Pimeäreaktioiden

Pimeäreaktiot, also known as the Calvin cycle or light-independent reactions, are a series of biochemical reactions that occur during photosynthesis. They take place in the stroma of chloroplasts and do not directly require light energy, although they depend on the products of the light-dependent reactions (ATP and NADPH). The primary function of the Calvin cycle is to fix atmospheric carbon dioxide into organic molecules, specifically sugars.

The cycle begins with carbon fixation, where an enzyme called RuBisCO attaches a molecule of carbon dioxide to a five-carbon sugar called ribulose-1,5-bisphosphate (RuBP). This unstable six-carbon compound immediately splits into two molecules of a three-carbon compound, 3-phosphoglycerate (3-PGA). In the next stage, reduction, ATP and NADPH from the light-dependent reactions are used to convert 3-PGA into glyceraldehyde-3-phosphate (G3P), a three-carbon sugar. For every three molecules of CO2 fixed, six molecules of G3P are produced. One of these G3P molecules exits the cycle to be used by the plant for the synthesis of glucose and other organic compounds.

The final stage is regeneration, where the remaining five G3P molecules are rearranged, using more ATP, to reform the initial three molecules of RuBP. This allows the cycle to continue. The Calvin cycle is essential for life on Earth as it produces the carbohydrates that form the base of most food chains and releases oxygen as a byproduct.