C4fotosyntese

C4 fotosyntese is a metabolic pathway that occurs in certain plants as an adaptation to hot and dry environments. It is a variation of the more common C3 photosynthetic pathway. The primary difference lies in the initial carbon fixation step. In C4 plants, carbon dioxide is first fixed by the enzyme PEP carboxylase in the mesophyll cells, forming a four-carbon compound, typically oxaloacetate. This compound is then converted to malate or aspartate and transported to specialized bundle sheath cells. Within the bundle sheath cells, the four-carbon compound is decarboxylated, releasing carbon dioxide. This concentrated CO2 is then refixed by the enzyme RuBisCO, which is primarily located in these bundle sheath cells, and enters the Calvin cycle.

This spatial separation of initial carbon fixation and the Calvin cycle allows C4 plants to maintain a high concentration of CO2 around RuBisCO, even when stomata are partially closed to conserve water. This mechanism significantly reduces photorespiration, a wasteful process that occurs when RuBisCO binds to oxygen instead of carbon dioxide, particularly at high temperatures. Consequently, C4 plants are more efficient in hot and arid conditions, often outcompeting C3 plants in such environments. Examples of C4 plants include corn (maize), sugarcane, sorghum, and many tropical grasses. The C4 pathway represents a significant evolutionary advantage for survival in challenging climates.