Chloroplasts and the overall reaction of photosynthesisEdexcel A-Level Biology A: Subtopic test

1. 1
   A student examines an electron micrograph of a palisade mesophyll cell from a spinach leaf. Each chloroplast is about 5 µm long and is surrounded by two membranes. Inside, there are flattened membrane-bound sacs arranged in stacks, linked by thin membranes, in a surrounding fluid that contains starch grains and a small circular piece of DNA.

   (a)

   In which part of the chloroplast are the enzymes of the light-independent reactions located?

   [1 mark]

   • AThe thylakoid membranes
   • BThe stroma
   • CThe granum
   • DThe chloroplast envelope

   (b)

   What is a granum?

   [1 mark]

   • AA fluid-filled space containing enzymes
   • BA circular strand of DNA
   • CThe double membrane around the chloroplast
   • DA stack of thylakoids

   (c)

   Explain how the stacking of the thylakoids into grana is related to the role of the chloroplast in the light-dependent reactions.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Two experiments were carried out with the alga Chlorella. In experiment 1 the water supplied contained the heavy isotope oxygen-18 (¹⁸O), and the carbon dioxide contained only oxygen-16 (¹⁶O). In experiment 2 the water contained only ¹⁶O and the carbon dioxide contained ¹⁸O. In each experiment the algae were illuminated and the oxygen gas that was released was analysed to find which isotope it contained.

   (a)

   Which result was obtained in experiment 1?

   [1 mark]

   • AThe oxygen gas contained ¹⁸O, so the oxygen came from the water
   • BThe oxygen gas contained only ¹⁶O, so the oxygen came from the carbon dioxide
   • CThe oxygen gas was a mixture of equal amounts of ¹⁶O and ¹⁸O
   • DNo oxygen gas was released because the carbon dioxide was not normal

   (b)

   Where was ¹⁸O found at the end of experiment 2?

   [1 mark]

   • AIn the oxygen gas that was released
   • BOnly in the chlorophyll
   • CIn the glucose and in the water produced
   • DIt was lost as carbon dioxide

   (c)

   Use the overall equation for photosynthesis to explain what these experiments show about the origin of the oxygen that is released and what happens to the hydrogen from water.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A biologist is preparing a talk on chloroplasts in the leaf cells of green plants. Chloroplasts are the site of photosynthesis, which uses light energy to make glucose from carbon dioxide and water.

   (a)

   Describe the structure of a chloroplast.

   [3 marks]

   (b)

   Explain how four features of the structure of a chloroplast are related to its role in photosynthesis.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A student isolates chloroplasts from spinach leaves by grinding them in ice-cold buffered sucrose solution, filtering through muslin and centrifuging. Samples of the chloroplast suspension are mixed with DCPIP, a blue dye that becomes colourless when it is reduced. Four tubes are set up and left for 10 minutes: A, chloroplasts and DCPIP in bright light; B, chloroplasts and DCPIP wrapped in foil; C, boiled chloroplasts and DCPIP in bright light; D, buffer solution and DCPIP in bright light. Only tube A turned colourless.

   (a)

   Explain why the chloroplasts were isolated in ice-cold, buffered sucrose solution and why DCPIP was used.

   [6 marks]

   (b)

   Explain the results in tubes A, B, C and D and state what the investigation shows about the light-dependent reactions.

   [6 marks]

   Total for question 4: 12 marks

End of questions