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Photosynthesis: Question 5

Syllabus 13.2

Multiple choice A2 1 mark

A plant is photosynthesising steadily in bright light with a plentiful supply of carbon dioxide. The concentration of carbon dioxide around the plant is then suddenly and substantially reduced, while light intensity and temperature are kept constant.

What is the immediate effect on the concentrations of glycerate 3-phosphate (GP) and ribulose bisphosphate (RuBP) in the Calvin cycle?

Choose an answer to check it, then compare with the worked solution below.
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Worked solution

Step 1: Track what happens to RuBP

RuBP is used up when it combines with carbon dioxide (catalysed by rubisco) to form GP, and it is regenerated from triose phosphate using ATP from the light-dependent stage.

Because light intensity and temperature stay the same, the light-dependent stage keeps supplying ATP and reduced NADP at the same rate as before, so RuBP continues to be regenerated at roughly its previous rate. However, with much less carbon dioxide now available, rubisco has far less substrate to react with, so RuBP is used up (fixed) more slowly than before.

Since RuBP is still being made at close to the old rate but is being consumed more slowly, RuBP accumulates, so its concentration increases.

Step 2: Track what happens to GP

GP is formed when carbon dioxide combines with RuBP, and it is used up when it is reduced to triose phosphate using ATP and reduced NADP.

With less carbon dioxide available, less new GP is formed by carbon fixation. But because the light-dependent stage is unaffected, ATP and reduced NADP are still available at the same rate, so existing GP continues to be reduced to triose phosphate at roughly its previous rate.

Since GP is now being formed more slowly than it is being used up, GP concentration decreases.

Step 3: Evaluate each option

  • A: correct. GP falls (formed more slowly than it is consumed) and RuBP rises (regenerated at the same rate but consumed more slowly), matching the reasoning above.
  • B: incorrect. This is the pattern expected if light intensity (not carbon dioxide) were suddenly reduced: GP would then be formed at roughly the same rate but reduced to TP more slowly (due to less ATP/reduced NADP), so GP would rise, and RuBP would then fall as it is used up in carbon fixation faster than it can be regenerated.
  • C: incorrect. RuBP does not decrease here, since its regeneration is unaffected by the fall in carbon dioxide while its use in carbon fixation slows down, causing it to build up rather than fall.
  • D: incorrect. GP does not increase here, since less carbon dioxide means less new GP is being formed, while GP already present continues to be used up as normal.

Final answer

  • A sudden fall in carbon dioxide concentration slows carbon fixation (RuBP + CO₂ → GP) without slowing the light-dependent stage, so RuBP builds up (increases) while GP is used up faster than it forms (decreases), option A.