Lesson 8.3.1

8.3.1 Phytochrome and IAA responses to environmental cues Quiz: Pearson Edexcel Biology A (Salters-Nuffield), Unit 8

20 questions

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Lesson 8.3.1, Phytochrome and IAA responses to environmental cues: 20 multiple choice questions for the Pearson Edexcel Biology A (Salters-Nuffield) (9BI0), Unit 8: Grey Matter, written with Revision Ninja.

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The 20 questions

  1. What is the name of the plant hormone IAA also known as?

    • Ethene, a gaseous hormone that promotes fruit ripening and the abscission of leaves in plants
    • Abscisic acid, a hormone that promotes stomatal closure and seed dormancy during drought stress
    • Auxin, the main natural plant hormone that promotes cell elongation and responses to light
    • Gibberellin, a hormone that promotes stem elongation and breaks seed dormancy in many plant species

  2. Which wavelength of light converts inactive phytochrome Pr into active phytochrome Pfr?

    • Ultraviolet light, which is strongly absorbed by phytochrome and destroys the pigment in the plant
    • Red light, of wavelength about 660 nm, which converts the inactive Pr form into the active Pfr form
    • Far-red light, of wavelength about 730 nm, which converts the inactive form into the active form
    • Green light, which is reflected by the leaf and is therefore not absorbed by phytochrome at all

  3. Which wavelength reverses the conversion of Pfr back to Pr?

    • Far-red light, of wavelength about 730 nm, which converts the active Pfr form back to the inactive Pr form
    • Red light, of wavelength about 660 nm, which converts the active Pfr form back to the inactive Pr form
    • Blue light, which is absorbed by a different pigment and does not affect phytochrome at all
    • Ultraviolet light, which breaks down the phytochrome molecule so that neither form remains in the plant

  4. What is the main effect of the active form Pfr in plants?

    • It stops the synthesis of all proteins in the plant cell, which halts growth in the shoot
    • It breaks down all IAA in the plant, which stops all growth responses to light and shade
    • It increases the transcription of genes involved in responses to the light environment
    • It blocks the xylem vessels, which prevents water from reaching the leaves during growth

  5. Why does a seed germinate in response to red light but not far-red light?

    • Red light stops the seed producing any enzymes, which means germination can happen only in the dark
    • Far-red light is absorbed by the seed coat only, which prevents the light from reaching the embryo inside
    • Red light is absorbed by chlorophyll, which kills the embryo and so the seed cannot germinate
    • Red light converts phytochrome to Pfr, which triggers germination, but far-red converts it back to Pr

  6. Which statement describes the role of IAA in phototropism?

    • IAA is pumped out of the shaded side, causing that side to elongate less than the lit side
    • IAA is destroyed in light, so the shoot grows straight and the plant does not bend towards the light source
    • IAA stops cell elongation on both sides equally, which makes the shoot grow straight towards the light
    • IAA accumulates on the shaded side, causing cells there to elongate more and the shoot to bend towards the light

  7. Which mechanism explains how IAA promotes cell elongation?

    • It breaks down cellulose in the cell wall completely, which removes the wall and lets the cell swell
    • It causes cell walls to loosen, allowing cells to take up water and expand
    • It converts starch into cellulose directly, which strengthens the cell wall and so promotes elongation
    • It stops water uptake into the cell, which keeps the cell small and stops it from expanding further

  8. Which statement describes how phytochrome can affect gene transcription?

    • Pr binds to ribosomes and stops translation, which prevents the synthesis of growth proteins in the cell
    • Pfr may activate transcription factors that switch on genes for growth responses in the plant
    • Phytochrome is itself a DNA polymerase, which copies the genome during cell division and growth
    • Pfr destroys DNA in the nucleus, which switches off the genes that control growth and development

  9. A plant grown in a shaded area has more far-red light reaching it than a plant in full sunlight. What does this do to the proportion of Pfr?

    • It has no effect on Pfr, since far-red light does not interact with phytochrome in any way
    • It decreases the proportion of Pfr, because far-red light converts Pfr back to Pr
    • It increases the proportion of Pfr, since far-red light is absorbed by the active form and converts it to Pfr
    • It makes all phytochrome Pfr permanently, which locks the plant into a fixed growth response

  10. A student places a shoot in unilateral light. Which observation is most consistent with IAA theory?

    • The shoot bends towards the light because cells on the shaded side elongate more
    • The shoot stops growing completely, since unilateral light removes all IAA from the plant
    • The shoot bends away from the light, since IAA moves towards the lit side and promotes growth there
    • The shoot grows straight because IAA is not produced when a shoot is exposed to any light

  11. Which statement is the best evaluation of the claim that IAA always stimulates growth in all parts of a plant?

    • Incorrect, because IAA only affects the roots and has no measurable effect on shoots or leaves at any level
    • Correct, because IAA always promotes growth in every organ at every concentration, with no exceptions
    • Incorrect, because the effect of IAA depends on concentration and the target tissue, and high levels can inhibit some growth
    • Correct, because IAA is the only hormone involved in growth, so its effect is the same in every organ

  12. Which of the following is a response to a change in light cues that is mediated by phytochrome?

    • Contraction of skeletal muscle, which occurs in animals when a nerve impulse arrives at the neuromuscular junction
    • Fertilisation of ovules, which occurs when pollen grains land on the stigma and grow a pollen tube
    • Shade avoidance, in which a plant grows taller when neighbouring plants shade it and reduce the red light
    • Osmotic lysis of red blood cells, which occurs when cells are placed in pure water with no solutes

  13. A plant's phytochrome ratio of Pfr to total phytochrome changes from 0.6 to 0.2 after a period in shade. What is the percentage fall in Pfr as a proportion of the original?

    • 0.4 per cent, found by dividing the difference of 0.4 by the original ratio of 100 per cent
    • 66.7 per cent, found by dividing the fall of 0.4 by the original ratio of 0.6, then multiplying by 100
    • 20 per cent, found by taking the final ratio of 0.2 as a percentage of the original ratio of 0.6
    • 40 per cent, found by subtracting the final ratio of 0.2 from the original ratio of 0.6

  14. Why is IAA transported polarly in shoots and roots?

    • Because it moves in a directed way from the apex of the shoot downwards through parenchyma cells
    • Because it is stored in the phloem and does not move, which keeps the hormone in fixed locations
    • Because it is carried passively in the xylem by water flowing up the stem from the roots
    • Because it moves only by evaporation from the leaves, which carries the hormone out of the plant

  15. Which of the following would most likely result from removing the tip of a shoot?

    • Phytochrome is produced in greater amounts, which causes the lateral buds to grow in response to the wound
    • The shoot grows faster because IAA is removed, which stops the hormone from inhibiting elongation
    • Growth of lateral buds may be promoted because IAA from the apex is removed
    • The shoot stops taking up water because the apex is the only point where water enters the plant

  16. A student expects a seedling to respond to light only by changes in IAA. What is a limitation of this expectation?

    • Seedlings do not contain any hormones, so the IAA expectation cannot apply to them at all
    • Light has no effect on plant growth, since all growth responses are controlled by temperature alone
    • IAA cannot respond to light at all, so the hormone is irrelevant to any light response in seedlings
    • Phytochrome also controls responses to light through gene transcription, not only through IAA

  17. Which pair correctly matches a light wavelength with its effect on phytochrome?

    • Red light: converts Pfr to Pr
    • Far-red light: converts Pr to Pfr
    • Red light: converts Pr to Pfr
    • Blue light: converts Pr to Pfr

  18. A researcher measures the rate of seed germination under two conditions: red light and far-red light. Under which condition is germination expected to be higher, and why?

    • Both equal, because phytochrome has no effect on seeds and so light colour makes no difference at all
    • Red light, because it produces active Pfr, which promotes germination in light-responsive seeds
    • Far-red light, because it destroys the seed coat, which allows the embryo to start growing more quickly
    • Far-red light, because it activates phytochrome and so starts the germination response in the seed

  19. Which statement best describes the function of IAA in the roots compared with the shoot?

    • IAA at the concentration favouring shoot growth can inhibit root elongation, so roots and shoots respond differently
    • IAA has no effect on roots, which are controlled entirely by gibberellins and cytokinins in the soil
    • IAA always stimulates root growth in the same way as shoot growth, so the two organs respond identically
    • IAA only affects the roots and never the shoots, which is why shoots grow without any auxin at all

  20. A seedling receives red light for part of the day and far-red light for the rest. Which statement best predicts the final Pfr proportion?

    • It is determined by the last light received, because far-red converts Pfr back to Pr
    • It is always 0 per cent Pfr because far-red light destroys the phytochrome molecule completely
    • It is unaffected by the light, because Pfr is made only in darkness and not in any light
    • It is always 100 per cent Pfr because red light is the stronger signal over the whole day

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