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How do C4 plants reduce the loss of water during photosynthesis?

C4 plants reduce water loss during photosynthesis by using a two-step process that minimises photorespiration.

C4 plants, such as maize and sugarcane, have evolved a unique mechanism to reduce water loss during photosynthesis, particularly in hot and dry conditions. This mechanism is known as C4 photosynthesis, a two-step process that effectively minimises photorespiration, a process that can lead to significant water loss.

In the first step of C4 photosynthesis, carbon dioxide is absorbed in the mesophyll cells, the cells located between the leaf's upper and lower epidermis. Here, the enzyme phosphoenolpyruvate carboxylase combines carbon dioxide with phosphoenolpyruvate (PEP) to form a four-carbon compound, oxaloacetate. This is why the process is referred to as C4 photosynthesis. The formation of this four-carbon compound occurs in the presence of low carbon dioxide concentrations, allowing C4 plants to absorb more carbon dioxide than C3 plants, even under conditions of drought and high temperatures.

In the second step, the four-carbon compound is transported to the bundle-sheath cells, which are tightly packed around the plant's veins. Here, the compound is broken down to release carbon dioxide, which is then used in the Calvin cycle to produce glucose. This process occurs in an environment with high carbon dioxide concentration, which suppresses photorespiration.

Photorespiration is a process that occurs in C3 plants when the enzyme RuBisCO, which is responsible for fixing carbon dioxide in the Calvin cycle, binds with oxygen instead of carbon dioxide. This process not only wastes energy but also causes water loss through the stomata, the tiny pores in the leaf surface. By minimising photorespiration, C4 plants can close their stomata to reduce water loss without significantly affecting their photosynthetic efficiency.

In summary, C4 plants have evolved a unique two-step photosynthetic process that allows them to absorb more carbon dioxide and minimise photorespiration, thereby reducing water loss. This adaptation makes C4 plants particularly efficient in hot and dry environments where water is scarce.

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