Calvin cycle, animation. The Calvin cycle is the sequence of light-independent reactions that take place during photosynthesis. Photosynthesis is the process plants use to synthesise carbohydrates from carbon dioxide and water using sunlight, it occurs in chloroplasts (top left). It products can also be used to synthesis other biological molecules. The three stages of the Calvin cycle are shown. During the first stage, carbon fixation, carbon dioxide (CO2) is added to the five-carbon sugar ribulose bisphosphate to form a transient six-carbon sugar, which splits into two molecules of the three-carbon sugar 3-phosphoglycerate with the addition of water (H2O). This sugar can be used to synthesis amino acids, the building blocks of proteins, and fatty acids. During the second stage, reduction, 3-phosphoglycerate is converted to phosphoglyceraldehyde. Two molecules of this can combine to form one molecule of glucose, which can go on to form starch. It can also synthesis glycerol, which can combine with the fatty acids produced in the first stage to produce fats. In stage three, regeneration, some of the phosphoglyceraldehyde is converted back to ribulose bisphosphate, which can be used to start the cycle again.
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