C3, C4, CAM Plants

C3 PLANTS:
In C3 plants, it's initial fixation of carbon occurs in the rubisco(the enzyme that catalyzes the first step of carbon fixation), because the first organic product of carbon fixation is a three-carbon compound, 3- phosphoglycerate. C3 plants thrive in cool, wet, and cloudy climates, where light levels may be low, because the metabolic pathway is more energy efficient. If there seems to be a good amount of water in the area, the stomata can stay open and let in more carbon dioxide. C3 plants produce less food when their stomata closes on hot,dry days. When O2 concentrations are higher than CO2 concentrations photorespiration occurs and thus generates no ATP, and produces no food for the organism. Examples of C3 plants include rice, wheat, and soybeans.

C4 PLANTS:
Within C4 plants, they use the Calvin cycle with an alternate mode of carbon fixation that forms a four-carbon compound as its first product. In C4 plants, there are two types of photosynthetic cells: bundle-sheath cells and mesophyll cells. In the process, the enzyme PEP carboxylase adds CO2 to PEP. PEP carboxylase has a very high affinity for CO2, and can fix CO2 efficiently when rubisco cannot. When temperatures are hot and dry and the stomata are partially closed, this causes C02 concentration in the leaf to fall and the O2 concentration to rise. In this way, C4 photosynthesis minimizes photorespiration and enhances sugar production. Examples of these plants include sugarcane, corn, and members of the grass family.

CAM PLANTS:
CAM Plants open their stomata during the night and close them during the day, basically the reverse of what other plants normally do. Closing the stomata during the day helps desert plants conserve water, but it also prevents CO2 from entering the leaves. When it's night the stomata of the plant are open and they take up CO2 and make them into a variety of organic acids. This type of carbon fixation is called CAM. The CAM plants store the acids that they make during the night into their vacuoles until the morning when the stomata closes. Then during the day when light reactions can provide ATP and NADPH for the Calvin cycle, CO2 is released from the organic acids that were made during night and made into sugars. Examples of CAM plants include many cacti, and pineapples.

Sources: Campbell Book.
Picture from: Campbell online book.