C4 Plants

In most plants, called C3 plants, the first product of carbon fixation is GP, a 3-carbon compound. RUBISCO has a drawback: at high temperatures and low CO₂ concentrations it increasingly combines with oxygen instead of CO₂. This process, photorespiration, wastes energy and releases fixed carbon. It is a serious problem in hot, bright conditions, where stomata close to save water and the CO₂ concentration in the leaf falls.C4 plants, such as maize and sugar cane, avoid this by fixing CO₂ twice, in two different cell types (Kranz anatomy).1. In the mesophyll cells, CO₂ combines with a 3-carbon compound, PEP, catalysed by PEP carboxylase. This enzyme has a high affinity for CO₂ and does not react with oxygen. The product is oxaloacetate, a 4-carbon compound, which gives C4 plants their name.
2. Oxaloacetate is converted to malate, which passes through plasmodesmata into the bundle sheath cells around the vascular bundles.
3. In the bundle sheath cells, malate releases CO₂, keeping its concentration around RUBISCO very high. The Calvin cycle then runs as normal, with little photorespiration.
4. The remaining 3-carbon pyruvate returns to the mesophyll and is converted back to PEP, using ATP.This extra ATP is a cost, so C4 plants have the advantage only in hot, bright conditions, where C3 plants would lose much fixed carbon to photorespiration. In cool climates, such as the UK, most plants are C3.