The Calvin cycle: Light independent reaction for A-Level Biology
The Calvin cycle is the light-independent reaction of photosynthesis. It takes place in the stroma of the chloroplast and uses CO2, ATP, and reduced NADP from the light-dependent reaction to make glucose precursors. The three stages are fixation (CO2 combines with RuBP), reduction (GP is reduced to TP), and regeneration (RuBP is remade so the cycle continues).
This guide explains each stage in turn, covers the role of the enzyme RuBisCO, shows what happens to the triose phosphate (TP) produced, and walks through the type of 6-mark question AQA loves to set. By the end you will be ready for any question on Topic 3.5.1 (Photosynthesis) of the AQA A-Level Biology specification.
Three stages, one cycle
Fixation, reduction, regeneration. Six turns produce one molecule of glucose. Name all three stages correctly to pick up the marks.
RuBisCO is the key enzyme
RuBisCO catalyses the first step, joining CO2 to RuBP. It is the most abundant enzyme on Earth and the rate-limiting step of the cycle.
Powered by the light reaction
The Calvin cycle needs ATP and reduced NADP from the thylakoid reactions. Without them the cycle stops within seconds.
Where the Calvin cycle fits in photosynthesis
Photosynthesis has two stages. The light-dependent reaction happens in the thylakoid membranes and uses light energy to split water, releasing oxygen, generating ATP, and reducing NADP. The light-independent reaction (the Calvin cycle) happens in the stroma and uses the ATP and reduced NADP to fix CO2 into organic molecules.
The Calvin cycle is called light-independent because it does not directly need light to run, but it stops quickly without it because the ATP and reduced NADP supply runs out. This is a classic AQA exam trick: A graph shows photosynthesis dropping to zero when light is removed and asks you to explain why the Calvin cycle slows even though it does not use light directly.
The exam wording examiners want Describe the Calvin cycle as the "light-independent reaction". Do not call it the "dark reaction" (the old name). It runs in daylight using the products of the light-dependent reaction; it just does not use light energy directly.
Stage 1: Fixation of CO2
Carbon dioxide diffuses into the stroma and combines with a 5-carbon molecule called ribulose bisphosphate (RuBP). The reaction is catalysed by the enzyme RuBisCO (ribulose bisphosphate carboxylase/oxygenase). The unstable 6-carbon intermediate immediately splits into two molecules of a 3-carbon compound called glycerate-3-phosphate (GP).
This is the carbon-fixing step: Inorganic CO2 has now been joined to an organic molecule. AQA mark schemes want "CO2 combines with RuBP", "catalysed by RuBisCO", and "forming two molecules of GP". Three phrases, three marks.
Stage 2: Reduction of GP to TP
GP is reduced to triose phosphate (TP) using energy from ATP and hydrogen from reduced NADP, both supplied by the light-dependent reaction. TP is a 3-carbon sugar phosphate and the first product of the Calvin cycle that can be used to build larger carbohydrates.
Reduced NADP becomes NADP and gets recycled back to the light-dependent reaction. ATP becomes ADP and inorganic phosphate, also recycled. This coupling is what makes the two halves of photosynthesis interdependent.
Stage 3: Regeneration of RuBP
Of every six TP molecules made, five are used to regenerate three molecules of RuBP using more ATP. The sixth TP molecule leaves the cycle and is used to make hexose sugars (like glucose), starch, amino acids, or lipids.
The Calvin cycle has to turn six times to make one glucose molecule. Each turn fixes one CO2 and produces two TP, so six turns make 12 TP in total. Ten of those 12 are recycled to regenerate the six RuBP, and the remaining two leave the cycle and join up to form one glucose (each TP has 3 carbons, so two together make a 6-carbon hexose).
| Stage | What happens | Inputs and outputs |
|---|---|---|
| Fixation | CO2 combines with RuBP, catalysed by RuBisCO, forming two molecules of GP | Inputs: CO2, RuBP. Output: 2x GP |
| Reduction | GP is reduced to TP using ATP and reduced NADP | Inputs: GP, ATP, reduced NADP. Outputs: TP, ADP, Pi, NADP |
| Regeneration | Five out of every six TP are used to regenerate RuBP, using ATP | Inputs: 5x TP, ATP. Outputs: RuBP, ADP |
Counting the ATP and NADPH used
To produce one molecule of glucose, the Calvin cycle must turn six times. Each turn fixes one CO2 and uses three ATP and two reduced NADP. Over six turns that is 18 ATP and 12 reduced NADP.
AQA loves to ask candidates to calculate these numbers. A typical question gives you the number of CO2 molecules used and asks for the number of turns, ATP, or reduced NADP. Memorise the ratios and the maths becomes mechanical.
The numbers worth memorising Per turn: One CO2 fixed, three ATP used, two reduced NADP used, two TP made. Per glucose: Six CO2 fixed, 18 ATP used, 12 reduced NADP used, 12 TP made (two leave the cycle to form glucose, ten regenerate RuBP).
What happens to triose phosphate (TP)
TP is the central building block. Two TP molecules combine to form a hexose sugar like glucose or fructose. TP can also be polymerised into starch for storage in the chloroplast, or transported as sucrose around the plant.
TP can also be converted into glycerol (for lipids) and combined with nitrogen-containing groups to make amino acids. This is why photosynthesis indirectly produces every major class of organic molecule in a plant, not just sugars.
Why RuBisCO matters RuBisCO is the most abundant enzyme on Earth and the rate-limiting step of photosynthesis. It also has an oxygenase activity, meaning it can fix O2 instead of CO2 (a wasteful process called photorespiration). This is why crops like rice and wheat are less efficient at photosynthesis than they could be, and why engineering a better RuBisCO is a major research goal.
Limiting factors and the Calvin cycle
If you suddenly cut off the CO2 supply to a plant, GP levels fall and RuBP levels rise (TP also falls). This is because no new GP can be made without CO2 to combine with RuBP, so existing GP continues to be reduced to TP and used up, while RuBP keeps being regenerated but is no longer being consumed in fixation.
If you suddenly cut off the light, GP levels rise (because no ATP or reduced NADP is available to reduce it) and TP and RuBP levels fall. AQA loves these change-the-variable graph questions, so practise reasoning through them carefully.
| Variable removed | GP level | TP level | RuBP level |
|---|---|---|---|
| CO2 removed | Falls (no new GP made) | Falls (less GP to reduce) | Rises (TP keeps regenerating RuBP) |
| Light removed | Rises (no ATP or NADPH to reduce it) | Falls (less reduction happening) | Falls (less TP to regenerate RuBP) |
| Temperature drops sharply | Rises slightly (enzymes slow but GP still made for a moment) | Falls (enzymes for reduction slow) | Falls (enzymes for regeneration slow) |
Worked example: A 6-mark Calvin cycle question
Question: "Describe how the Calvin cycle produces a molecule of triose phosphate." (6 marks)
Model answer points: CO2 combines with RuBP, a 5-carbon molecule, in a reaction catalysed by RuBisCO (1). This produces an unstable 6-carbon intermediate which splits into two molecules of GP (1). GP is reduced to TP (1). The reduction uses ATP from the light-dependent reaction (1). The reduction also uses hydrogen from reduced NADP (1). Five out of six TP molecules are used to regenerate RuBP so the cycle can continue, with the sixth TP being available for synthesis of glucose (1).
Six clear, separately credited points equals six marks. Always tag the source of the ATP and reduced NADP (the light-dependent reaction).
Common mistakes that lose marks Calling the Calvin cycle the "dark reaction". Forgetting that GP is a 3-carbon compound and the unstable intermediate is 6-carbon. Saying TP becomes glucose directly (it takes two TP molecules to make one hexose). Writing "reduced NADP" as "NADPH" without explanation (AQA accepts both but mark schemes prefer reduced NADP). Missing the regeneration stage out altogether.
Key facts to memorise for the exam
- The Calvin cycle is the light-independent reaction of photosynthesis and takes place in the stroma of the chloroplast
- Stage 1 (fixation): CO2 combines with RuBP, catalysed by RuBisCO, producing two GP molecules
- Stage 2 (reduction): GP is reduced to TP using ATP and reduced NADP from the light-dependent reaction
- Stage 3 (regeneration): Five out of every six TP are used to regenerate RuBP using ATP
- Per turn: One CO2 fixed, three ATP and two reduced NADP used, two TP made
- Per glucose: Six turns, 18 ATP and 12 reduced NADP used, 12 TP made (two leave to form glucose, ten regenerate RuBP)
- TP can be made into glucose, starch, sucrose, amino acids, or lipids
- Removing CO2 makes GP fall and RuBP rise; removing light makes GP rise and RuBP fall
