The Calvin cycleEdexcel A-Level Biology A: Subtopic test

1. 1
   In the 1950s Melvin Calvin and his team supplied the green alga Chlorella with carbon dioxide containing radioactive carbon-14, in bright light. After a measured time, samples of the algae were dropped into hot ethanol. The compounds in each sample were separated by chromatography and the radioactive ones were identified. After 5 seconds nearly all the radioactivity was in one compound, but after 30 seconds it was found in many compounds.

   (a)

   Which compound was the first to contain radioactive carbon?

   [1 mark]

   • AGALP
   • BRibulose bisphosphate (RuBP)
   • CGlycerate 3-phosphate (GP)
   • DGlucose

   (b)

   Why were the algae dropped into hot ethanol after each measured time?

   [1 mark]

   • ATo increase the rate of photosynthesis in the sample
   • BTo provide ethanol as a substrate for the Calvin cycle
   • CTo make the cells absorb more carbon dioxide
   • DTo denature the enzymes so that the reactions stop and the compounds present at that time can be identified

   (c)

   Suggest why, after 30 seconds, the radioactive carbon was found in many different compounds.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A suspension of Chlorella is kept in bright light with a constant supply of carbon dioxide, and the concentrations of GP and RuBP inside the cells stay steady. The light is then switched off, while the carbon dioxide supply continues unchanged, and the concentrations of GP and RuBP are measured over the next minute.

   (a)

   What happens to the concentrations of GP and RuBP shortly after the light is switched off?

   [1 mark]

   • AGP increases and RuBP decreases
   • BBoth GP and RuBP increase
   • CGP decreases and RuBP increases
   • DBoth GP and RuBP decrease

   (b)

   Which enzyme catalyses the reaction between carbon dioxide and RuBP?

   [1 mark]

   • AATP synthase
   • BRubisco
   • CATP hydrolase
   • DDNA polymerase

   (c)

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

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A tomato grower raises the carbon dioxide concentration in a greenhouse from 0.04% to 0.1%, in bright light at 25 °C. In the Calvin cycle in the leaf cells, each molecule of GP is reduced to one molecule of GALP using one molecule of ATP and one molecule of reduced NADP, and the regeneration of each molecule of RuBP from GALP uses one molecule of ATP.

   (a)

   Explain how the increase in carbon dioxide concentration affects the concentrations of GP and RuBP in the leaf cells, and why the rate of carbon fixation does not keep on increasing as carbon dioxide is added.

   [3 marks]

   (b)

   Calculate the total number of molecules of ATP and of reduced NADP used in the Calvin cycle when six molecules of carbon dioxide are fixed and one hexose sugar is made. Show your working.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Sugars first made in the chloroplasts of a clover leaf are used by the plant, and by a rabbit that eats the leaf, as a source of energy and as raw materials for building new molecules.

   (a)

   Describe how the Calvin cycle uses the products of the light-dependent reactions to make sugars from carbon dioxide.

   [6 marks]

   (b)

   Explain how the plant and the rabbit use the sugars to release energy and to make polysaccharides, lipids, amino acids and nucleic acids.

   [6 marks]

   Total for question 4: 12 marks

End of questions