Photosynthesis Question 12

12. Photosynthesis in $C_4$ plants is relatively less limited by atmospheric $CO_2$ levels because of :

[2005]

(a) Effective pumping of $CO_2$ into bundle sheath cells.

(b) Rubisco in $C_4$ plants has higher affinity for $CO_2$.

(c) Four carbon acids are the primary initial $CO_2$ fixation products.

(d) The primary fixation of $CO_2$ is mediated via PEP carboxylase.

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Answer : d

Hints & Solution

(d) In $C_4$ plants, initial fixation of carbon dioxide occurs in mesophyll cells. The primary acceptor of $CO_2$ is phosphoenol pyruvate or PEP. It combines with carbon dioxide in the presence of PEP carboxylase to form oxaloacetate. $C_4$ plants are more efficient in picking up $CO_2$ even when it is found in low concentration because of the high affinity of PEP case.