The primary function of the Calvin cycle is to convert carbon dioxide in the air into sugar, which plants and algae use as food. Plants depend directly on the Calvin cycle for the energy they need to grow and reproduce. Because plant life is at the bottom of the food chain, the Calvin cycle is responsible for providing all organisms with food and nutrients in some way.
The energy that fuels the Calvin cycle is provided by two chemicals, nicotinamide adenine dinucleotide phosphate and adenosine triphosphate, which is a nucleotide that holds a large amount of chemical energy within its phosphate bonds. Both chemicals contain energy captured from sunlight.
The Calvin cycle has four main phases: carbon fixation or carboxylation, reduction, carbohydrate formation and regeneration. In the carboxylation phase, carbon dioxide is added to a five-carbon sugar-phosphate to create a six-carbon compound that quickly splits into two three-carbon sugar-phosphates. During the reduction phase, the new compound is reduced to glyceraldehyde-3-phosphate, also known as triose phosphate. This chemical is fueled by the energy-providing compounds that drive the Calvin cycle. In the carbohydrate formation phase, the trios phosphates are used to create other carbohydrates, such as sucrose and starch. During the regeneration phase, enzymes convert the remaining triose phosphates into additional five-carbon sugar-phosphates that are used in the carboxylation phase.