Explain Alternate Photosynthetic Pathways?

An alternate photosynthetic pathway is the C4 pathway, where plants incorporate carbon into four-carbon compounds instead of 3PG. This pathway was found when plants were exposed to 14CO₂, a radioactive substance used to trace the steps in biochemical reactions.

In these plants, the CO₂ acceptor is phosphoenolpyruvate, or PEP, for short. The four-carbon compounds formed by the C4 pathway reactions are taken up by another type of cell, that releases CO₂. The CO₂ is taken up by RuBP and used in the Calvin cycle to form sugars used to synthesize and store starches. The reaction catalyzed by the enzyme PEP carboxylase can proceed in very low concentrations of CO₂. This represents an adaptation to dry environments, where plants conserve water by keeping the stomata, openings in the leaves where gas exchange occurs, closed as much as possible.

The CAM, or crassulacean acid metabolism, pathway, occurs in cacti and other desert plants. It is similar to the C4 pathway, except that the PEP carboxylase reaction occurs at night and the Calvin cycle predominates in the daytime. During the daytime, the stomata close to preserve water, and the plant uses CO₂ trapped in the four-carbon compounds for its metabolism.