Photosynthesis Question 52
52. In Hatch and Slack pathway, the primary $CO 2$ acceptor is
[2019]
(a) Rubisco
(b) Oxaloacetic acid
(c) Phosphoglyceric acid
(d) Phosphoenol pyruvate
Show Answer
Answer : d
Hints & Solution
(d) Hatch and Slack pathway is a cyclic pathway for $CO_2$ fixation. The primary $CO_2$ acceptor is a 3-carbon compound phosphoenol pyruvate (PEP) which is present in mesophyll cells. PEP is converted to oxaloacetic acid (OAA), which is then further converted into a 4-carbon compound such as malic acid or aspartic acid, which is then transported to the bundle sheath cells. In the bundle sheath cells, it is again broken down into a 3-carbon compound with the release of $CO_2$. The $CO_2$ released enters the Calvin cycle in the bundle sheath cells, while the 3-carbon compound is transported back to the mesophyll cells. In the mesophyll cells, the 3 -carbon compound is converted back to PEP, thus completing the cycle. $C_4$ plants capture carbon dioxide in their mesophyll cells forming oxaloacetate. This oxaloacetate is then converted to malate and is transported into the bundle sheath cells, where oxygen concentration is low to avoid photorespiration. So, $C_4$ plants are more efficient in photosynthesis than $C_3$ plants.
