Question

What are the disadvantages of photorespiration? Explain how photorespiratory losses are overcome in plants like sugarcane.

Answer 1

Hint: Photorespiration is often referred to as the oxidative photosynthetic carbon process. It is a mechanism that requires the removal of solid carbon as carbon dioxide in plants in the presence of light.

Complete Answer:
Photorespiration is a metabolic mechanism that happens in plants in the presence of light, in which rubisco, the enzyme implicated in the fixation of carbon dioxide with ribulose bisphosphate, absorbs oxygen instead of carbon dioxide, leading to the formation of a two-carbon compound, glycolate.

Disadvantages of Photorespiration:
- It decreases the effectiveness of photosynthesis for a few reasons. First of all, oxygen is added to the carbon. In other words, carbon is oxidised, which is the inverse of photosynthesis — the conversion of carbon to glucose.
- Second, it is now essential to re-synthesize ribulose bisphosphate and to minimise phosphoglycolate.

- Sugarcane has developed a method to resolve photo-respiratory losses. Sugarcane is a plant of $C_4$. These plants have quite a unique leaf anatomy called anatomy of Kranz. They have two groups of cells, mesophyll cells and bundle sheath cells, which are necessary for the $C_4$ pathway.
- Especially large cells across the vascular clusters of $C_4$ plants are called bundle sheath cells. These cells can form multiple layers around the vascular bundles, they are distinguished by a large number of dimorphic chloroplasts, a lack of grana, thick walls impervious to gas exchange and no intercellular spaces.
- As a consequence of this morphology, photorespiration does not take place in $C_4$ plants. As a result, these plants demonstrate better productivity and yield relative to $C_3$ plants.

Note: The effect of photorespiration in $C_4$ plants is insignificant as the affinity of phosphoenolpyruvate carboxylase to carbon dioxide is incredibly high. Photorespiration can increase as oxygen concentration raises or as carbon dioxide concentration reduces.