Plant responses to environmental cuesEdexcel A-Level Biology A: Subtopic test

1. 1
   Lettuce seeds were kept in the dark and given brief flashes of red light (R) and far-red light (FR) in different sequences. Seeds given R alone, or R then FR then R, mostly germinated (92% and 90%). Seeds given R then FR germinated poorly (8%). The germination of the seeds depended on the last flash that they received.

   (a)

   Which form of phytochrome stimulates the germination of lettuce seeds?

   [1 mark]

   • APfr, the form that absorbs far-red light
   • BPr, the form that absorbs red light
   • CBoth forms equally
   • DNeither form; phytochrome is not involved in germination

   (b)

   Which type of light converts Pfr to Pr?

   [1 mark]

   • ARed light of wavelength 660 nm
   • BFar-red light of wavelength 730 nm
   • CBlue light
   • DUltraviolet light

   (c)

   Explain why seeds given red light then far-red light germinated poorly, but seeds given red, far-red and then red light germinated well.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Young oat seedlings were illuminated from the right. Normal seedlings bent towards the light. Seedlings whose tips had been removed did not bend. Seedlings with an opaque cap over the tip did not bend, but seedlings with a cap over the lower part of the shoot did bend towards the light.

   (a)

   How does the shoot tip cause the seedlings to bend towards the light?

   [1 mark]

   • AIt makes IAA only on the lit side
   • BIt absorbs light and converts it to heat, which warms the lit side
   • CIt sends action potentials down the shoot to the lit side
   • DIt makes IAA, which is moved to the shaded side of the shoot by carrier proteins

   (b)

   What is the effect of IAA on cells on the shaded side of the shoot?

   [1 mark]

   • AIt stops the cells absorbing water
   • BIt causes the cells to divide faster so that the shoot thickens
   • CIt causes hydrogen ions to be pumped into the cell wall, which loosens the wall so the cells elongate
   • DIt breaks down cellulose in the cell walls so the cells burst

   (c)

   Explain why the seedlings with the tips removed did not bend.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Seedlings of a plant species were grown for ten days in white light with a normal ratio of red to far-red light, and in white light with extra far-red added to give a low red to far-red ratio, as occurs under the leaves of neighbouring plants. The mean stem length of seedlings in normal light was 4 cm. The mean stem length with extra far-red was 11 cm.

   (a)

   Explain the difference in mean stem length between the two groups of seedlings.

   [3 marks]

   (b)

   Calculate the percentage increase in mean stem length with extra far-red light, and suggest one advantage and one disadvantage to the plant of the increase in stem length.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Chrysanthemums are short-day plants. A grower in a glasshouse wants to supply flowers all year, so controls the length of the dark period using blackout blinds. The plants flower only when the continuous dark period is longer than a critical length of about ten hours. The grower also lays some pots on their sides during transport, and notes that within a day the shoots bend upwards and the roots bend downwards.

   (a)

   Explain how phytochrome allows chrysanthemums to detect the length of the night, and how this leads to flowering, including its effect on transcription.

   [6 marks]

   (b)

   Explain how IAA causes the shoots to bend upwards and the roots to bend downwards when the pots are laid on their sides.

   [6 marks]

   Total for question 4: 12 marks

End of questions