The Calvin Cycle: A Vital Process for Photosynthesis and Carbon Conversion in Plants and Bacteria

Calvin cycle

the reactions of photosynthesis that use the energy stored by the light-dependent reactions to form glucose and other carbohydrate molecules

The Calvin cycle is a set of chemical reactions that occur in the stroma of chloroplasts, the organelles responsible for photosynthesis in plants, algae, and some bacteria. It is also known as the dark reaction or the light-independent reaction, as it does not directly require light to occur (although it is indirectly dependent on the products of the light-dependent reactions).

The Calvin cycle involves a series of enzyme-catalyzed reactions that convert carbon dioxide (CO2) and other molecules into glucose, a type of sugar that serves as a source of energy and building material for the organism. The overall reaction can be summarized as:

6 CO2 + 18 ATP + 12 NADPH + 12 H+ → C6H12O6 + 18 ADP + 18 Pi + 12 NADP+

In other words, six molecules of CO2 are combined with 18 molecules of ATP (adenosine triphosphate) and 12 molecules of NADPH (nicotinamide adenine dinucleotide phosphate) to produce one molecule of glucose, as well as 18 molecules of ADP (adenosine diphosphate), 18 molecules of inorganic phosphate (Pi), and 12 molecules of NADP+ (nicotinamide adenine dinucleotide phosphate).

The Calvin cycle is divided into three main stages: carbon fixation, reduction, and regeneration. In the carbon fixation stage, an enzyme called Rubisco (ribulose-1,5-bisphosphate carboxylase/oxygenase) catalyzes the addition of CO2 to a five-carbon sugar called ribulose bisphosphate (RuBP), forming a six-carbon intermediate that immediately splits into two molecules of 3-phosphoglycerate (3-PGA). Each cycle fixes one molecule of CO2, and the cycle must turn six times to produce one molecule of glucose.

In the reduction stage, ATP and NADPH are used to convert 3-PGA to glyceraldehyde-3-phosphate (G3P), a three-carbon sugar that can be used to form glucose or other organic molecules. Some of the G3P molecules are exported to the cytosol of the cell to be used for energy production, while others are used to regenerate RuBP in the next stage of the cycle.

In the regeneration stage, some of the G3P molecules are converted back into RuBP, using ATP and additional reactions, in order to restart the cycle and continue fixing CO2. This stage also involves the rearrangement and recycling of other molecules that were used in the earlier stages.

Overall, the Calvin cycle is a complex and highly regulated process that plays a crucial role in sustaining life on Earth by converting inorganic carbon (CO2) into organic carbon (glucose) that can be used by organisms as a source of energy and nutrients.

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