The Calvin cycle: carbon fixation, triose phosphate and RuBP

Molecules (Interaction and interdependence) · Photosynthesis · note 8 of 9

Spec C1.3.15, C1.3.16, C1.3.17

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The Calvin cycle: carbon fixation, triose phosphate and RuBPSpec C1.3.15, C1.3.16, C1.3.17

In short

In the Calvin cycle in the stroma, the enzyme Rubisco fixes carbon dioxide by combining it with RuBP, forming glycerate 3-phosphate (GP). GP is converted to triose phosphate (TP) using reduced NADP and ATP. Five molecules of TP are converted to three molecules of RuBP using ATP, so the cycle continues; to make glucose, five-sixths of TP is recycled.

Carbon fixation by Rubisco

In the stroma, the enzyme Rubisco catalyses the combination of carbon dioxide with RuBP (ribulose bisphosphate, 5C). The product splits immediately into two molecules of glycerate 3-phosphate (GP), a 3C compound. This is carbon fixation: inorganic carbon becomes part of an organic molecule.

RuBP (5C) + CO₂ → 2 glycerate 3-phosphate (2 × 3C)

Rubisco is the most abundant enzyme on Earth. High concentrations of it are needed in the stroma because it works relatively slowly and is not effective in low carbon dioxide concentrations.

Synthesis of triose phosphate

Glycerate 3-phosphate is converted into triose phosphate (TP) using reduced NADP and ATP from the light-dependent reactions. Reduced NADP supplies the hydrogen (GP is reduced) and ATP supplies energy.

Regeneration of RuBP

Most triose phosphate is used to regenerate RuBP, using more ATP. You do not need the individual reactions, but five molecules of triose phosphate are converted to three molecules of RuBP (5 × 3C = 15C = 3 × 5C), allowing the Calvin cycle to continue.

Why five-sixths of the triose phosphate is recycled

Show that if glucose is the product, five-sixths of the triose phosphate must be converted back to RuBP.

One glucose (6C) needs six carbon atoms, so 6 CO₂ must be fixed by 6 RuBP.
6 RuBP + 6 CO₂ → 12 GP → 12 TP (36 carbon atoms).
To regenerate the 6 RuBP (30 carbon atoms), 10 TP are needed (10 × 3C = 30C).
That leaves 2 TP (6C) to make one glucose.
10 of 12 TP are recycled: 10 ÷ 12 = 5/6.

Answer: Five-sixths of the TP is used to regenerate RuBP and one-sixth forms glucose.

The Calvin cycle in the stroma: CO₂ combines with RuBP (5C), catalysed by Rubisco, to form 2 × glycerate 3-phosphate (GP, 3C); GP is reduced to triose phosphate (TP, 3C) using ATP and reduced NADP; one-sixth of TP leaves to form glucose and other carbon compounds; five TP are converted to three RuBP using ATP. — The Calvin cycle: carbon fixation by Rubisco, reduction of GP to TP, and regeneration of RuBP.

Exam tip:

Use the IB names: RuBP, glycerate 3-phosphate (GP) and triose phosphate (TP). The intermediate between RuBP and GP is not required.

Written and checked against the IB Biology HL specification · Updated October 2026

Frequently asked questions

Why do plants absorb red and blue light but not green?

Chlorophyll absorbs mainly red and blue light because only photons with energy matching the gaps between its electron energy levels can excite its electrons. Green light does not match well, so most of it is reflected or transmitted, which is why leaves look green. Accessory pigments such as carotenoids absorb some other wavelengths.

What is the difference between an absorption spectrum and an action spectrum?

An absorption spectrum shows the percentage of light a pigment absorbs at each wavelength, while an action spectrum shows the rate of photosynthesis at each wavelength. Both peak in blue and red light. The action spectrum stays above zero in green light because accessory pigments absorb some of it, and the similar shapes show absorbed light drives photosynthesis.

How do you calculate Rf values in chromatography?

Divide the distance moved by the pigment by the distance moved by the solvent front, both measured from the origin line. Rf values lie between 0 and 1 and have no units. Pigments are identified by their colour and by comparing their Rf values with reference values for the same solvent and medium.

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