The key differences between C3, C4, and CAM photosynthesis are seen in the way that carbon dioxide is extracted from sunlight. Plants, algae, and many species of bacteria utilize one of these photosynthetic processes in a chemical reaction that creates energy. Whether an organic compound uses C3, C4, or CAM photosynthesis depends largely on the conditions of the organic compound's habitat.
In photosynthesis, plants and other organic compounds use the energy from sunlight to extract nutrients from air and water. Photosynthetic organisms feature a green compound known as chlorophyll that contains the enzymes ATP and NADPH. With the energy absorbed from sunlight, photosynthetic compounds convert these enzymes to ADP and NADP+. The energy from the converted enzymes is used to extract carbon dioxide from air and water, which is then used to produce sugar molecules such as glucose. Through photosynthesis, plants excrete waste molecules including oxygen, which makes the air breathable for animal organisms.
Photosynthetic organisms that undergo C3 photosynthesis begin the process of energy conversion, known as the Calvin cycle, by producing a three-carbon compound called 3-phosphoglyceric acid. This is the reason for the title "C3." C3 photosynthesis is a one-stage process that takes place inside of the chloroplast organelles, which act as storage centers for sunlight energy. The energy is then used to combine ATP and NADPH into ordered sugar molecules. Roughly 85 percent of the plants on earth utilize C3 photosynthesis.
C4 photosynthesis is a two-stage process in which a four-carbon intermediate compound is produced. The photosynthetic process occurs in the chloroplast of a thin-walled mesophyll cell. Once created, the intermediate compound is pumped into a thick-walled bundle sheath cell, where the compound is split into carbon dioxide and a three-carbon compound. The carbon dioxide then undergoes the Calvin cycle, as in C3 photosynthesis. The benefit of C4 photosynthesis is that it produces a higher concentration of carbon, making C4 organisms more adept at surviving in habitats with low light and water.
CAM is an abbreviation of crassulacean acid metabolism. In this type of photosynthesis, organisms absorb sunlight energy during the day, then use the energy to fix carbon dioxide molecules during the night. During the day, the organism's stomata close up to resist dehydration, while the carbon dioxide from the night prior undergoes the Calvin cycle. CAM photosynthesis allows plants to survive in arid climates, and therefore is the type of photosynthesis used by cacti and other desert plants. However, CAM photosynthesis is also observed in non-desert plants including pineapples and epiphyte plants such as orchids.