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What is the Calvin cycle?

Class 11 BiologyNCERT chapter: Photosynthesis in Higher Plantsइसे हिंदी में पढ़ें

Exam-ready answer

The Calvin cycle is the biosynthetic phase of photosynthesis that takes place in the stroma of chloroplasts. It uses ATP and NADPH from the light reaction to fix CO₂ into carbohydrate. It has three stages: carboxylation of RuBP by RuBisCO, reduction of 3-PGA to triose sugar, and regeneration of RuBP.

Explanation

Melvin Calvin and his co-workers traced the path of carbon using radioactive ¹⁴C in algae. They found that the first stable product of CO₂ fixation is 3-phosphoglyceric acid (3-PGA), a 3-carbon compound, so this pathway is called the C₃ pathway. The cycle runs in the stroma and occurs in all photosynthetic plants, whether they follow the C₃ or the C₄ pathway.

In carboxylation, CO₂ combines with ribulose-1,5-bisphosphate (RuBP), a 5-carbon acceptor, in a reaction catalysed by the enzyme RuBisCO, forming two molecules of 3-PGA. In reduction, 3-PGA is converted into triose phosphate, using 2 ATP and 2 NADPH for each CO₂ fixed. This sugar leads on to the formation of glucose.

In regeneration, the remaining triose molecules are rearranged to form RuBP again, using 1 ATP per CO₂, so the cycle can continue. In total, fixing one CO₂ needs 3 ATP and 2 NADPH. Making one glucose needs six turns of the cycle, that is 6 CO₂, 18 ATP and 12 NADPH. This is why the light reaction must keep supplying ATP and NADPH.

Points to remember

  1. Calvin cycle: the biosynthetic phase of photosynthesis; site: stroma of the chloroplast.
  2. First stable product: 3-PGA (3-carbon), hence the C₃ pathway; CO₂ acceptor: RuBP (5-carbon).
  3. Enzyme of carboxylation: RuBisCO (RuBP carboxylase-oxygenase).
  4. Three stages: carboxylation, reduction and regeneration.
  5. Per CO₂ fixed: 3 ATP + 2 NADPH; per glucose: 6 turns, 18 ATP + 12 NADPH.

Exam tip

Draw the cycle as a circle with the three stages labelled and show the ATP and NADPH used at each stage; state 18 ATP and 12 NADPH per glucose.

Questions students also ask

Why is the Calvin cycle called the C₃ cycle?
Because the first stable product formed after CO₂ fixation is 3-phosphoglyceric acid, which has three carbon atoms. Melvin Calvin identified it with the help of radioactive carbon. Plants in which this is the first product are called C₃ plants.
Is the Calvin cycle directly dependent on light?
It does not use light directly, but it depends on the ATP and NADPH produced by the light reaction. When light is not available these supplies soon run out and the cycle stops. That is why calling it the 'dark reaction' is misleading.
What is RuBisCO?
RuBisCO stands for RuBP carboxylase-oxygenase. It is the enzyme that combines CO₂ with RuBP in the carboxylation step. It is the most abundant enzyme in the world, and it can also bind O₂, which leads to photorespiration in C₃ plants.

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