfaces
25. Exoskeletons can be advantageous because they are very strong and both provide protection support for the body. They also offer places where muscles can anchor in order to move appendages. Furthermore, exoskeletons are useful because they prevent dessication. However, exoskeletons can also be a disadvantage because they must be molted to accommodate for the growth of the body. During molting, while a new and larger exoskeleton is secreted in ecdysis, the animal is left vulnerable.
26. The hemocoel—sinuses connected to arteries—exists in open circulatory systems. It surrounds tissues and organs and contains hemolymph fluid. The hemocoel is not a coelom; the true coelom is very reduced in most arthropods.
27. It is believed that arthropods diverged early on into four main evolutionary lineages
- Cheliceriformes (sea spiders, horseshoe crabs, scorpions, ticks, spiders)
o named is derived from chelicerae, clawlike feeding appendages that serve as pincers/fangs
o have anterior cephalothorax and posterior abdomen
o lack sensory antennae
o most have simple eyes (with a single lens)
o earliest form was eurypterids (water scorpions) = marine and freshwater predators
o modern forms include sea spiders (pycnogonids) and horseshoe crabs
o living majority = arachnids (includes scorpions, spiders, ticks, mites)
- Myriapods (centipedes and millipedes)
o terrestrial
o millipedes = class Diplopoda
§ two pairs of walking legs on each of trunk segments, formed by two fused segments
§ eat decaying leaves and plant matter
§ among the earliest land animals
o centipedes (class Chilopoda)
§ carnivores
§ head has pair of antennae & three pairs of appendages modified as mouth parts, including jawlike mandibles
§ trunk region segments all have one pair of walking legs
§ poison claws on anteriormost trunk segment
· paralyze prey
· defense
- Hexapods (insects and their wingless, six-legged relatives)
o more species-rich than all other forms of life combined
o in almost all habitats (land, water, air)
o some are able to fly
o diverse mouths
o complex organ systems
§ regionally specialized
§ Malpighian tubules = outpockets of digestive tract, removes metabolic wastes from hemolymph
§ tracheal system (respiration) = branched, chitin-lined, carries O2 from spiracles directly to cells
§ nervous system = ventral nerve cords with several segmental ganglia
· two cords meet in the head
· ganglia from several anterior segments fuse into cerebral ganglion (brain)
· structure is close to the antennae, eyes, and other sense organs
o development through metamorphosis
§ incomplete à young are smaller with different body proportions, molting occurs until adult body and size are reached
§ complete à three stages = larval, pupal, adult
o reproduction usually sexual
§ individuals have separate sexes
§ attracted by color, odor, sound
§ females store sperm in spermatheca, in some cases holding enough sperm from a single mating to last a lifetime
§ eggs laid on food source
o important natural and agricultural pollinators
o carriers for many diseases (i.e. malaria and African sleeping sickness)
- Crustaceans (crabs, lobsters, shrimps, barnacles, and many others)
o most = marine/freshwater environment
o typically have biramous (branched), specialized appendages
§ two pairs of antennae
§ at least three pairs of mouthparts, including hard mandibles
§ walking legs present on thorax
§ appendages for swimming or reproduction found on abdomen
§ able to regenerate lost appendages during molting
o gas exchange differs based on size
§ small à exchange gases across thin areas of the cuticle
§ large à gills used
o open circulatory system
§ heart pumps hemolymph into short arteries and then into sinuses that bathe the organs
§ nitrogenous wastes excreted by diffusion through thin areas of cuticle
§ glands regulate salt balance of hemolymph
o sexual reproduction
§ most species have separate sexes
§ aquatic species have several larval stages
28. Spider have three specialized features. To begin with, they are able to inject poison into their prey to immobilize it. The poison is located in glands on their chelicerae. While consuming their prey, spiders spill some of their digestive juices into the tissues. The liquid is sucked up as their meal. Spiders also carry out gas exchange by book lungs, which are stacked plates with an extensive surface area contained in an internal chamber. Lastly, spiders are able to catch flying insects in their silk webs. The production of the protein occurs in abdominal glands. The silk begins as a liquid, but solidifies as it is spun in fibers by spinnerets. Silk fibers also function as egg covers, drop lines for a rapid escape, and “gift wrapping” for nuptial gifts.
29. Insects vary greatly and are the most species rich phylum because of two main things: their ability to fly and the diversification of their mouthparts. Insect flight evolved during the Carboniferous and Permian periods and what followed was an explosion in insect variety because new adaptive zones opened. Flying also enabled insects to escape predators and find more food and mates.
Then, insect mouthparts began to diversify due to different needs (feeding on gymnosperms and Carboniferous plants). Adaptive radiation occurred.
30. Distinguishing Characteristics of Echinoderms (i.e. sea stars)
- deuterostomes
- radial cleavage à secondary radial symmetry
- development of coelom from archenteron
- formation of anus from blastopore
- water vascular system
o network of hydraulic canals
o have tube feet, branched extensions, that function in movement, feeding, gas exchange
- thin skin covering endoskeleton of hard calcareaou plates
- most are sessile (slow moving marine animals)
- many have skeletals bumps/spines that make them prickly
- sexual reproduction à gametes released by males and females into seawater
- internal & external parts radiate from center (generally as 5 spokes)
- larvae have bilateral symmetry
- adults not perfectly radial
31. Living echinoderms are divided into six classes...
- Asteroidea (sea stars)
o multiple arms radiate from central disk
§ undersides have rows of tube feet
· act like suction disks
· controlled by hydraulic & muscular action
· can grasp substrates and prey, & creep over surfaces
§ able to pull apart bivalves (prey)
o can evert stomach through mouth
o can regernerate limbs
- Ophiuroidea (brittle stars)
o distinct central disk
o long, flexible arms
o tube feet lack suckers
o move by a serpentine lashing of arms
o suspension feeders, scavengers, or predators
- Echinoidea (sea urchins and sand dollars)
o no arms
o five rows of tube feet used for locomotion
o pivoting of long spines also enables movement
o mouths may be ringed with complex jawlike structures
o either spherical or flattened and disk-shaped
- Crinoidea (sea lilies and feather stars)
o lilies attach to substratum by stalks
o feather stars crawl using long, flexible arms
o mouth directed upward (award from substrate)
o arms (for suspension feeding) circle mouth
o very conservative evolution
- Holothuroidea (sea cucumbers)
o lack spines
o very reduced endoskeleton
o elongated oral-aboral axis
o five rows of tube feet à some function as feeding tentacles (suspension or deposit feeding)
- Concentricycloidea (sea daisies)
o discovered in 1986 à only two known species
o bodies = armless, disk-shaped, five-fold symmetry
o less than a centimeter in diameter
o absorb nutrients through the membrane surrounding body
o considered to be highly derived sea stars
32. Chordata are included in a chapter on invertebrates because the phylum does include invertebrate subphyla (although it is only 2 out of many). However, they are linked with Echidnoderms, because both are bilateral deuterostomes.
33. Echidnoderms and Chordates are developmentally similar because they are both bilateral deuterostomia. They are coelomates, with radial cleavage. Also, their coeloms develop from their archenterons and their anuses form from their blastopores.
1. Parts of a Sponge
spongocoel- water is drawn through the pores into this central cavity
porocyte- cell with pores that allow water into the sponge
epidermis- layer of cells that covers the outer surface of the sponge
choanocyte- cholar cells; create a flow of water through the sponge with their flagella and trap food with their collars
mesohyl- gelanitous region separating two cell layers of body of sponge
amoebocyte- wander through mesohyl, take up food from water and from choanocytes, digest it, and carry nutrients to other cells, secrete tough skeletal fibers within the mesohyl
osculum- large opening coming from central cavity, allows water to flow out
spicules- sharp spikes (made of calcium carbonate) located in the mesohyl; form the "skeleton" of many sponges
2. Distinguishing Characteristics of Phylum Cnidaria (hydras, jellies, sea anemones, and coral animals)
- radial symmetry
- gastrovascular cavity
- cnidocytes
o have batteries that may help with defense/attack
o can inject poison, stick to, or entangle the target
- mostly marine
- relatively simple body construction
o diploblastic
o sac with central digestive compartment
o single opening functions as both mouth and anus
o two variations
§ sessile polyp
· adhere to the substratum by the aboral end
· extend their tentacles, waiting for prey
§ floating medusa
· flattened, mouth-down versions of polyps
· move by drifting passively and contracting
· bell-shaped bodies
- carnivores à capture prey
- tentacles arranged in a ring
- nuscles and nerves exist in simplest forms
- four major classes: Hydrozoa, Scyphozoa, Cubozoa, and Anthozoa
· Most hydrozoans alternate polyp and medusa forms, as in the life cycle of Obelia.
° The polyp stage, often a colony of interconnected polyps, is more conspicuous than the medusa.
· Hydras, among the few freshwater cnidarians, are unusual members of the class Hydrozoa in that they exist only in the polyp form.
° When environmental conditions are favorable, a hydra reproduces asexually by budding, the formation of outgrowths that pinch off from the parent to live independently.
° When environmental conditions deteriorate, hydras form resistant zygotes that remain dormant until conditions improve.
· The medusa generally prevails in the life cycle of class Scyphozoa.
° The medusae of most species live among the plankton as jellies.
· Most coastal scyphozoans go through small polyp stages during their life cycle.
° Jellies that live in the open ocean generally lack the sessile polyp.
· Cubozoans have a box-shaped medusa stage.
° They can be distinguished from scyphozoans in other significant ways, such as having complex eyes in the fringe of the medusae.
· Cubozoans, which generally live in tropical oceans, are often equipped with highly toxic cnidocytes.
· Sea anemones and corals belong to the class Anthozoa.
° They occur only as polyps.
° Coral animals live as solitary or colonial forms and secrete a hard external skeleton of calcium carbonate.
° Each polyp generation builds on the skeletal remains of earlier generations to form skeletons that we call coral.
· In tropical seas, coral reefs provide habitat for a great diversity of invertebrates and fishes.
3. Specialized cells found in cnidaria include cells of the epidermis and gastrodermis, which have bundles of microfilaments arranged into contractile fibers. Cnidarians also have muscles and nerves (in very simple forms). Their movements are controlled by their noncentralized nerve net and simple sensory receptors
4. The two basic body plans in Cnidaria are sessile polyps and floating medusas. The cylindrical polyps, such as hydras and sea anemones, use their aboral ends to adhere to their substratum. They wait for their prey with extended tentacles. The medusas are flattened, mouth-down versions of polyps that move by drifting passively and by contracting their bell-shaped bodies. Some cnidarians exist only as polyps, while others exist only as medusas. Some pass sequentially through both a medusa stage and a polyp stage in their life cycle.
5. Four Classes of Cnidaria
- Hydrozoa
o alternate polyp and medusa forms
o colony of interconnected polyps is more conspicuous than medusa
o favorable environmental conditions à asexual reproduction through budding
o bad conditions à hydras form resistant zygotes that remain dormant until conditions improve
- Scyphozoa
o medusa generally prevails in the life cycle
o small polyp stages
- Cubozoans
o boxed shaped medusa stage
o complex eyes embedded
o often have highly toxic cnidocytes
- Anthozoas
o occur only as polyps
o each generation builds on skeletal remains of earlier generations
o form skeletons called coral
o provide habitat for a great diversity of invertebrates and fishes
6. The term diploblastic refers to an animal with only 2 germ layers, such as the cnidarian. An animal with 3 germ layers is triploblastic.
Coeloms form from mesoderm tissue. Animals with a true coelom are known as coelomates. Animals that lack a coelom are known as acoelomates. They have a solid bodies without body cavities.
A gastrovascular cavity is a central digestive compartment while an alimentary canal is a digestive tract with a separate mouth and anus.
Protostomes undergo spiral cleavage, in which planes of cell division are diagonal to the vertical axis of the embryo. Some protostomes also show determinate cleavage, where the fate of each embryonic cell is determined early in development. Many deuterostomes undergo radial cleavage in which the cleavage planes are parallel or perpendicular to the vertical egg axis. Most deuterostomes show indeterminate cleavage, whereby each cell in the early embryo retains the capacity to develop into a complete embryo.
7. Distinguishing Characteristics of Platyhelminthes
- live in marine, freshwater, and damp terrestrial habitats
- thin bodies, ranging in size
- triploblastic
o middle embryonic tissue layer (a mesoderm)
o contributes to more complex organs/organ systems and to true muscle tissue
- gastrovascular cavity with only one opening
- lack a coelom
- flat shape
o places cells close to surrounding water
o enables gas exchange & elimination of nitrogenous wastes (ammonia) by diffusion across body surface
- no specialized organs for gas exchange/circulation
- relatively simple excretory apparatus mainly maintains osmotic balance
8. Four Classes of Platyhelminthes
- Turbellaria (planarians)
o nearly all free-living/nonparasitic
o mostly marine
o carnivores or scavengers
o move using cilia on ventral epidermis
o glide along secreted film of mucus
o muscles can be used for undulatory swimming
o pair of eyespots detect light
o lateral flaps function mainly for smell
o nervous system = complex & centralized à able to modify responses to stimuli
o reproduce asexually through regeneration (parent constricts in the middle, each half regenerates the missing end)
o able to reproduce sexually (hermaphrodites cross-fertilize)
- Monogenia
o parasites in/on host
o tough protective covering
o suckers help with attachment
o large reproductive organs take up most of body
o mostly external parasites of fishes
o simple life cycles
§ ciliated, free-living larva that infects host
- Trematoda
o parasites in/on wide range of hosts
o suckers help with attachment
o complex life cycles with alternation of sexual & asexual stages
§ many need intermediate host for larvae development
§ adult worm appears in final host (normally vertebrate)
§ large reproductive organs take up most of body
o release molecules that manipulate the host’s immune system
o tough protective covering
- Cestoidea (tapeworms)
o parasitic, mostly in vertebrates, including humans
o scolex, suckers and hooks on head, anchor worm in host’s digestive tract
o lack a gastrovascular cavity à absorb food particles
o series of proglottids, sacs of sex organs, lie posterior to the scolex
§ eggs develop into larvae à encyst hosts’s muscles
§ larvae develop into mature adults
§ mature proglottids= loaded with thousands of eggs
§ mature worms release eggs from posterior end
§ eggs leave with host’s feces
9. Trematodes live as parasites in or on other animals, and have very complicated life cycles. Most species have alternations of sexual and asexual stages. They require an intermediate host where the larvae can develop before being able to infect a final host (usually a vertebrate) where the adult worms lives. The blood fluke, Schistosoma, is an example of one such parasite. This organisms causes body pains and dysentery. It uses snails as an intermediate host before infecting humans. Thus, it must evade the immune systems of two very different hosts. These blood flukes create a partial immunological camouflage by mimicking their host’s surface proteins. They are also able to release molecules that manipulate the host’s immune system.
10. Trematodes evade detection by mimicking their host’s surface proteins and releasing molecules that manipulate the host’s immune system. Thus, they create a partial immunological camouflage
11. Tapeworms, of the class Cestoidea, are parasitic. They are mostly invertebrates and live mostly in vertebrates. They have suckers and hooks on the head, or scolex, which anchor them in the digestive tract of their host. They lack a gastrovascular cavity, but absorb food particles from their host. A long series of proglottids, sacs of sex organs, lie posterior to the scolex. Mature proglottids loaded with thousands of eggs are released from the posterior end of the tapeworm and leave with the host’s feces. The eggs are then ingested by intermediary hosts, such as pig and cattle. The eggs develop into larvae that encyst in the muscle of the host. Undercooked meat may cause the cysts to be transferred to the human where the larvae develop into mature adults inside the body.
12. Rotifers are tiny animals (ranging in size from 5 µm to 2 mm), most of which live in freshwater. They are smaller than many protists but are truly multicellular, with specialized organ systems. Rotifers have an alimentary canal, a digestive tract with a separate mouth and anus. Internal organs lie in the pseudocoelom, a body cavity that is not completely lined with mesoderm. The fluid in the pseudocoelom serves as a hydrostatic skeleton. Through the movements of nutrients and wastes dissolved in the coelomic fluid, the pseudocoelom also functions as a circulatory system. The word rotifer refers to the crown of cilia that draws a vortex of water into the mouth. Food particles drawn in by the cilia are captured by the jaws (trophi) in the pharynx and ground up. Some rotifers exist only as females that produce more females from unfertilized eggs, a type of parthenogenesis. Other species produce two types of eggs that develop by parthenogenesis. One type forms females, and the other forms degenerate males that survive just long enough to fertilize eggs. The zygote forms a resistant stage that can withstand environmental extremes until conditions improve. The zygote then begins a new female generation that reproduces by parthenogenesis until conditions become unfavorable again.
13. Parthenogenesis is a type of reproduction where the female produces offspring from unfertilized eggs. Some rotifers produce more females from unfertilized eggs; some produce two types of eggs. One forms females and the other forms degenerate males that survive just long enough to fertilize eggs. Species that reproduce asexually tend to accumulate harmful mutations in their genomes faster than sexually reproducing species. As a result, asexual species experience higher rates of extinction and lower rates of speciation. It is puzzling that so many rotifers survive without males.
14. Lophophores are horseshoes-shaped or circular fold of the body wall bearing ciliated tentacles that surround and draw water towards the mouth. Tentacles trap suspended food particles. There are three lophophore phyla: Ectoprocta, Phoronida, and Brachiopoda.
15. Nemertea have bodies much like flatworms. However, they have small fluid-filled sacs that may be a reduced version of a true coelom. The sac and fluid hydraulics operate an extensible proboscis, which the worm uses to capture prey. These organisms have an alimentary canal and a closed circulatory system in which the blood is contained in vessels. Nemerteans have no heart, and the blood is propelled by muscles squeezing the vessels. Nearly all nemerteans are marine.
16. Nemerteans and flatworms have very similar bodies. They have similar excretory, sensory, and nervous systems. However, what separates Nemerteans from flatworms is that they have an alimentary canal and a closed circuit system where blood is contained in vessels. Nemerteans have small fluid-filled sacs that may be a reduced version of a true coelom. Nemerteans also have no heart, the blood is propelled by muscles squeezing the vessels
17. Mollusca is an invertebrate phyla that includes snails and slugs, oysters and clams, and octopuses and squids. There are several characteristics that distinguishing them from other animal phyla.
- generally marine, but some may have fresh water or terrestrial habitat
- soft bodied... most protected by hard shell made of calcium carbonate (some shells have been lost/reduced in evolution)
- muscular foot typically for movement
- visceral mass with most of the internal organs
- mantle à secretes shell, drapes over visceral mass, creates water-filled chamber with gills, anus, and excretory pores
- feed using radula, a straplike rasping organ that scrapes up food
- generally have separate sexes, with gonads found in visceral mass*many snails, however, are hermaphrodites
- trochophore (ciliated larva stage) found in life cycle*also in marine annelids and certain lophotrochozoans
18. The basic molluscan body plan has evolved into distinct ways that constitute the 8 classes of the phylum. The are 4 prominent classes.
- Bivalvia (clams, oysters, mussels, and scallops)
o shells divided into 2 halves, hinged at the mid-dorsal line
o powerful adductor muscles close shell tightly for protection
o most = suspension feeders à trap fine particles in mucus that coats their gills
· cilia convey the particles to the mouth
· water flows into mantle cavity through incurrent siphon, passes over gills, exits through excurrent siphon
· generally have sedentary lives
· attach to surface, move using muscular foot or swim (flap shells, jet out water)
o no distinct head or radula
o mantle
· outer edge may have eyes/sensory tentacles
· cavity contains gills used for feeding/gas exchange
- Cephalopoda (squids, octopuses, cuttlefish, and chambered nautiluses)
o ancestors probably were shelled molluscs that took up predatory lifestyle
o only molluscs with closed circulatory system
o have well-developed sense organs and complex brain
o capture prey with long tentacles
o move quickly by contracting mantle cavity, firing stream of water out of excurrent siphon
o foot modified into muscular siphon, parts of tentacles, and head
o mantle covers visceral mass
o shell maybe be reduced/internal/missing
- Gastropoda (snails and slugs)
o make up three-quarters of all living species of molluscs
o shells formation = independent developmental process, generally conical, may be flattened (i.e. abalones)
o radulas used to graze on algae/plants
· in predators, modified to bore holes in the shells or to tear apart tough animal tissues
· teeth can form separate poison darts, which penetrate and stun prey
o have distinct heads with eyes at the tips of tentacles
o movement created by a rippling motion of foot or by cilia
o undergo torsion during embryonic development
· visceral mass is rotated up to 180 degrees
· anus and mantle cavity are above the head (in adults)
· some of the organs that were bilateral are reduced/lost on one side of the body
o aquatic species = gills
o terrestrial species = lining of the mantle cavity functions as lung
- Polyplacophora (chitons)
o oval shaped
o unsegmented bodies
o shells divided into eight dorsal plates
o muscular foot used to grips rock substrates and creep over them
o grazers à used radulas to scrape & ingest algae
19. There are two major characteristics that distinguish Annelida from other animal phyla, being that they have segmented bodies and live in moist habitats (sea, freshwater, damp soil)
20. The phylum Annelida is divided into three classes...
- Oligochaeta
o segmented worms
o named for their relatively sparse chaetae (bristles made of chitin)
o Earthworms
· eat their way through soil and digest it in their ailementary canal à undigested material is egested as castings, which enrich the tilled soil
· cross-fertilizing hermaphrodites
Ø 2 earthworms exchange sperm, separate, store sperm
Ø clitellum (special organ) secretes mucous cocoon
Ø cocoon slides along worm’s body, picking up stored eggs and sperm
· some are asexual and reproduce through fragmentation followed by regeneration
- Polychaeta
o polychaetes = “many setae”
o marine (w/ some exceptions)
o pair of paddle/ridgelike parapodia (“almost feet”)
· function in locomotion
· have several chitinous setae
· rich blood vessel inside function as gills (w/ some exceptions)
- Hirudinea (leeches)
o majority inhabit fresh water
o land leeches move through moist vegetation
o range in size from about 1 to 30 cm
o generally feed on other invertebrates
o some are blood-sucking parasites
· feed by attaching temporarily to host
· use blade-like jaws or enzymes to get through skin to blood
· anesthetic keeps host unaware
· secrete hirudin, an anticoagulant
21. Leeches are able to feed on blood due to their special adaptations, such as their feeding “technique.” To get through their hosts’ skin, leeches are equip with either blade-like jaws or enzymes that they can secrete to get to the blood source. Leeches also secrete anesthetics to keep their hosts unaware, so that they can suck longer and get more blood. Their hirudin secretions also are an adaptation, as they prevent blood from coagulating and allow the leeches to feed longer.
22. Characteristics Distinguishing Nematoda (Roundworms) from Other Wormlike Animals
- tough cuticle coating cylindrical body
o exoskeleton periodically shed
o new one is excreted
- alimentary tract
o use the fluid in their pseudocoelom to transport nutrients
o lack a circulatory system
- move by contracting longitudinal muscles (thrashing motion)
- reproduce sexually
o sexes generally separate
o fertilization = internal
o females can lay over 100,000 eggs per day
o zygote = resistant cell, able to survive harsh conditions
- major role in decomposition/nutrient recycling
- parasitize animals or attack plant roots
23. Over 50 species of nematodes—such as pinworms and hookworms—parasitize humans. For example, Trichinella spiralis is a species of nematodes that causes trichinosis and is acquired by consuming undercooked infected meat. The worms encyst in a variety of human organs, including skeletal muscle. These worms can hijack some of their hosts’ cellular function, thus effecting gene expression and protein coding. Nematodes do have a positive effect on the environment, however. Free-living nematodes help with decomposition and nutrient recycling. Caenorhabditis elegans, a soil organism, is a model organism in developmental biology.
24. Characteristics Distinguishing Anthropods from Other Animal Phyla
- seen as most successful phylum (three reasons in italics)
- diverse
- widely distributed à represented in nearly all habitats in the biosphere
- large population
- body segmentation and jointed appendages
o segments & appendages become specialized for variety of functions
o labor division is efficient among regions
- hard exoskeleton
o body completely covered by cuticle made of protein and chitin
o very strong – offers protection
o provides points of attachment for muscles that move appendages
o relatively impermeable to water – prevent desiccation, provides support
o can be thick & inflexible or thin & flexible – ce dépend
o must be molted during growth
§ new, larger on is secreted = ecdysis
§ animal temporarily vulnerable
- well-developed sense organ
o eyes (vision)
o olfactory receptors (smell)
o antennae (touch & smell)
o generally located at anterior end à shows extensive cephalization
- open circulatory system
o hemolymph fluid propelled by heart through short arteries into sinuses (hemocoel... not a coelom) surrounding tissues & organs
o hemolymph returns to heart through valved pores
- reduced true coelom
- specific organs for gas exchange
o aquatic à gills
§ thin, feathery extensions
§ extensive surface area in contact with water
o terrestrial à specialized internal surfaces
25. Exoskeletons can be advantageous because they are very strong and both provide protection support for the body. They also offer places where muscles can anchor in order to move appendages. Furthermore, exoskeletons are useful because they prevent dessication. However, exoskeletons can also be a disadvantage because they must be molted to accommodate for the growth of the body. During molting, while a new and larger exoskeleton is secreted in ecdysis, the animal is left vulnerable.
26. The hemocoel—sinuses connected to arteries—exists in open circulatory systems. It surrounds tissues and organs and contains hemolymph fluid. The hemocoel is not a coelom; the true coelom is very reduced in most arthropods.
27. It is believed that arthropods diverged early on into four main evolutionary lineages
- Cheliceriformes (sea spiders, horseshoe crabs, scorpions, ticks, spiders)
o named is derived from chelicerae, clawlike feeding appendages that serve as pincers/fangs
o have anterior cephalothorax and posterior abdomen
o lack sensory antennae
o most have simple eyes (with a single lens)
o earliest form was eurypterids (water scorpions) = marine and freshwater predators
o modern forms include sea spiders (pycnogonids) and horseshoe crabs
o living majority = arachnids (includes scorpions, spiders, ticks, mites)
- Myriapods (centipedes and millipedes)
o terrestrial
o millipedes = class Diplopoda
§ two pairs of walking legs on each of trunk segments, formed by two fused segments
§ eat decaying leaves and plant matter
§ among the earliest land animals
o centipedes (class Chilopoda)
§ carnivores
§ head has pair of antennae & three pairs of appendages modified as mouth parts, including jawlike mandibles
§ trunk region segments all have one pair of walking legs
§ poison claws on anteriormost trunk segment
· paralyze prey
· defense
- Hexapods (insects and their wingless, six-legged relatives)
o more species-rich than all other forms of life combined
o in almost all habitats (land, water, air)
o some are able to fly
o diverse mouths
o complex organ systems
§ regionally specialized
§ Malpighian tubules = outpockets of digestive tract, removes metabolic wastes from hemolymph
§ tracheal system (respiration) = branched, chitin-lined, carries O2 from spiracles directly to cells
§ nervous system = ventral nerve cords with several segmental ganglia
· two cords meet in the head
· ganglia from several anterior segments fuse into cerebral ganglion (brain)
· structure is close to the antennae, eyes, and other sense organs
o development through metamorphosis
§ incomplete à young are smaller with different body proportions, molting occurs until adult body and size are reached
§ complete à three stages = larval, pupal, adult
o reproduction usually sexual
§ individuals have separate sexes
§ attracted by color, odor, sound
§ females store sperm in spermatheca, in some cases holding enough sperm from a single mating to last a lifetime
§ eggs laid on food source
o important natural and agricultural pollinators
o carriers for many diseases (i.e. malaria and African sleeping sickness)
- Crustaceans (crabs, lobsters, shrimps, barnacles, and many others)
o most = marine/freshwater environment
o typically have biramous (branched), specialized appendages
§ two pairs of antennae
§ at least three pairs of mouthparts, including hard mandibles
§ walking legs present on thorax
§ appendages for swimming or reproduction found on abdomen
§ able to regenerate lost appendages during molting
o gas exchange differs based on size
§ small à exchange gases across thin areas of the cuticle
§ large à gills used
o open circulatory system
§ heart pumps hemolymph into short arteries and then into sinuses that bathe the organs
§ nitrogenous wastes excreted by diffusion through thin areas of cuticle
§ glands regulate salt balance of hemolymph
o sexual reproduction
§ most species have separate sexes
§ aquatic species have several larval stages
28. Spider have three specialized features. To begin with, they are able to inject poison into their prey to immobilize it. The poison is located in glands on their chelicerae. While consuming their prey, spiders spill some of their digestive juices into the tissues. The liquid is sucked up as their meal. Spiders also carry out gas exchange by book lungs, which are stacked plates with an extensive surface area contained in an internal chamber. Lastly, spiders are able to catch flying insects in their silk webs. The production of the protein occurs in abdominal glands. The silk begins as a liquid, but solidifies as it is spun in fibers by spinnerets. Silk fibers also function as egg covers, drop lines for a rapid escape, and “gift wrapping” for nuptial gifts.
29. Insects vary greatly and are the most species rich phylum because of two main things: their ability to fly and the diversification of their mouthparts. Insect flight evolved during the Carboniferous and Permian periods and what followed was an explosion in insect variety because new adaptive zones opened. Flying also enabled insects to escape predators and find more food and mates.
Then, insect mouthparts began to diversify due to different needs (feeding on gymnosperms and Carboniferous plants). Adaptive radiation occurred.
30. Distinguishing Characteristics of Echinoderms (i.e. sea stars)
- deuterostomes
- radial cleavage à secondary radial symmetry
- development of coelom from archenteron
- formation of anus from blastopore
- water vascular system
o network of hydraulic canals
o have tube feet, branched extensions, that function in movement, feeding, gas exchange
- thin skin covering endoskeleton of hard calcareaou plates
- most are sessile (slow moving marine animals)
- many have skeletals bumps/spines that make them prickly
- sexual reproduction à gametes released by males and females into seawater
- internal & external parts radiate from center (generally as 5 spokes)
- larvae have bilateral symmetry
- adults not perfectly radial
31. Living echinoderms are divided into six classes...
- Asteroidea (sea stars)
o multiple arms radiate from central disk
§ undersides have rows of tube feet
· act like suction disks
· controlled by hydraulic & muscular action
· can grasp substrates and prey, & creep over surfaces
§ able to pull apart bivalves (prey)
o can evert stomach through mouth
o can regernerate limbs
- Ophiuroidea (brittle stars)
o distinct central disk
o long, flexible arms
o tube feet lack suckers
o move by a serpentine lashing of arms
o suspension feeders, scavengers, or predators
- Echinoidea (sea urchins and sand dollars)
o no arms
o five rows of tube feet used for locomotion
o pivoting of long spines also enables movement
o mouths may be ringed with complex jawlike structures
o either spherical or flattened and disk-shaped
- Crinoidea (sea lilies and feather stars)
o lilies attach to substratum by stalks
o feather stars crawl using long, flexible arms
o mouth directed upward (award from substrate)
o arms (for suspension feeding) circle mouth
o very conservative evolution
- Holothuroidea (sea cucumbers)
o lack spines
o very reduced endoskeleton
o elongated oral-aboral axis
o five rows of tube feet à some function as feeding tentacles (suspension or deposit feeding)
- Concentricycloidea (sea daisies)
o discovered in 1986 à only two known species
o bodies = armless, disk-shaped, five-fold symmetry
o less than a centimeter in diameter
o absorb nutrients through the membrane surrounding body
o considered to be highly derived sea stars
32. Chordata are included in a chapter on invertebrates because the phylum does include invertebrate subphyla (although it is only 2 out of many). However, they are linked with Echidnoderms, because both are bilateral deuterostomes.
33. Echidnoderms and Chordates are developmentally similar because they are both bilateral deuterostomia. They are coelomates, with radial cleavage. Also, their coeloms develop from their archenterons and their anuses form from their blastopores.
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