C4 and CAM Plants

C4 and CAM plants are plants that use certain special compounds to gather carbon dioxide (CO₂) during photosynthesis. Using these compounds allows these plants to extract more CO₂ from a given amount of air, helping them prevent water loss in dry climates.

All photosynthetic plants need carbon to build sugars, and all get their carbon from CO₂ in the air. CO₂ must first be bound, or "fixed," to another molecule inside the plant cell in order to begin its transformation into sugar. In most plants, carbon fixation occurs when CO₂ reacts with a five-carbon compound called RuBP (ribulose 1,5-bisphosphate). The product splits immediately to form a pair of three-carbon compounds, and therefore this pathway is called the C3 pathway. Further reaction leads to the creation of a sugar (glyceraldehyde-3-phosphate) and the regeneration of RuBP. This series of reactions is known as the Calvin-Benson cycle after the two scientists who elucidated it.

The enzyme that catalyzes the joining of RuBP and CO₂ is known as RuBP carboxylase, also called Rubisco. Rubisco is believed to be the most abundant protein in the world. However, Rubisco is not very efficient at grabbing CO₂, and it has an even worse problem. When the concentration of CO₂ in the air inside the leaf falls too low, Rubisco starts grabbing oxygen instead. The ultimate result of this process, called photorespiration, is that sugar is burned up instead of being created. Photorespiration becomes a significant problem for plants during hot, dry days, when they must keep their stomates (leaf pores) closed to prevent water loss.

Diverse groups of plants have evolved different systems for coping with the problem of photorespiration. These plants, called C4 plants and CAM plants, initially bind carbon dioxide using a much more efficient enzyme. This allows a more efficient harvest of CO₂, allowing the plant to trap sufficient CO₂ without opening its stomates too often. Each then uses the CO₂ in the Calvin-Benson cycle.

C4 ("four-carbon") plants initially attach CO₂ to PEP (phosphoenolpyruvate) to form the four-carbon compound OAA (oxaloacetate) using the enzyme PEP carboxylase. This takes place in the loosely packed cells called mesophyll cells. OAA is then pumped to another set of cells, the bundle sheath cells, which surround the leaf vein. There, it releases the CO₂ for use by Rubisco. By concentrating CO₂ in the bundle sheath cells, C4 plants promote the efficient operation of the Calvin-Benson cycle and minimize photorespiration. C4 plants include corn, sugar cane, and many other tropical grasses.

CAM ("crassulacean acid metabolism") plants also initially attach CO₂ to PEP and form OAA. However, instead of fixing carbon during the day and pumping the OAA to other cells, CAM plants fix carbon at night and store the OAA in large vacuoles within the cell. This allows them to have their stomates open in the cool of the evening, avoiding water loss, and to use the CO₂ for the Calvin-Benson cycle during the day, when it can be driven by the sun's energy. CAM plants are more common than C4 plants and include cacti and a wide variety of other succulent plants.

see also Leaves; Photosynthesis; Water Movement in plants

Richard Robinson