Question

Loss of water in $C_4$ plants as compared to $C_3$ plants for the same amount of $CO_2$ fixed is
A. Double
B. One third
C. One fourth
D. Half.

Answer 1

Photorespiration begins in the chloroplast, when rubisco attaches $O_2$ to RuBP in its oxygenase reaction. The loss of water in $C_4$ ​plants is much lesser than the loss of water in $C_3$ ​plants.

Complete step by step answer:
To answer this question, first, we need to know about the process of photorespiration. Photorespiration is a wasteful pathway that occurs when the Calvin cycle enzyme rubisco acts on oxygen rather than carbon dioxide. It utilizes fixed carbon, wastes energy, and likely occurs when plants block their stomata (leaf pores) to decrease water loss. High temperatures make it even worse. The most of plants are $C_3$ plants that have no specific features to conflict photorespiration. $C_4$ plants lower the photorespiration by isolating starting of $CO_2$ fixation and the Calvin cycle in the region, processing these steps in other cell types.
Now, let us find the solution from the option.
The loss of water in $C_4$​plants is reduced as they do not require to keep their stomata open for a long time to take in carbon dioxide due to the efficient $C_4$​pathway and hence, reduce the loss of water via transpiration by one third.
The loss of water in $C_4$​plants is one third that of $C_3$​plants.

Thus, the correct option is B.i.e. One third.

Note: $C_3$, $C_4$ and CAM plants all utilize the Calvin cycle to produce sugars from $CO_2$. These processing paths for fixing $CO_2$ have different benefits and drawbacks and produce plants favorable for different environments. The $C_3$ mechanism functions properly in cold environments, whilst $C_4$ and CAM plants are adapted to dry, hot environments. Both the $C_4$ and CAM pathways have developed separately over two dozen times, which means they may provide plant species in hot climates which is a unique evolutionary benefit.