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Chapter 39 Reading Quiz. What are the three parts to a signal-transduction pathway? What is the general term for chemicals that coordinate the parts of an organism? Which hormone promotes the ripening of fruits? What effect do gibberellins have on seeds? What is “apoptosis”?.

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Chapter 39 Reading Quiz

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Chapter 39 Reading Quiz

  • What are the three parts to a signal-transduction pathway?

  • What is the general term for chemicals that coordinate the parts of an organism?

  • Which hormone promotes the ripening of fruits?

  • What effect do gibberellins have on seeds?

  • What is “apoptosis”?

1. Explain how a hormone may cause its effect on plant growth and development.

  • Hormone  chemical signals that coordinate the parts of an organism

  • It is a compound that is produced by one part of the body and then transported to other parts

  • Here it triggers responses in the target cells and tissues 

2. Describe phototropism.

  • Phototropism  the growth of a shoot toward or away from light

    - positive  towards light

    - negative  away from light 

3. Describe what the hormone auxin does and effects that it has on plants.

  • Auxin  describes any chemical substance that promotes the elongation of coleoptiles (the sheath that encloses the plant shoot)

  • Have multiple functions in both monocots & dicots

  • Stimulates stem elongation, root growth, differentiation, and branching

  • Development of fruit, apical dominance, phototropism & gravitropism 

4. Describe what cytokinins do to a plant.

  • They are synthesized in the roots and transported to the other organs

  • Affect root growth & differentiation

  • Stimulates cell division and growth

  • Stimulates germination and can delay senescence 

5. Explain how the ratio of cytokinin to auxin affects cell division and cell differentiation.

  • If more cytokinin than auxin  shoot buds develop from the callus

  • If more auxin than cytokinin  roots form

  • Cytokinin alone in a culture will have no effect 

6. Define apical dominance and describe the check-and-balance control of lateral branching by auxins and cytokinins.

  • Apical dominance  the ability of the terminal bud to suppress the development of axillary buds

  • The auxins & gibberellins are antagonistic in this case

  • Auxin transported down the shoot from the terminal bud restrains axillary buds from growing, causing the shoot to lengthen

  • Cytokinins entering the shoot system from the roots counter the action of auxin by signaling axillary buds to begin growing

    - auxin cannot suppress the growth of these buds once it has begun 

7. List several factors besides auxin from the terminal bud that may control apical dominance.

  • As you’ve seen, cytokinins also play a role in apical dominance through stimulating the lateral branches to begin, and overpowering auxin

  • This ends the apical dominance that the terminal bud once had 

8. Describe gibberellins and their effects on plants.

  • Gibberellins  form in the meristems of apical buds and roots, young leaves, and the plant embryo

  • Promote seed & bud germination

  • Causes stem elongation and leaf growth

  • Stimulates flowering and development of fruit

  • Affects root growth & differentiation 

9. Describe how stem elongation and fruit growth depend upon a synergism between auxin and gibberellins.

  • Stem elongation 

    - auxins stimulate stem elongation

    - gibberellins also stimulate stem elongation

  • Fruit growth 

    - dual control by auxins & gibberellins

    - in some plants, both must be present for fruit to set 

10. Explain the probable mechanism by which gibberellins trigger seed germination.

  • After water is imbibed, the release of gibberellins from the embryo signals the seeds to break dormancy and germinate

  • Gibberellins support the growth of seedlings by stimulating the synthesis of digestive enzymes to mobilize food supply 

11. Describe how abscisic acid (ABA) helps prepare a plant for winter.

  • It is advantageous for plants to be dormant at times

  • ABA is produced in leaves, stems, roots, seeds

  • ABA inhibits growth, closes stomata during water stress, and counteracts the breaking of dormancy

  • Suspends primary & secondary growth 

12. Explain the antagonistic relationship between ABA and gibberellins.

  • In seed growth, the seed will germinate when ABA is overcome by its inactivation or removal or by the increased activity of gibberellins

  • The ratio of gibberellins to ABA determines whether the seed will remain dormant or germinate 

13. Give an example of how ABA can act as a stress hormone.

  • It helps plants cope with adverse conditions

  • When a plant begins to wilt, ABA accumulates in the leaves and causes stomata to close, preventing further water loss 

14. Describe the role of ethylene in plant senescence, fruit ripening and leaf abscission.

  • Ethylene gas  a gaseous hormone

  • Senescence  the progression of irreversible change that eventually leads to death (ex: autumn leaves, withering petals)

  • Fruit ripening  degradation of cell walls, decrease chlorophyll content, dropping

  • Leaf abscission  the loss of leaves every autumn (keeps deciduous trees from desiccating during winter 

15. Discuss how the study of mutant varieties of plants has heightened our understanding of plant hormones.

  • Mutant varieties that don’t develop correctly tell us about plant hormone function

  • Ex: one tomato variety has fruit that never ripens (it is missing the ethylene receptor)

  • By studying these mutations scientists are beginning to understand the signal-transduction pathways that link hormonal signals to cellular responses 

16. List two environmental stimuli for leaf abscission.

  • Cooler temperatures

  • Less water 

19. Explain how light causes a phototropic response.

  • The cells on the darker side of the plant stem elongate faster than the cells on the bright side, curving the stem toward the light source 

18. Define circadian rhythm and explain what happens when an organism is artificially maintained in a constant environment.

  • Circadian rhythm  a physiological cycle with a frequency of about 24 hours

  • Rhythms could be internal, or responses to an environmental cycle (rotation of earth)

  • Rhythms persist even if organism is sheltered from environmental cues

  • Deviation from 24-hour clock can be 21 – 27 hours when sheltered

  • All research thus far points

    to an internal clock 

19. List some common factors that entrain biological clocks.

  • Day length

  • Synthesis of a protein that regulates it’s own production through feedback control

  • It’s possible that all eukaryotes are prone to jet lag, the way we are when the clock on the wall doesn’t match our internal clock 

20. Define photoperiodism.

  • Photoperiodism  a physiological response to photoperiod

  • Seasonal events  seed germination, flowering, onset and breaking of bud dormancy 

21. Distinguish among short-day plants, long-day plants, and day-neutral plants; give common examples of each; and explain how they depend upon critical night length.

  • Short-day  requires a light period shorter than a critical length to flower

    ex: poinsettias (flower in late summer, fall, or winter)

  • Long-day  flower only when the light period is longer than a certain number of hours

    ex: spinach, lettuce, irises (flower in late spring, early summer)

  • Day-neutral  unaffected by day-length; flower at a certain stage of maturity

    ex: tomatoes, rice, dandelions 

22. Explain how the interconversion of phytochrome can act as a switching mechanism to help plants detect sunlight and trigger many plant responses to light.

  • How does a plant measure a photoperiod?

    - pigments called phytochromes

  • Red light (660 nm) is the best at interrupting dark periods & can induce a long-day plant to flower even if it shouldn’t 

23. Using photoperiodism as an example, explain how an integrated control system can regulate a plant process such as flowering.

  • Photoperiodism is a series of complicated systems working together

  • Phytochromes detect the amount & quality of light to determine if flowering will happen

  • Plant hormones may also play a large role, as plants can communicate the flower-inducing substance 

24. Describe how plants apparently tell up from down, and explain why roots display positive gravitropism and shoots exhibit negative gravitropism.

  • Gravitropism functions as soon as a seed germinates

  • Statoliths (specialized plastids containing dense starch grains) are located in certain cells of the root cap

  • The aggregation of these statoliths at the low point may cause a redistribution of calcium, triggering growth 

25. Distinguish between thigmotropism and thigmomorphogenesis.

  • Thigmotropism  the directional growth in response to touch

    ex: vine coiling response

  • Thigmomorphogenesis  a developmental response to mechanical perturbation, usually results from an increased production of ethylene to chronic mechanical stimulation

    ex: stunted trees in windy places 

26. Describe how motor organs within pulvini can cause rapid leaf movements and sleep movements.

  • Pulvini  specialized motor organs located at the joints of leaves

  • Results from a rapid loss of turgor by cells within these pulvini

  • The motor cells become flaccid after stimulation because they lose potassium, which causes water to leave the cells

  • Sleep movements are powered by daily changes in the turgor pressure of the pulvini

  • The potassium gain & loss governs water gain & loss, drooping or raising the leaves 

27. Provide a plausible explanation for how a stimulus that causes rapid leaf movement can be transmitted through the plant.

  • From the point of stimulation, the message that produces this response travels wavelike through plant 1 cm/s

  • Chemical messengers probably have a role in this transmission

  • An electrical impulse can also be detected (action potential) 

28. Explain the molecular basis of resistance to nonvirulent pathogens.

  • Pathogens & plants have coevolved to compromise

  • Specific resistance to a disease is based on gene-for-gene recognition

  • Requires a precise match-up between an allele in the plant and the allele in the pathogen

  • This renders the pathogen “nonvirulent” and it doesn’t destroy the plant or itself in the process 

29. Describe the local and systemic response to virulent pathogens.

  • Localized  the cells attacked release molecular signals that sets the rest into action and forms a barricade of sorts to slow pathogen

  • Systemic  the same response in localized can produce a signal molecule that triggers generalized defense responses in the organs distant from the original site of infection  The End!

18. Define tropism and list three stimuli that induce tropisms and a consequent change of body shape.

  • Tropism  growth responses that result in curvatures of whole plant organs toward or away from stimuli

  • Light

  • Gravity

  • Touch 

17. Describe the components of a signal-transduction pathway.

  • Reception  hormone binds to the specific hormone receptor on cell surface

  • Signal transduction  information is taken into the cell

  • Induction  the activation of the desired cellular responses 

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