Question

${C_4}$ plants ultimately use RUBISCO, yet have a lower rate of photorespiration than do ${C_3}$ plants. Explain.

Answer 1

Hint:-Plants use light energy for the production of ATP and NADPH, this is called photophosphorylation. It occurs in the chloroplast of plant cells. ATP and NADPH are produced during photophosphorylation via a specific pathway. These products of photophosphorylation are then used in the formation of food during the dark reaction. The dark reaction operates in different photosynthetic organisms by three different pathways named as ${C_3}$, ${C_4}$ and CAM pathways. Due to the type of dark reaction present in plants, they are of majorly three types named as ${C_3}$, ${C_4}$ and CAM plants.

Complete answer:
In the ${C_3}$ cycle, there occurs a total of three steps- carboxylation, reduction and regeneration. There is an enzyme called RUBISCO, found in chloroplasts of photosynthetic plants. This enzyme binds with carbon dioxide during the carboxylation step of the pathway and the products formed during the pathway ultimately produce glucose and starch, by using ATP and NADPH. During the ${C_4}$ pathway, atmospheric carbon dioxide is fixed by another enzyme called PEPcase. During this cycle, carbon dioxide is released into bundle sheath cells of these plants. This carbon dioxide is then utilised in the ${C_3}$ pathway and ultimately glucose and starch are produced in these plants. Photorespiration is a cyclical pathway in which there is neither synthesis of sugars nor of ATP and NADPH. Rather, it results in the release of carbon dioxide with the utilisation of ATP.
RUBISCO has the affinity to bind with both oxygen and carbon dioxide. This binding is competitive with its active sites. RUBISCO has more affinity to bind with carbon dioxide but when oxygen concentration is more than carbon dioxide, more and more oxygen will bind with it. This increases the process of photorespiration. Photorespiration requires the input of oxygen, thus if the amount of oxygen is increased photorespiration will also get increased. RUBISCO is present in bundle sheath cells in ${C_4}$ plants and also carbon dioxide is being released in bundle sheath cells in these plants. This increases the binding of carbon dioxide with RUBISCO in bundle sheath cells thus, minimising photorespiration as no oxygen is available for performing photorespiration.
Thus ${C_4}$ plants have lower rates of photorespiration.

Note:-
The evolution of ${C_4}$ photosynthetic systems is probably one of the strategies for maximising the availability of carbon dioxide while minimising water loss. ${C_4}$ plants are twice as efficient as ${C_3}$ plants in terms of fixing carbon (making sugars). ${C_4}$ plants lose only half as much water as a ${C_3}$ plant for the same amount of carbon dioxide fixed.