In which cells of the leaf, pyruvate is converted to PEP in $C_4$ pathway:

Question

Accepted Answer

The correct option is 2:
In the $C\_4$ pathway, the conversion of pyruvate to phosphoenolpyruvate (PEP) is a crucial step that occurs specifically within the mesophyll cells of the leaf. This reaction is catalyzed by the enzyme pyruvate phosphate dikinase (PPDK), and it requires energy in the form of ATP. The PEP then acts as the primary CO2 acceptor in these mesophyll cells, where it is carboxylated by PEP carboxylase to form a four-carbon compound, oxaloacetate. This spatial separation of initial carbon fixation (in mesophyll cells) and the Calvin cycle (in bundle sheath cells) is a key adaptation of $C\_4$ plants to reduce photorespiration and enhance photosynthetic efficiency in hot, arid environments.

The incorrect option is 1: Epidermal cells
Epidermal cells form the outermost layer of the leaf and primarily function in protection and gas exchange regulation through stomata. They do not contain the necessary enzymes or organelles (like chloroplasts in most cases, or specialized enzymes for the C4 pathway) to carry out the conversion of pyruvate to PEP.

The incorrect option is 3: Bundle sheath cells
While bundle sheath cells are integral to the $C\_4$ pathway, they are where the four-carbon compound (like malate or aspartate) is decarboxylated to release CO2, which then enters the Calvin cycle. The initial conversion of pyruvate to PEP, which regenerates the primary CO2 acceptor, occurs upstream in the mesophyll cells, not in the bundle sheath cells.

The incorrect option is 4: Guard cells
Guard cells are specialized epidermal cells that surround the stomata and regulate their opening and closing, thereby controlling gas exchange and transpiration. They play a role in gas exchange but are not directly involved in the biochemical reactions of the $C\_4$ photosynthetic pathway, including the conversion of pyruvate to PEP.