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Light-independent reaction

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The Calvin cycle

In photosynthesis, a light-independent reaction takes place in plant chloroplasts. In this process, sugars are made from carbon dioxide.

The process, known as the Calvin cycle, uses products of the light-dependent reactions (ATP and NADPH) and various enzymes. Therefore, the light-independent reaction cannot happen without the light-dependent reaction.

Sugars made in the light-independent reactions are moved around the plant (translocation). This takes place in the grana region of the chloroplast. It is anaerobic (does not use oxygen).

The Calvin cycle

  1. A sugar (Ribulose biphosphate or RuBP) made of 5 carbon atoms combines with carbon dioxide to form a 6-carbon sugar (phosphoglycerate). An enzyme called RuBisCO speeds this reaction up.
  2. Phosphoglycerate is reduced with hydrogen atoms from the light-dependent reaction to form two molecules of triose phosphate (each has 3 carbon atoms). ATP is needed for this to occur.
  3. Some triose phosphate is converted (using ATP) back into Ribulose Biphosphate (which is why this is called a cycle).
  4. The rest left over is used to produce glucose.

The sum of reactions in the Calvin cycle is the following:

3 CO2 + 6 NADPH + 5 H2O + 9 ATPglyceraldehyde-3-phosphate (G3P) + 2 H+ + 6 NADP+ + 9 ADP + 8 Pi   (Pi = inorganic phosphate)

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