pimeäreaktiot

Pimeäreaktiot, also known as the Calvin cycle or light-independent reactions, are a series of biochemical reactions that occur during photosynthesis. They do not directly require light energy, but instead use the energy carriers ATP and NADPH produced during the light-dependent reactions to convert carbon dioxide into glucose. This process takes place in the stroma of chloroplasts. The Calvin cycle can be divided into three main stages: carbon fixation, reduction, and regeneration.

In the carbon fixation stage, carbon dioxide from the atmosphere is incorporated into an organic molecule. This is catalyzed by the enzyme RuBisCO, which attaches CO2 to a five-carbon sugar called ribulose-1,5-bisphosphate (RuBP). This forms an unstable six-carbon compound that immediately splits into two molecules of a three-carbon compound, 3-phosphoglycerate (3-PGA).

The reduction stage involves using the ATP and NADPH generated in the light-dependent reactions to convert 3-PGA into a higher-energy three-carbon sugar, glyceraldehyde-3-phosphate (G3P). For every three molecules of CO2 fixed, six molecules of G3P are produced.

Finally, in the regeneration stage, some of the G3P molecules are used to synthesize glucose and other organic compounds. The remaining G3P molecules are recycled, using more ATP, to regenerate the initial RuBP acceptor molecule, allowing the cycle to continue. This continuous regeneration of RuBP is crucial for the sustained fixation of carbon dioxide.