The light-independent reaction (Calvin cycle)AQA A-Level Biology: Subtopic test

1. 1
   A plant biochemist is studying the first step of the Calvin cycle in an extract of chloroplasts, using carbon dioxide and the five-carbon compound ribulose bisphosphate (RuBP).

   (a)

   In which part of a chloroplast does the Calvin cycle take place?

   [1 mark]

   • AThylakoid membrane
   • BSpace inside a thylakoid
   • CStroma
   • DInner envelope membrane

   (b)

   Which enzyme catalyses the reaction between carbon dioxide and RuBP?

   [1 mark]

   • AATP synthase
   • BRubisco
   • CCarbonic anhydrase
   • DDNA polymerase

   (c)

   Explain, using the number of carbon atoms in each compound, how one carbon dioxide molecule reacting with RuBP leads to the formation of two molecules of GP.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A culture of algae is kept in bright light with a constant supply of carbon dioxide. The concentrations of glycerate 3-phosphate (GP), reduced NADP and ribulose bisphosphate (RuBP) in the cells are steady. The carbon dioxide supply is then suddenly stopped while the light stays on.

   (a)

   Soon after the carbon dioxide supply stops, what happens to the concentration of GP?

   [1 mark]

   • AIt rises
   • BIt falls
   • CIt stays the same
   • DIt rises then falls to zero immediately

   (b)

   Soon after the carbon dioxide supply stops, what happens to the concentration of reduced NADP?

   [1 mark]

   • AIt falls because it is used up reducing GP
   • BIt rises because less is used in reducing GP
   • CIt stays the same because it is made in the light-independent reaction
   • DIt falls to zero because the light-independent reaction has stopped

   (c)

   Explain the change in the concentration of RuBP after the carbon dioxide supply is stopped.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A herbicide that inhibits the enzyme catalysing the reduction of glycerate 3-phosphate (GP) to triose phosphate (TP) is sprayed on a field of wheat in bright sunlight.

   (a)

   Predict and explain the effect of the herbicide on the concentrations of GP, triose phosphate and RuBP in the wheat leaves.

   [3 marks]

   (b)

   Suggest why the light-dependent reaction would also slow down in the treated wheat.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Chlorella algae in a lit, sealed flask were supplied with carbon dioxide containing radioactive carbon-14. Samples were killed at intervals and the compounds containing carbon-14 were identified. After 5 seconds nearly all the radioactivity was in glycerate 3-phosphate (GP). After 30 seconds it was also found in triose phosphate, ribulose bisphosphate (RuBP), hexose sugars, amino acids and lipids.

   (a)

   Explain what these results show about the light-independent reaction.

   [6 marks]

   (b)

   The light-independent reaction does not use light directly, yet it stops soon after the light is switched off. Explain why. Also explain why only some of the triose phosphate can be removed to make hexose sugars.

   [6 marks]

   Total for question 4: 12 marks

End of questions