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How does the Calvin cycle use ATP and reduced NADP to fix carbon dioxide into carbohydrate?

Describe the Calvin cycle (light-independent reactions), including carbon fixation, reduction and the regeneration of the carbon dioxide acceptor

A focused answer to the H2 Biology Energy and Equilibrium outcome on the light-independent reactions. Carbon fixation by rubisco, the reduction of glycerate phosphate to triose phosphate using ATP and reduced NADP, and the regeneration of the carbon dioxide acceptor.

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What this dot point is asking

SEAB wants you to describe the Calvin cycle (the light-independent reactions) in the stroma: carbon fixation by rubisco, the reduction of glycerate 3-phosphate to triose phosphate using ATP and reduced NADP, and the regeneration of the ribulose bisphosphate acceptor. You should also explain how it depends on the light-dependent reactions. This completes the second half of photosynthesis.

The answer

Where and what

The Calvin cycle occurs in the stroma of the chloroplast. It uses the ATP and reduced NADP made in the light-dependent reactions to fix carbon dioxide into carbohydrate. It has three stages.

Stage 1: Carbon fixation

Carbon dioxide combines with the five-carbon acceptor ribulose bisphosphate (RuBP), catalysed by the enzyme rubisco, forming an unstable six-carbon compound that immediately splits into two molecules of the three-carbon glycerate 3-phosphate (GP).

Stage 2: Reduction

GP is reduced to triose phosphate (TP). This uses ATP (for energy) and reduced NADP (for hydrogen and reducing power), both supplied by the light-dependent reactions.

Stage 3: Regeneration

Most of the triose phosphate is used, with more ATP, to regenerate RuBP, so the cycle can continue. A small proportion of the triose phosphate is used to build glucose and other organic molecules.

Dependence on the light stage

The cycle cannot run without the ATP and reduced NADP from the light-dependent reactions. This is why, although the Calvin cycle does not directly use light, it stops soon after the light is removed.

Examples in context

Example 1. Limiting factors. The rate of the Calvin cycle, and so of photosynthesis, can be limited by carbon dioxide concentration, temperature (which affects rubisco) or light intensity (which affects the supply of ATP and reduced NADP). Recognising the limiting factor from a graph is a frequent exam task.

Example 2. The fate of triose phosphate. Only one in six triose phosphate molecules leaves the cycle to make sugars; the rest regenerate RuBP. This balance keeps the cycle turning and explains why so much of the cycle's output is recycled rather than exported as carbohydrate.

Try this

Q1. State where in the chloroplast the Calvin cycle occurs. [1 mark]

  • Cue. In the stroma.

Q2. Name the enzyme that catalyses the fixation of carbon dioxide and the molecule it acts on. [2 marks]

  • Cue. Rubisco catalyses the combination of carbon dioxide with ribulose bisphosphate (RuBP).

Q3. Explain why the Calvin cycle stops shortly after a plant is placed in the dark. [2 marks]

  • Cue. The light-dependent reactions stop supplying ATP and reduced NADP, so GP cannot be reduced to TP and RuBP cannot be regenerated, halting the cycle.

Exam-style practice questions

Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

Original6 marksDescribe the three stages of the Calvin cycle and explain how it depends on the products of the light-dependent reactions.
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Examiners want the three stages and the dependence on ATP and reduced NADP.

The Calvin cycle occurs in the stroma of the chloroplast. In carbon fixation, carbon dioxide combines with the five-carbon acceptor ribulose bisphosphate (RuBP), catalysed by the enzyme rubisco, to form an unstable six-carbon compound that breaks down into two molecules of the three-carbon glycerate 3-phosphate (GP).

In the reduction stage, GP is reduced to triose phosphate (TP). This uses both ATP (to provide energy) and reduced NADP (to provide hydrogen and reducing power) from the light-dependent reactions.

In regeneration, most of the triose phosphate is used to regenerate the RuBP acceptor, using more ATP, so the cycle can continue. A small proportion of the triose phosphate is used to make glucose and other organic molecules.

The cycle depends on the light-dependent reactions for ATP and reduced NADP; without them the reduction of GP and the regeneration of RuBP cannot occur. Markers reward the three stages, the role of rubisco, the use of ATP and reduced NADP in reduction, the regeneration of RuBP, and the dependence on the light stage.

Original3 marksA plant is suddenly deprived of carbon dioxide while remaining in the light. Explain what happens to the levels of ribulose bisphosphate and glycerate 3-phosphate in the Calvin cycle.
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The answer should reason from the blocked fixation step.

Without carbon dioxide, the fixation step cannot occur, so ribulose bisphosphate (RuBP) is no longer converted to glycerate 3-phosphate (GP).

RuBP therefore accumulates, because it is still being regenerated from triose phosphate but is no longer being used up by reaction with carbon dioxide.

GP falls, because it is no longer being produced from RuBP and carbon dioxide, while it continues to be converted to triose phosphate using ATP and reduced NADP.

Markers reward the blocked fixation, the rise in RuBP because it is not consumed, and the fall in GP because it is not being made.

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