Question

In ${C_4}$ plants, calvin cycle occurs in
A. Stroma of bundle sheath chloroplasts
B. mesophyll chloroplasts
C. Grana of bundle sheath chloroplasts
D. Does not occur as $C{O_2}$ is fixed mainly by PEP and no $C{O_2}$ is left for Calvin cycle

Answer 1

Photorespiration occurs when instead of carbon dioxide, enzyme RUBISCO attacks on oxygen. This is a harmful process for plants. To combat this, ${C_4}$ plants perform initial $C{O_2}$ fixation and calvin cycle in different cells namely, mesophyll cells and bundle sheath cells respectively.

Complete answer:
In ${C_4}$​ plants, the Calvin cycle occurs in the stroma of bundle sheath chloroplasts.
In ${C_4}$ plants, mesophyll cells (present in the middle of the leaf as spongy tissue) are the site of light-dependent reactions whereas calvin cycle operates in the stroma of thick-walled bundle-sheath cells (specialized cells present around the veins of leaf).
In ${C_4}$ plants, the atmospheric $C{O_2}$ is fixed in the mesophyll cells with the help of a non-rubisco enzyme namely PEP carboxylase to produce a simple, 4-carbon compound, oxaloacetate. This PEP carboxylase cannot bind with ${O_2}$. Oxaloacetic acid is then transformed to malic acid which can be transported to the bundle-sheath cells. Inside the stroma of bundle sheath cells, breakdown of malic acid takes place releasing $C{O_2}$. This $C{O_2}$ released is fixed by enzyme RUBISCO and the sugars are produced via the calvin cycle. Malic acid produced is returned back to the mesophyll cells.
Other options are incorrect as in ${C_4}$ plants, initial $C{O_2}$ fixation takes place in the mesophyll cells which convert $C{O_2}$ into a 4C stable compound called Oxaloacetate which is then converted to a similar compound called malate. This malate is transferred to the stroma of bundle sheath cells where the Calvin cycle takes place. Also, these bundle sheath cells lack grana in their chloroplast.

The correct answer is option A.

Note: Photosynthesis occurs in two steps: First is the light reaction in which light energy is trapped in the grana of chloroplast for production of ATP and NADPH whereas second step is the dark reaction which occurs in the in the stroma of the chloroplasts. In this ATP and NADPH produced during light reaction are utilized to form glucose.