AP Psychology Ch17 and 18 vocab

self concept
This component of self involves the beliefs people hold about who they are, as well as what characteristics they have.

reference groups
Festinger’s theory of social comparison states that people assess their own value based on those around them. This terms refers to the people to whom you feel are similar to you that serve as your basis of comparison.

relative deprivation
This phenomenon is produced when individuals feel their status is unfavorable compared to that of others. It is the belief that no matter how much a person gets, they deserve more.

social identity
This term refers to a person’s beliefs about the groups they belong to (nationally, religiously, etc.); it is a large part of their self concept.

self-schema
This term refers to a person’s mental representation of himself, which may shape that individual’s thoughts, experiences, and emotions.

self-fulfilling prophecy
This process occurs when a person does something that causes others to confirm that person’s impressions or beliefs or when schemas cause people to inadvertently lead people to behave according to our expectations.

attribution
This term deserves the process people go through to explain the causes of behaviors, both their own and that of others. This process helps us to understand and predict later actions, as well as try to control situations.

fundamental attribution error
This term describes people’s tendency to overattribute others’ behaviors to internal factors (i.e. their personality), instead of external factors (i.e. stress).

actor-observer bias
This term describe people’s tendency to attribute their own behavior to external causes, even while attributing the behavior of others to internal causes, due to the different kinds of information they have about the behavior of themselves versus the behavior of others. This phenomenon is especially common inappropriate or inadequate behavior is concerned.

self-serving bias
This term describes the tendency people have to take personal credit for success (internal cause), while blaming failures on external causes.

attitude
This tendency to act, feel, or think negatively or positively towards things in our environment is a part of social cognition that has been studied for an extremely long time. This tendency guides how we react to others, what we do, what we support, and so on. There are three components: cognitive (beliefs), affective/emotional (feelings), and behavioral (actions).

cognitive dissonance theory
This theory states that people want their attitudes, beliefs, and thoughts to be consistent with each another and with the individual’s behavior. When inconsistencies are noticed, people feel anxious and attempt to reduce the dissonance—often by changing attitudes rather than behaviors.

self-perception theory
This theory challenges the cognitive dissonance theory. It states that individuals often find themselves unsure about their attitudes, and thus must reflect on their behaviors under certain circumstances and then infer what their attitude should be. There is no tension in this process.

contact hypothesis
This belief is based on the idea that prejudices and stereotypes towards certain people will diminish as contact with the people increases.

matching hypothesis
This belief is based on the idea that individuals have a tendency to date, marry, or form relationships with people of the same level of physical attractiveness. People compromise because they are afraid of rejection from those with physical appeal greater than their own.

norms
This term refers to learned, socially based rules that prescribe what people should or shouldn’t do in certain situations. We inherit them from our teacher, parents, and peers. They describe what is expected of people and help make social situations less ambiguous and more comfortable.

deindividuation
This psychological state occurs when a person—often a member of a group—loses his sense of individuality. This loss heightens feelings of cohesiveness with his group and increases emotional arousal. The focus of attention on membership in the group and the values of group serves to reduce a sense of personal responsibility by creating a feeling of anonymity, and also shifts attention away from internal thoughts to external environment.

empathy-altruism theory
This beliefs states that the likelihood of people engaging in unselfish helping acts (altruism) will do so even when the cost is high when they feel empathetic toward the person in need.

social loafing
This terms describes how people have a tendency to not work as hard when in a group, as it is difficult to identity an individual’s contributions. Individuals often exert more effort when performing alone.

conformity
This terms describes when a person changes their beliefs or behavior to match those around them. It is the result of group pressure.

compliance
This terms describes when people adjust their behavior after requested to do so. Explicit requests are spoken or directly indicated. Implicit requests are unspoken but understood.

frustrating-aggression hypothesis
Originally developed by John Dollard, this idea states that frustration always results in aggression, and that aggression will not occur unless someone is frustrated. This hypothesis, however, is said to be too simply and general.

prisoner’s dilemma game
This situation occurs when two criminals are separated after a crime. They have the choice of confessing or remaining silent. If neither confess, they will mostly likely be jailed for only 1 year for a minor offense. If both confess, they are likely to get a 5 year sentence. However, if one confesses and the other does not, the one that confessed will be released while the non-confessor will be jailed for up to 10 years. Responses can be cooperative or competitive. It is a mixed-motive conflict.

altruism
This term describes an unselfish concern for another’s welfare. It is a helping behavior.

arousal: cost-reward theory
This belief states that people find the sight of a victim anxiety-provoking and distressing. Their emotions motivate them to reduce their unpleasant arousal, namely the victim’s situation.
As physiological arousal of bystanders increases, so does the likelihood of them offering assistance.

group think
This phenomenon occurs when members of a group are not able to completely or realistically evaluate the potential negative consequences of a decision or the options open to them. Three conditions increase the likelihood of this to occur: isolation from outside influence, time pressure or extremely stress, biased leadership.

zero-sum games
This terms describes situations in which conflict is extremely likely. In these cases, the gains of one person is subtracted from another person’s resources; the sum of losses and gains is zero.

AP Psychology Ch16 vocab

CHAPTER 16 - Treatment and Therapy

Psychoanalysis
This method of treatment was developed by Sigmund Freud, and focuses on the affect of the ego (a referee between superego and id) and unconscious conflicts on a client. Freud examined the relationship between a person’s history and their current problems, and searched for hidden meanings in people’s dreams and actions.

Client-Centered Therapy
This treatment method is part of the phenomenological approach. It focuses on how therapists needs to establish relationships with their clients through the use of positive regard, empathy, and congruence. In this method, a client decides when to talk about whatever they want without being directed, judged, or interpreted by the therapist. The client solves his own problem with little advice from his therapist.

Unconditioned Positive Regard
This attitude involves treating a client as valued person, no matter what. Therapists should listen to their clients and accept their statements without interrupting or judging.

Empathy
This attitude involves appreciation of how the world looks from client’s perspective. Therapists must look at the client with an internal frame of reference and try to gain an emotional understanding of client’s thoughts and feelings.

Reflection
This feature of client-centered therapy helps a patient focus the thoughts and feelings they are expressing by confirming communication between the patient and therapist. The therapist confirms communication by paraphrasing his patient’s words, their meaning, and their emotions.

Congruence
This attitude is also known as “genuineness,” and involves consistency between a therapist’s feelings and actions.

Gestalt Therapy
This method, developed by Frederick S. Perls and his wife Laura, is direct and dramatic. It tries to help people grow by making them more self accepting, self aware, and unified. Clients are prodded towards certain feelings and impulses. Incongruities between what a person says and does are pointed out.

Cognitive-Behavioral Therapy
This type of behavioral treatment focuses on changing thinking patterns as well as behaviors.

Systematic Desensitization
This method of modifying behavior (specifically phobias and anxieties) involves visualizing the negative stimuli while remaining relaxed in order to unlearn the learned association between the stimuli and the negative response.

Modeling
In this technique, therapists teach their clients certain behaviors by demonstrating them themselves. Skills are learned by the client vicariously.

Token Economy
This technique of positive reinforcement involves rewarding desired behaviors with items (tokens) that can later be exchanged for actual rewards (i.e. a cookie).

Extinction
This technique involves operant conditioning and is a process of removing reinforcers to make undesired behaviors stop.

Flooding
This method of behavior therapy keeps people in feared but harmless situations, depriving them of their normal escape patterns. Slowly, their negative response diminishes and eventually is extinguished.

Aversive Conditioning
This classical conditioning technique involves turning habitual but undesirable behaviors into less attractive options so that a client will be less likely to perform the behaviors.

Rational Emotive therapy
This form of cognitive behavior therapy is based upon the idea that depression, guilt, anxiety, etc. are caused by people’s thoughts and interpretations of events. Individuals must learn to recognize self-defeating thoughts and replace them with beneficial ones.

Neuroleptics
This psychoactive drug, also called an antipsychotic, helps decrease symptoms of several mental disorders, including incoherence, paranoia, and hallucinations. Phenothiazines are the most common type.

Antidepressants
This drug helps individuals with mood disorders by helping them resume normal eating and sleeping habits as well as improving their disposition.

Anxiolytics
This drug helps individuals with anxiety disorders by reducing mental and physical tension (symptoms of anxiety).

AP Psychology Ch15 vocab, test notes

Psychological Disorders

Culture-general disorders - appear almost worldwide, but symptoms differ according to cultural backgrounds.

Culture-specific disorders – observed in only certain areas and are unique to certain cultures.

Diathesis-stress model - focuses on biological imbalances, inherited traits, brain damage, enduring psychological traits, and early learning experiences (biopsychosocial causes) that may create a predisposition for a psychological disorder; appearance of a disorder depends on stressors encountered by individual… when stress levels exceed coping capacity, panic response is triggered and psychological disorders may arise

Positive symptoms: unwanted additions to a person’s mental life... include distortions or exaggerated behavioral, perceptual, or cognitive functioning (i.e. delusions or hallucinations)

Negative symptoms: take away from elements of normal mental life; decrease or cause a loss of normal functioning... [ex: flat affect (restricted emotional expression), alogia (lack of speech)]

Personality disorder - long-standing, inflexible patterns of behavior, affects all areas of functioning
10 types: paranoid, schizoid (detached, restricted emotions), schizotypal (detached, odd perceptions/thoughts/ behaviors), dependent, obsessive-compulsive, avoidant, histrionic (overly dramatic, shallow, desire to be center of attention), narcissistic, borderline (unstable, impulsive, angry, suicidal), antisocial

General anxiety disorder – aka free-floating anxiety, involves non-specific, excessive, and long-lasting anxiety; individuals feel worried, jumpy, and irritable, believing disaster is imminent

Abnormal behavior
-not psychologically healthy, prevent effective everyday functioning
-socially unacceptable
-faulty reality perception
-self defeating, causes personal destress

Historical treatment of the insane
-prehistoric: supernatural, valued
-Greek/Roman: God’s way, embraced mental problems, bodies different?, kind treatment
-Middle Ages: Satan made people that way, torture
-Colonial Times: “witches” often OCD

Criterion for Abnormal/Normal Behavior
statistical: anything seen only in small % of popu
concensual: whatever society dictates is normal is normal (general population’s beliefs)
functioning: if a person can function, they are normal
personal: how you feel about your situation

Models of Psychological Disorders
Biomedical: something physically wrong, i.e. chemical imbalances, genetics, endocrine system problems
Psychoanalytic: repressed past problems, unconscious conflict
Cognitive: faulty constructs, your perspective causes problems
Behavioral: learned to be sick to get rewards/attention
Sociological: has to do w/ social class and social stress (poor working class = lowest #)

Categorizing disorders
psychological – in the mind
systemic – disease/disorder
traumatic – environment or life experience

Diagnostic and Statistical Manual (DSM)
DSM: 1962, 60 disorders
DSM II: 1968, original found not adequate, 2 categories created: neurotic (stress) & psychotic (imbalances)
DSM III: 1980, 150 disorders, based on what doctors saw in patients, neurotic/psychotic categories thrown out (not enough), homosexuality removed as disoder
DSM III-R: 1987, 250 disorders, 17 categories
DSM IV: 1994, close to 500 disorders, based on patient symptoms directly
axis
1. major clinical symptoms, basic disorders
2. personality/development disorders
3. physical problems
4. stress level in past year
5. independence

ESSAY
Create a case history for each of the disorders on the test. Make sure to describe the cause, type of onset, symptoms, behaviors, and treatments

Conversion Hysteria/Disorder: loss of physical functions w/o physical cause
onset – immediate
cause – traumatic
treatment – psychoanalysis
**type of somatoform disorder; occurs when an individual appears to be blind, deaf, paralyzed, or insensitive to pain when they are not; symptoms tend to appear with severe stress; individuals show surprisingly little concern over condition

Bipolar Disorder: manic & depressive states
onset – gradual
cause – genetic and biochemical (monoamide oxide)
treatment – neuroleptics (antipsychotics), lithium
*characterized by the appearance of 2 alternating emotional extremes: depression & mania
Mania describes an extremely elated, energetic, impulsive, reckless, agitated emotional state

Seasonal Affective Disorder: mood shifts with season, depression, weight gain
onset – gradual, episodic
cause – environmental
treatment – antidepressants, light therapy

Schizoprenia: hallucinations & delusions
1) paranoid – delusions of grandeur 2) catatonic – stupor/frozen to excited states
3) hebephrenic/disorganized – childish, word salads,
onset – can be gradual, reaction/immediate, or just happen upon adulthood
cause – organic brain disease, genetic, imbalance of dopamine
treatment – neuroleptics, pherothiazines

Paranoid Reaction: no hallucinations, delusional, lack insight
1) state: trauma and acute 2) true: always been a little off
onset – organized, gradual
cause – biochemical
treatment – neuroleptics, phenothiazines

OCD: thoughts that won’t go away & repetitive ritualistic behaviors that serve no purpose
onset –gradual or immediate
cause – antibodies for strep, chemical imbalance of serotonin
treatment – SSRI antidepressants, systematic desensitization, lobotomy (rare)

Panic Attacks: heart palpitations, chest pain, dizziness, difficulty breathing, episodic w/o cause or warning
onset – immediate, reactive?
cause – unequal blood flow to right side of brain, genetic?
treatment – drug therapy (Xanax), antidepressants, lobotomy

Post Traumatic Stress Disorders: flashbacks, nightmares
onset – reactive, immediate
cause – repression surrounding traumatic event
treatment – SSRI’s, psychoanalysis, EMDR

Phobias: unreasonable fears
onset –gradual or immediate
cause – trauma, associated with panic attacks
treatment – MAO inhibitors, systematic desensitization

Dissociative Disorders
1) amnesia: memory loss 2) fugue: memory loss plus flight, end up somewhere else 3) identity disorder: alters splitting from core personality 4) depersonalization: lose control of body
onset –gradual or acute
cause – repression, childhood abuse, trauma
treatment – psychoanalysis

AP Psychology Ch12 Test notes/essay

Development Unit Essay

COMPARE AND CONTRAST ERIKSON'S PSYCHOSOCIAL THEORY OF DEVELOPMENT (8 stages) WITH FREUD'S PSYCHOSEXUAL THEORY OF DEVELOPMENT (6 stages)

Both Freud and Erikson’s theories on development have stages that match up age and principal behaviors established. Freud, however, believed that in each stage we have a libidinal focus, while Erikson believed we go through stages based on our social interactions. Freud’s stages also stop after age 12, whereas Erikson’s continue throughout life and don’t end until we die.

Both Freud and Erikson’s first stages deal with trust as the principal behavior established. Freud focused on the mouth and believed that people in this stage get their arousal orally, generally through breastfeeding. Freud believed that in moments of stress or hardship, we may regress back to earlier stages. This can be seen when six year olds children get frightened and suck on their thumb, or when an teenager has a hard day and goes out for 3 Whoopers. Erikson’s first stage is trust vs. mistrust and deals with having our needs met (“I am what I am given”). If we our needs are met (i.e. hungry, get fed; cold, get blanket), we develop into trusting individuals. If we don’t have needs our met, we feel worthless and fail to develop trust and may be suspicious of people later in life, even if we have strong bonds with them. Erikson believed that people who were given too much early on—for example, they had parents that would give them food before they even felt hungry—will become gullible and overly trusting people.

Both Freud and Erikson’s second stages deal with competency as the principal behavior established. Freud believed that in this stage we get our arousal anally, especially people in this stage are generally being potty trained. The potty training results in either gained feelings or control or a feeling of lack of control. According to Freud, if someone were to have difficulty with this stage—or any other stage—they may become fixated or stuck. Anally fixated people fall under two catergories: those who are anal expulsive are very messy because they believe they have no control over their lives, and those who are anal retentive, who are extremely neat perfectionists who desperately seek control over everything in their lives. People can also anally regressed and become very neat in times of stress (i.e. when a student cleans his room before a big exam, instead of studying). Erikson’s second stage deals with autonomy vs doubt (“I am what I will”). We develop into independent people if our parents allowed us to fail. If a child’s parents were constantly doing everything for them then they always feel like they are not good enough or not competent enough, and will learn to doubt themselves.

Two of Freud’s stages actually match up with the principal behavior established in Erikson’s third stage. The first of Freud’s stages involves phallic self-stimulation (3rd stage) and learning about one’s body and what feels good. The seconds of Freud’s stages involves Oedipal/Electra complexes (4th stage), in which children focus on their opposite sex parent. Boys become attracted to their mothers and fear their fathers because they think their fathers will castrate them. Girls become attracted to their fathers and begin to think women are not as important as men because men have penises. Girls develop penis envy (which Freud claimed girls never get over) and think that the only thing they can have in place of a penis is a baby. Thus, the principal behavior established is the learning of gender roles. Erikson’s third stage involves initiative vs guilt (“I am what I imagine”). Children either take the initiative to dream big and reach for the stars, or feel guilty for trying things because their parents do not support them or show approval. Children learn to feel shame.

Both Freud and Erikson’s next stages focus on learning as the principal behavior established. For Freud, the fifth stage is a latent stage, which involves no libidinal focus because children believe sex is “yucky” due to the guilt they feel after working through their complexes in the previous stage. Thus, this stage simply involves focusing on school and education. According to Freud, this is when children develop morality and learn things such as shame and disgust. Erikson’s parallel is his fourth stage, industry vs inferiority. Industry involves learning and feeling smart. Inferiority is felt by those in low groups (i.e. the “slower” readers or the “easy” math group) who are held back. The key phrase is “I am what I can learn.”

Then, Erikson’s theory has a fifth stage, for which Freud does not truly have a parallel. Erikson’s stage involves identity vs role confusion. During this period, individuals try to discover who they are and work through issues involving time, sexual polarization, and self confidence, among other things.

Freud’s last stage corresponds with the principal behaviors established in Erikson’s sixth stage. Freud’s stage involves genetalia as the libidinal focus, only unlike the phallic and complex stages, this time individuals want others (not parents) to stimulate them. In this stage, people learn to establish relationships. Erikson’s parallel stage involves intimacy versus isolation. We develop relationships with others in this stage, and must consider whether we want to get married and commit ourselves to someone else.

While Freud didn’t believe that development extended beyond his genitalia stage, Erikson still had two more stages, the seventh stage being generativity vs self absorption, in which we learn to become involved in the community. People who are on the generativity side contribute to the world around them, whereas those who are self absorbed only live in the small space that they occupy and do not add to the community around them. The stage in Erikson’s theory is integrity vs despair, in which a person must deal with accepting their life and eventual death. People will integrity will look back and see all the wonderful things they did. They will see accomplishments and will believe their life was meaningful. People who despair reflect on their life only to see failure and missed opportunities. They reflect on what they should have done or could have done better.

AP Psychology Theories on Development

Piaget’s Theory of Intellectual Development

1) Sensorimotor Stage – thinking confined to what is sensed physically, struggle with object permanence (mastery leads to separation anxiety)
2) Preoperational Stage – very egocentric, learn that symbols represent objects, struggle with conversation and reversibility (looking at things in 1-D instead of 2-D, “do you have a sister?” “does your sister have a sister?”), confuse reality with fantasy
3) Concrete Operational Stage – fully grasp conservation, reversibility, and absolutes; struggle with abstract concepts and hypotheticals
4) Formal Operational Stage – master abstract thinking, struggle with understanding things from another perspective (putting self into someone else’s shoes)
* little acronym: SPiCe F (the F is random, but the "spice" part really helped me)


Kohlberg’s Theory of Moral Development

*amoral before 4 years old
PRECONVENTIONAL
1) Might Makes Right – morality based on consequences; if caught, wrong
a. preoperational
b. egocentric
2) Marketplace Morality –do the right thing if it is beneficial to self, “what will I get out of it?”
CONVENTIONAL
3) Good Girl/Good Boy – total conformity, morality is what everyone does, go with crowd
a. early teens
b. may last a lifetime
4) Law and Order – law is the law, must follow rules, can’t always get what we want
POSTCONVENTIONAL
5) Common Good – majority can be wrong, change the system by working within it, “what will benefit most people?”
6) Ethical Principals – certain things are true regardless of laws, may need to change things to help everyone, MLK

AP Biology Plant Kingdom Notes

More Plant Kingdom Notes

COMMON CHARACTERISTICS
- eukaryotic, autotrophic, photosynthetic
o chloroplasts
o chlorophyll a, b
o carotenoids
- multicellular (evolved from chlorophyta/green algae)
- non motile
- cell wall of cellulose (different from bacteria and fungi)
- food reserve stored as starch

Problems w/ Life on Land
- water loss
- gas exchange
- gravitational pull

Terrestrial Changes (distinguished plants from algae)
- structural adaptations
o complex bodies w/ cell specialization for diff functions
o stomata: balances gas exchange and dehydration
o cuticle: waxy secretion, cover surface, prevents dessication
- chemical adaptations
o lignin: rigidifies cellulose into wood, holds plant upright against gravity, maximizes leaf exposure to sun
- reproductive adaptations
o embryotic reproduction
o embryo retained and protected w/in parent à won’t dry out
o allows for gamete dispersion/fertilization without water

Charophyceans
- green algae
- closest relative of land plants
o 4 key shared characteristics w/ plants
§ rose-shaped (rosette) complex for cellulose synthesis
§ structure of flagellated sperm
§ formation of a phragmoplast
§ peroxisome enzymes
o nuclear and chloroplast gene shown close relations
§ sequenced DNA = very similar
§ in chloroplast, layer of durable polymer called sporopollenin prevents exposed zygotes from drying out
o 5 not shared traits (in plants, but not charophyceans)
§ apical meristems
§ walled spores produced in sporangia
§ multicellular dependent embryos
§ alternation of generations
§ multicellular gametangia

Vascular Plants
1. structural support
a. rigid structure needed to stand upright
b. cellulose and lignin fortify plant
2. regional specialization of plant body
a. water and light segregated in terrestrial habitat (water=roots & light=plant top)
b. root, aerial shoots, stem, leaves evolve
c. but this specialization leads to problem of transportation
3. vascular system
a. moves substances btwn roots/leaves
b. xylem
i. conducts water and minerals from root to leaves
ii. dead at functional maturity
c. phloem
i. transports food down plants
ii. distributes amino acids, sugars
iii. live at functional maturity
iv. arranged in tubules
4. pollen
a. contains male gametophytes (which will produce sex gametes)
b. eliminates need for water for sexual reproduction
5. seeds
a. diploid next generation sporophytes (product of archegonium, female organ)
6. increased dominance of diploid sporophyte
a. branching sporophytes of vascular plants amplifies production of spores
b. more complex bodies becomes possible

Seedless Vascular Plants
3 divisions
- lycophytes (lycophyta)
o club moss, ground pines
o mostly homosporous
- horsetails (sphenophyta)
o have photosynthetic, free-living gametophytes not dependent on sporophyte for food


Seed Vascular Plants


Reproductive adaptations
1. gametophytes of seed plants become more reduced in size (compared to the seedless vascular plants) and are retained within moist reproductive tissue of the sporophyte, unlike seedless plant gametophytes, which are independent
2. seeds replace spores as main means of dispersing offspring
harsh terrestrial environment required resistant structure for dispersal
bryophytes & seed(less?) vascular plants release hard spores
seed = more hardy due to multicellularity
seed contains sporophyte embryo, food supply, surrounding protective coat
all seed plants = heterosporous (have mega and micro sporangia)
seed develops into megasporangia
3. pollen became vehicle for sperm cells in seedplants
microspores due to pollen grains which mature to male gametophyte
coated w/ polymer sporopollenin
carried by wind/animals following release

GYMNOSPERMS = lack enclosed chambers (ovaries) in which seeds develop à open seed

needle-shaped leaves = adapted to dry conditions
v thick cuticle
v stomata are in pits reducing water loss
v despite its different shape, it has megaphylls like all SP leaves

LIFE CYCLE OF A PINE
v sporophyte dominate
v sporangia located on cones
v multicellular sporophyte reduced, develops from haploid spores retained in sporangia
v male gametophyte consists of multicell nutritious tissue
v archegonium developes w/in ovule
v heterosporous
v takes nearly 3 yrs to complete life cycle

Angiosperms -> flowering plants
division – Anthophyta, 2 classes: monocotyledones & dicotyledones
a) mostly use insects and animals to increase efficiency of pollenation
b) terrestrial adaptation refined vascular tissue
c) conifers have Tracheids (sp?)
i. water conducting cells, early xylem
ii. elongated, tapered à function in both support and water movement
d) have vessel elements
i. shorter and wider
ii. arranged end to end forming continuous tubes
iii. more specialized for conducting water
e) reinforced by fibers
i. specialized for support
ii. thick lignified wall
iii. fibers evolved in conifers *some conifers have fibers & tracheids but not vessel elements

Flower -> defining reproduction adaptation
- compressed shoot w/ four whorls of _____ leaves
- parts of flower in 30.6 in Campbell
4 Evolutionary Trends
# of floral parts have become reduced
floral parts become fused
symmetry has changed from radial to bilateral
ovary drooped below petals and sepals à better protected

Life Cycle -> highly refined version of all
- heterosporous
- microsporangia in anthers produce microspores à form female gametophytes
- immature male gametophyte = pollen grains
- 2 haploid nuclei participate in double fertilization

Characteristics of Angiosperms
- female gametophyte
o don’t produce archegonium
o located in ovule
o consist of an embryo sac w/ 8 haploid nuclei in 7 cells
o large central ___ has 2 nuclei
o 1 cell is egg

Seed = Mature Ovule
1. embryo – develops from zygote w/ embryonic root and either 1 (monocot) or 2 (dicot) cotoyledons on seed leaves
2. endosperm – the triploid nucleus in embryo sac divides repeatedly forming ___ endosperm rich in starch à food reserves
3. seed coat

AP Biology Ch25, 26, 27 notes

Plants Notes

*taxonomy - in plant and fungi, phylums are called “divisions”

phylogeny = evolutionary history of species

SYSTEMATICS
- phylogeny and taxonomy
- classification reflects evolutionary affinities of species
- constantly changing
- fossil record serves are most evidence

fossils
- very rare -> record = spotty, incomplete
- only hard things like shells—nothing soft/fleshy... must be near water & sediment
- DATING
- relative = based on strata holding the fossils (oldest strata and fossils on the bottom)
- absolute = radiometric - unstable isotope with known half life can be tested for, i.e. Carbon-14

continental drift = as continents move, environments change... natural selection acts forces changes in species as they slowly adapt to their environment

massive extinctions create new adaptive zones by freeing up environmental niches and allowing for rapid adaptive radiation à major adaptations? insect wings (allows for new zones), shells (defense)

taxa should be monophyletic = SINGLE ancestor gives rise to all species in taxon

POLYPHYLETIC = members do not all share common ancestor

paraphyletic = taxon grouping consists of ancestor, but not all descendents (only some)

homology -> descent from common ancestor (shared phylogenetic history) can lead to morphological (i.e. bone structure) and molecular similarities (genes/DNA)
- shared primitive character = general, trait beyond the taxon being defined (i.e. horses and humans have hair, doesn’t mean their related because whales also have hair)
- shared derived character = unique, evolutionary novelty... distinct to clade, useful in establishing a phylogeny
analogy -> similarity due to convergent evolution (i.e. bat wing and bird wing function)
to construct phylogeny, differentiating between HOMOLOGY and ANALOGY is very important
homoplasies = analogous structures that evolved independently
branching sequences in phylogeny show time of evolution or divergence

cladogram = patterns of shared characteristics, doesn’t show evolutionary history or timing, merely chronological sequencing ->lade = group of species w/in tree, includes ancestors & descendents
ultrameric tree shows time
phylogenetic tree represents a hypothesis

Outgroup comparison
- differentiates btwn derived or primitive characters
- important step in cladistic analysis
- ingroup = species studies which display mix of shared primitive and shared derived characters outgroup = the species closely related to the species being studied, but less so than any in the ingroup

Orthologous genes - widespread, found in diff gene pools due to speciation (i.e. ß hemoglobin genes found in both humans & mice)... all living things share certain biochem/developmental pathwaysParalogous genes - result from gene duplication, found in more than one copy in the same genome (i.e. olfactory receptor genes)

AP Biology Ch39 Objectives

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.

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.

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.