The light independent reactions of photosynthesis, known as the dark reactions, use ATP and NADPH generated during the light reactions to synthesize sugar precursors. In the most common scenario, called the Calvin Cycle, carbon dioxide is incorporated into a stable three-carbon compound called phosphoglycerate (PGAL), which serves as the precursor of glucose and other sugars. Six turns of the Calvin Cycle generate the equivalent of one glucose molecule.
Biologists divide plants into three categories called C3, C4 and CAM, based on the mechanism and location of the dark reactions of photosynthesis. Most plants are designated C3, meaning the first stable compound they generate in the dark reactions is PGAL. This pathway is optimal for plants living in mild environments with abundant moisture and carbon dioxide. Examples of C3 plants include wheat, rice, oats and tomatoes.
In contrast, C4 plants tend to inhabit hotter, drier environments. Examples include corn, sugarcane and sorghum. In order to minimize water loss, C4 plants have a specialized structure called Kranz anatomy, in which photosynthesis takes place in cells arranged as a wreath or bundle sheath layer. C4 plants combine carbon dioxide with phosphoenolpyruvate to form OAA (oxaloacetate). OAA is converted to malate, which diffuses into the bundle sheath cells and releases carbon dioxide, which promptly enters the Calvin Cycle. By concentrating carbon dioxide in the innermost cell layer of the plant, less water is lost due to transpiration.
Finally, certain desert plants such as cacti and agave have developed a form of photosynthesis called CAM, which stands for Crassulacean Acid Metabolism. These plants must conserve water even more efficiently than C4 plants. Rather than keep their stomata (pore cells) open during the day like C3 and C4 plants, CAM plants keep them open at night in order to accumulate carbon dioxide, which they store as malate or aspartate in an intracellular vacuole. During the day, the plant taps these carbon dioxide reserves, channeling CO2 into the Calvin Cycle without losing any water to the environment. This explains why the chloroplasts of cacti are found in the body of the plant rather than in leaves. The CAM cycle also accounts for the ability of xerophytes (desert plants) to survive for months or years without rain.