Question

$C_4$ plants are more efficient in photosynthesis than $C_3$ plants due to

• A. higher leaf area
• B. presence of larger number of chloroplasts in the leaf cells
• C. presence of thin cuticle
• D. lower rate of photorespiration

Answer 1

The correct option is D lower rate of photorespiration

RuBisCO has both carboxylase and oxygenase activities, and hence in $C_3$ plants, some of the RuBP undergoes photorespiration instead of entering the Calvin cycle. On the other hand, in $C_4$ plants, the primary carbon acceptor is phosphoenolpyruvate (PEP) and not RuBP. The PEP, on carboxylation, is converted into oxaloacetate inside the mesophyll cells. This oxaloacetate is then converted to malate which is transported into the bundle sheath cells. Here, malate undergoes decarboxylation to release $C{O}_2$. The walls of the bundle sheath cells are thick, which prevents the escape of $C{O}_2$. Thus, $C{O}_2$ accumulates in these cells, and makes all the RuBP present there enter the Calvin cycle and not photorespiration. This makes $C_4$ plants more efficient in photosynthesis.