The Calvin cycle is also referred to as C3 metabolism because fixed CO2 first appears in the form of a three-carbon molecule. Carbon dioxide and other gases enter and exit the cell through tiny pores called stomata. Plants are able to regulate the movement of gases into and out of cells by controlling the opening and closing of stomata. The opening and closing of the stomata is a balance between taking up CO2 and losing water. All terrestrial plants use the Calvin cycle, but some plants initially fix carbon using alternate routes.
C4 and CAM metabolism are more derived photosynthetic pathways and, in certain environments, increase the fitness of plants that utilize these pathways. In both C4 and CAM plants, CO2 is initially fixed into four-carbon compounds; eventually these four carbon compounds release CO2to RUBP and the Calvin-cycle enzymes act as usual.
Many plants that are adapted to xeric (dry) environments, such as cacti, use a CAM pathway. In the case of CAM, carbon dioxide enters the open stomata during the night, is fixed, and the stomata are closed during the day when the Calvin-cycle operates.