Biology · Introductory biology · Concept

Photosynthesis and the Calvin cycle

Photosynthesis turns light energy into the chemical energy of sugar. In the thylakoid membranes of the chloroplast, the light-dependent reactions split water, release oxygen and make ATP and NADPH. In the stroma around them, the Calvin cycle spends that ATP and NADPH to fix carbon dioxide: the enzyme rubisco joins CO₂ to RuBP, the product is reduced to the three-carbon sugar G3P, and most of the G3P is rebuilt into RuBP so the cycle can continue.

The overall reaction

Overall, photosynthesis reverses respiration’s equation, with light as the energy source: carbon dioxide and water become glucose and oxygen. The oxygen released comes from water, not from carbon dioxide.

6 {CO}_{2} + 6 H_{2} O \to C_{6} H_{12} O_{6} + 6 O_{2}

\begin{array}{c} 6 {CO}_{2} + 6 H_{2} O \\ \to C_{6} H_{12} O_{6} + 6 O_{2} \end{array}

The light-dependent reactions

In the thylakoid membranes, chlorophyll in photosystems II and I absorbs light. Electrons taken from water pass along an electron transport chain to NADP⁺, making NADPH; splitting the water releases O₂ and protons, and the proton gradient drives ATP synthase. These reactions make ATP and NADPH but no sugar.

The Calvin cycle

In the stroma, the Calvin cycle runs in three stages. Fixation: rubisco joins CO₂ to the five-carbon sugar RuBP, and the six-carbon product at once splits into two molecules of 3-PGA. Reduction: ATP and NADPH convert 3-PGA into G3P. Regeneration: more ATP rearranges most of the G3P back into RuBP.

The Calvin cycle: three turns fix three CO₂

Stage	What happens	Uses	Makes
Fixation	Rubisco joins CO₂ to RuBP; each product splits in two	3 CO₂, 3 RuBP	6 3-PGA
Reduction	3-PGA is reduced to G3P	6 ATP, 6 NADPH	6 G3P
Regeneration	Five G3P are rearranged into RuBP	5 G3P, 3 ATP	3 RuBP
Export	One G3P leaves to build sugars	—	1 G3P

Counting carbons

Three turns fix 3 CO₂ onto 3 RuBP, 15 carbons, to make 6 G3P, 18 carbons. Five G3P, 15 carbons, rebuild the 3 RuBP, and one G3P leaves the cycle to make sugars. A glucose is two G3P, so it takes six turns.

3 (1) + 3 (5) = 6 (3)

Why the two stages need each other

The Calvin cycle uses no light directly, but it slows and stops in the dark once ATP and NADPH run out, and the light reactions stall without the ADP and NADP⁺ the cycle returns. Rubisco can also bind O₂ in place of CO₂, which wastes energy in photorespiration, most of all when it is hot and dry.

Common mistakes

Calling the Calvin cycle the dark reactions and expecting it to run all night: it needs the ATP and NADPH the light reactions make, so it slows and stops in the dark.
Saying the oxygen released comes from CO₂: it comes from splitting water.
Thinking each turn of the Calvin cycle makes a sugar: three turns export one G3P, and a glucose takes six turns.
Forgetting the ATP spent to regenerate RuBP: each exported G3P costs 9 ATP, not 6.

Key terms

Photosynthesis: Conversion of light energy into chemical processes that support carbon fixation. Light-dependent reactions supply energy carriers used by subsequent carbon-assimilation pathways.
Calvin cycle: A photosynthetic carbon-fixation pathway using ATP and reducing power to incorporate CO₂ and regenerate its carbon acceptor. It is not simply a nighttime process.
Chloroplast: A photosynthetic organelle in plants and algae. Its internal membranes and surrounding compartments support distinct light-dependent and carbon-fixation processes.
Thylakoid: A flattened membrane sac inside a chloroplast; stacks of them are called grana. Its membrane holds chlorophyll, the photosystems and ATP synthase for the light-dependent reactions, and the fluid around the thylakoids, the stroma, is where the Calvin cycle runs.
Rubisco: The enzyme that fixes carbon in the Calvin cycle by joining CO₂ to the five-carbon sugar RuBP. It can also bind O₂ in place of CO₂, which wastes energy in photorespiration, especially in hot, dry conditions.
Electron transport chain: A series of proteins and carriers in a membrane that pass electrons along, pumping protons across the membrane as they go. In respiration it takes electrons from NADH and FADH₂ and hands them to oxygen; in a chloroplast it carries them from water to NADP⁺.
Reduced electron carrier: A molecule carrying reducing equivalents between reactions. Different carriers serve different metabolic roles; their presence does not identify a complete pathway by itself.
Adenosine triphosphate: A nucleotide used in cellular energy coupling. Its hydrolysis can drive an otherwise unfavorable process when reactions are coupled under suitable conditions.
Biological assimilation: Incorporation of acquired material into an organism’s molecules. Carbon fixation specifically incorporates inorganic carbon into organic compounds; subsequent processing can involve additional pathways.

Work through an example

How many CO₂, ATP and NADPH does the Calvin cycle use to make one G3P that leaves the cycle? How many does it use to make one glucose?

Count what the Calvin cycle spends on a sugar →

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Open the Calvin cycle Open worked example on a board Molecular cycles in Biology Reference

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