Calvin Cycle

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Reduction: In the second stage of the Calvin cycle, 3-PGA molecules are converted into glyceraldehyde-3-phosphate (G3P), a sugar molecule with three carbon atoms, using ATP and NADPH molecules. The name of this step comes from the fact that NADPH forms G3P by donating electrons to a three-carbon intermediate.

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Regeneration: While certain G3P molecules may renew the RuBP acceptor by forming glucose, other G3P molecules must be regenerated. Regeneration necessitates ATP and entails a convoluted chain of processes known as the "carbohydrate scramble."

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Carbon Fixation (RuBisCO Reaction): The Calvin cycle starts with the enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO), catalyzing the fixation of carbon dioxide (CO2) onto a five-carbon compound, ribulose-1,5-bisphosphate (RuBP). This reaction produces two molecules of 3-phosphoglycerate (3-PGA), a three-carbon compound.

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Reduction Phase: In this phase, ATP (adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate) generated during the light-dependent reactions of photosynthesis provide the energy and reducing power needed to convert 3-PGA molecules into another three-carbon compound, glyceraldehyde-3-phosphate (G3P). This conversion occurs through a series of enzyme-catalyzed reactions, consuming ATP and NADPH.

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Regeneration of RuBP: Some of the G3P molecules produced in the reduction phase are used to regenerate RuBP through additional enzyme-catalyzed reactions. These reactions ensure that the Calvin cycle can continue, as RuBP is essential for the initial carbon fixation step.

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Glucose Formation: From every three molecules of CO2 that enter the Calvin cycle, one molecule of G3P is ultimately converted into glucose or other carbohydrates. This occurs when two G3P molecules combine and undergo further reactions to form glucose and other sugars.

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Release of One Molecule: Out of the six G3P molecules generated in the cycle, five are used for RuBP regeneration, and one is used to produce glucose or other carbohydrates. Therefore, it takes six turns of the Calvin cycle to produce one glucose molecule.