Light-independent reactions and the Calvin cycleEdexcel International A Level Biology: Subtopic test

1. 1
   A student isolates intact chloroplasts from spinach leaves and suspends them in a buffer containing hydrogencarbonate ions, which act as a source of carbon dioxide. In the light, the chloroplasts take up carbon dioxide and make GALP.

   (a)

   In which part of the chloroplast does carbon fixation by the Calvin cycle take place?

   [1 mark]

   • AThylakoid membrane
   • BLumen of a granum
   • CStroma
   • DInner chloroplast membrane

   (b)

   Which reaction is catalysed by the enzyme RUBISCO?

   [1 mark]

   • AReduction of GP to GALP
   • BCombination of carbon dioxide with RuBP
   • CRegeneration of RuBP from GALP
   • DHydrolysis of ATP to ADP and Pi

   (c)

   Explain why isolated chloroplasts kept in the dark in this buffer cannot make GALP, even though carbon dioxide is available.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A plant biochemist is calculating the energy requirements of the Calvin cycle in a leaf. She assumes that, for every carbon dioxide molecule fixed, the cycle uses 3 molecules of ATP and 2 molecules of reduced NADP.

   (a)

   How many molecules of ATP are used in the Calvin cycle when 6 molecules of carbon dioxide are fixed?

   [1 mark]

   • A9
   • B12
   • C15
   • D18

   (b)

   When 6 carbon dioxide molecules are fixed, 12 molecules of GALP are made. How many of these 12 GALP molecules are needed to regenerate RuBP?

   [1 mark]

   • A10
   • B2
   • C6
   • D12

   (c)

   Explain why most of the GALP made in the Calvin cycle is not used to make carbohydrate but is used in another process.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Tomato plants grown in a glasshouse are supplied with carbon dioxide containing radioactive ¹⁴C for a short time. Later, radioactivity is found in the starch in the leaves, the cellulose in the cell walls, the proteins in the fruits and the lipids in the seeds.

   (a)

   Describe how carbon from carbon dioxide is incorporated into GALP in the Calvin cycle.

   [3 marks]

   (b)

   Use your knowledge of the fate of GALP to suggest why radioactivity is found in the starch, proteins and lipids of these plants, and why radioactive carbon dioxide is also released from the leaves.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A researcher grows the green alga Chlorella in bright light with a high concentration of carbon dioxide and measures the concentrations of GP and RuBP. In one experiment the light is switched off while the carbon dioxide concentration stays high: the GP concentration rises and the RuBP concentration falls. In a second experiment the light stays on but the carbon dioxide concentration is lowered: the GP concentration falls and the RuBP concentration rises.

   (a)

   Explain the changes in the concentrations of GP and RuBP when the light is switched off.

   [6 marks]

   (b)

   Explain the changes in the concentrations of GP and RuBP when the carbon dioxide concentration is lowered and the light stays on.

   [6 marks]

   Total for question 4: 12 marks

End of questions