A-Level Biology October/November 2024 Q7(c): The light-dependent stage of photosynthesis produces ATP and reduced NADP, which are us…

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  The light-independent stage, also known as the Calvin cycle, occurs in the stroma of the chloroplast.

  1. The cycle begins as carbon dioxide combines with a 5-carbon compound, ribulose bisphosphate (RuBP). This process is known as carbon fixation.
  2. This reaction is catalysed by the enzyme rubisco.
  3. This forms an unstable 6-carbon intermediate which immediately breaks down to form two molecules of a 3-carbon compound, glycerate 3-phosphate (GP).
  4. GP is then reduced to triose phosphate (TP) in a two-step reaction. This reduction is carried out by reduced NADP and requires energy supplied by the hydrolysis of ATP, both of which are products of the light-dependent stage.
  5. For every 6 molecules of TP produced, 5 are used to regenerate the starting molecule, RuBP. This regeneration also requires ATP.
  6. The remaining 1 molecule of TP is the net product and is used to synthesise other organic molecules. For example, two TP molecules can combine to form a hexose sugar like glucose, which can then be converted to sucrose, starch or cellulose. TP can also be used to synthesise lipids and amino acids.

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  How the marks are awarded

  • B1 — Stating that carbon dioxide reacts/combines with ribulose bisphosphate / RuBP.
  • B1 — Identifying the enzyme for carbon fixation as rubisco.
  • B1 — Stating that two molecules of glycerate 3-phosphate (GP) are formed.
  • B1 — Describing the reduction of glycerate 3-phosphate (GP) to triose phosphate (TP).
  • B1 — Stating that this reduction uses both reduced NADP and ATP.
  • B1 — Explaining that triose phosphate (TP) is used to regenerate RuBP.
  • B1 — Stating that some TP is used to form other organic compounds, with a valid example such as hexoses, starch, or lipids.

  Common mistakes

  • Confusing the order of the cycle, for example stating that GP is converted to RuBP or that TP is fixed with CO2.
  • Incorrectly stating the roles of ATP and reduced NADP, such as saying they are used for carbon fixation or only mentioning one of them for the reduction of GP.
  • Failing to mention the regeneration of RuBP, which is the main fate of the triose phosphate produced and makes the process a cycle.
  • Mixing up the number of carbons in the molecules, for example stating that GP is a 6C compound or that RuBP is a 3C compound.

  Examiner tip: For cyclical processes like the Calvin cycle, drawing a simplified, annotated diagram is the most effective way to memorise the sequence of intermediates and the inputs/outputs at each stage.

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