Calvin cycle

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Related to Calvin-Benson cycle: photosynthesis
Calvin cycleclick for a larger image
Fig. 86 Calvin cycle . The ‘dark’ reactions of the Calvin cycle.

Calvin cycle

a series of chemical reactions, first described by Melvin CALVIN, which take place in the watery matrix of CHLOROPLASTS, where carbon dioxide is incorporated into more complex molecules and eventually carbohydrate. Energy for the reactions is supplied by ATP with NADPH (see NADP acting as a reducing agent, both having been produced in the light reactions of PHOTOSYNTHESIS. Since light is not required for the Calvin cycle to continue (provided CO2 ,ATP and NADPH are present) the steps are called the ‘dark’ reactions.

Every turn of the cycle fixes one molecule of carbon dioxide by producing two molecules of PGA and then two molecules of PGAL; three turns are necessary to release one molecule of PGAL (C3) for the glucose pathway with the remaining five PGAL molecules remaining within the cycle. Thus six turns produce sufficient quantities of PGAL for the production of one molecule of glucose (C3 + C3 = C6).

Calvin cycle

see dark reaction.