C4 plants: the Hatch-Slack pathway

C₄ plantshave evolved a special metabolic adaptation which reduces photorespiration. They do not use ribulose biphosphate (RuBP) to fix carbon dioxide directly into the Calvin cycle. Instead, they use phosphoenolpyruvate (PEP) to fix carbon dioxide as a four-carbon compound, oxaloacetate. The reaction is catalysed by phosphoenolpyruvate carboxylase (PEP, carboxylase). This enzyme cannot combine with oxygen. Consequently C₄ plants can continue to fix carbon dioxide even when its concentration is very low.

The leaves of C₄ plants are specially adapted to carry out this initial fixation. A ring of large closely packed cells called the bundle sheath surrounds the leaf veins. Surrounding the bundle sheath is a smaller ring of mesophyll cells. The distinctive arrangement is called Kranz anatomy and can be used to identify C₄ plants (“Kranz” means crown or halo and refers to the two distinctive rings). The initial fixation of carbon dioxide into oxaloacetate takes place in the small ring of mesophyll cells. Then the oxaloacetate is converted to malate, another four-carbon compound. Malate is transported into the bundle sheath cells where it releases carbon dioxide. Once released, the carbon dioxide is reassimilated by RuBP and enters the Calvin cycle in the same way as described for C₃ plants. The metabolic pathway that transports carbon dioxide into the bundle sheath cells is called the Hatch-Slack pathway. As a result of this pathway, the concentration of carbon dioxide in the bundle sheath cells is 20 to 120 times higher than normal.

C₄ plants have two main advantages in hot, dry environments. First, because PEP carboxylase has a high affinity for carbon dioxide and does not combine with oxygen, C₄ plants can continue to photosynthesise even when their stomata are closed for long periods. This reduces water loss and photorespiration. C₄ plants need only about half as much water as C₃ plants for photosynthesis. Secondly, because high carbon dioxide concentrations can be maintained in the bundle sheath cells, C₄ plants can increase their photosynthetic efficiency.

These adaptations enable C₄ plants to outcome C₃ plants in hot and very sunny conditions, but not in temperate conditions. Fewer than 0.5 per cent of plant species are C₄ plants, yet they include economically important crops such as maize, sugar cane, and millet.