The Calvin cycle is a process in which stored energy is utilized to create usable compounds in a photosynthetic organism. This chemical reaction is sometimes known as the Calvin-Bensom-Bassham cycle, after Melvin Calvin, James Bassham, and Andrew Benson, the University of California researchers who first described the process. The chemistry behind this cycle is extremely complex, but the basics can be understood by laypeople.
Photosynthetic organisms use sunlight to create energy which is stored in the form of several chemical compounds. For the organism, this is just the first step, as the energy does not do anything for the organism once it has been stored. In the Calvin cycle, the energy is used to transform carbon dioxide into a sugar which can be used by the plant, in a process which is sometimes referred to as carbon fixation.
This process takes place inside the stroma, a network of connective tissue in the chloroplasts of the organism. Chloroplasts are specialized organelles found in the cells of photosynthetic organisms. For the Calvin cycle to occur, certain enzymes must be triggered through exposure to sunlight. These enzymes bind to the carbon in carbon dioxide, setting off a chain reaction which results in the creation of a sugar.
Some people refer to the Calvin cycle as a dark reaction or light-independent reaction, referencing the fact that it can take place whether or not sunlight is present. However, sunlight still plays a critical role, because the enzymes responsible for the Calvin cycle will not be active unless they have been stimulated by sunlight. While this cycle can take place in the dead of night, the organism still needs regular sun exposure to store energy and activate the enzymes so that the Calvin cycle will keep going.
For the organism, the Calvin Process is critical, because it provides compounds which are necessary to the organism's survival. The Calvin Process is of interest to humans both because of general curiosity about how the world works, and because it explains how plants use carbon. Plants are famous for intaking carbon dioxide and releasing oxygen, a trait which has a tremendous impact on the world's climate. Carbon fixation can help scrub carbon dioxide from the air while creating a supply of usable oxygen, and some research has suggested that the Earth's atmosphere was formed through this process, as the result of an explosion of photosynthetic organisms which created enough oxygen for other forms of life to appear.