Plant responses and plant hormonesEdexcel International A Level Biology: Subtopic test

1. 1
   Oat seedlings are grown in complete darkness so that their coleoptiles (the protective sheaths around the shoots) grow straight upwards. A student then lights a group of seedlings from one side only with a dim lamp. After several hours the coleoptiles have curved towards the lamp. The student knows that the response depends on the plant hormone indoleacetic acid (IAA), which is made in the coleoptile tip.

   (a)

   In the curving coleoptiles, where is the concentration of IAA highest?

   [1 mark]

   • AOn the side facing the lamp
   • BOn the side away from the lamp
   • CEqual on both sides of the coleoptile
   • DOnly in the cells at the base of the coleoptile

   (b)

   According to the acid growth hypothesis, how does IAA make cells of the coleoptile elongate?

   [1 mark]

   • AIt makes the cell walls more alkaline so cellulose microfibrils slide apart
   • BIt prevents water entering the cells by osmosis
   • CIt activates proton pumps that move H⁺ into the cell wall, lowering its pH
   • DIt stimulates the hydrolysis of cellulose to glucose

   (c)

   Explain why the coleoptiles curve towards the lamp.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Seeds of a light-sensitive lettuce variety are soaked in the dark and then given brief flashes of red light (660 nm) and far-red light (730 nm) in different orders, before being returned to the dark. Germination is scored after three days. Batches whose final flash was red germinated; batches whose final flash was far-red did not.

   (a)

   Which statement about phytochrome is correct?

   [1 mark]

   • APfr absorbs red light and is converted to Pr
   • BPr absorbs far-red light and is converted to Pfr
   • CPfr absorbs red light and remains as Pfr
   • DPr absorbs red light and is converted to Pfr

   (b)

   Another batch receives flashes in the order red, far-red, red, far-red, red and is then returned to the dark. What would be expected?

   [1 mark]

   • AGermination, because the final flash leaves most of the phytochrome as Pfr
   • BNo germination, because the first flash was red
   • CNo germination, because far-red light was given twice
   • DGermination only if the seeds are returned to the light

   (c)

   Explain how the formation of Pfr leads to the germination of the lettuce seeds.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   In Core Practical 18, a student cuts barley grains in half across the middle. Half grains with the embryo and half grains without the embryo are placed cut side down on starch agar plates and incubated for 48 hours at 25 °C. The plates are then flooded with iodine solution and the diameter of the clear zone around each half grain is measured. Mean diameters of clear zone: embryo half grains on water 14 mm; embryo-less half grains on water 0 mm; embryo-less half grains on gibberellin solution 12 mm; embryo-less half grains on gibberellin solution plus actinomycin D (an inhibitor of transcription) 0 mm.

   (a)

   Explain why a clear zone forms around the half grains with an embryo but not around the embryo-less half grains on water.

   [3 marks]

   (b)

   (i) Assuming the clear zones are circular, calculate the percentage by which the area of the clear zone around the embryo half grains exceeds that around the embryo-less half grains on gibberellin. Give your answer to 3 significant figures. (2 marks)
   (ii) Explain what the result with actinomycin D suggests about how gibberellin brings about amylase production. (2 marks)

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A glasshouse grower produces chrysanthemums, which are short-day plants: they flower only when the continuous dark period is longer than a critical length. To prevent flowering until a particular sale date, the grower interrupts the dark period with a few minutes of red light. The grower also uses plant hormones, including a gibberellin spray to lengthen the stems of cut flowers.

   (a)

   Explain, in terms of phytochrome, how a short exposure to red light in the middle of the dark period prevents flowering in a short-day plant, including the effect on gene expression.

   [6 marks]

   (b)

   Compare the ways in which auxin (IAA) and gibberellin promote stem elongation, including their effects on transcription.

   [6 marks]

   Total for question 4: 12 marks

End of questions