The Calvin cycle is a metabolic process that occurs in the chloroplasts of plant cells. Its main function is to create sugar from carbon dioxide for the plant to use as a source of energy.
Plants obtain all of their energy from the Sun. Each plant cell has a small organelle known as a chloroplast that captures energy from the Sun. Inside the chloroplasts, a plant's chlorophyll pigments undergo photosynthesis to convert the energy from the Sun into sugar and carbon dioxide. The carbon dioxide is released into the atmosphere and then recaptured by the plant to undergo the Calvin cycle. Carbon dioxide enters the chloroplasts and combines with a five-carbon sugar known as ribulose 1,5-bisphosphate. The enzyme RuBisCo catalyzes this reaction. Next, the reaction forms a six-carbon intermediate that decays into two molecules of the three-carbon compound 3-phophglyceric acid. Next, the acid is converted into glyceraldehyde 3-phosphate, which is a precursor to glucose. The glyceraldehyde 3-phosphate is converted back to ribulose 1,5-bisphosphate to complete the cycle. In total, for every three molecules of carbon dioxide that enter the Calvin cycle, one molecule of glyceraldehyde 3-phosphate is formed. This molecule can then be converted into glucose for the plant to use for energy.