C₄ pathway takes place in

The correct answer is Mesophyll cells.

Step-by-Step Explanation:

1. Understanding the C₄ Pathway:
The C₄ pathway (also known as the Hatch-Slack pathway) is a photosynthetic process used by certain plants (called C₄ plants, like maize, sugarcane, and sorghum) to minimize photorespiration and optimize carbon dioxide capture under high temperatures and light intensities.

2. Spatial Separation of Reactions:
In C₄ plants, photosynthesis is divided spatially between two distinct types of photosynthetic cells to concentrate CO₂ around the enzyme RuBisCO. These two cell types are:

• Mesophyll cells: Located towards the outer layers of the leaf.
• Bundle sheath cells: Large cells arranged in a wreath-like manner (Kranz anatomy) around the vascular bundles.

3. First Step of C₄ Pathway:
The initial fixation of carbon dioxide occurs in the mesophyll cells. Here, the primary CO₂ acceptor is phosphoenolpyruvate (PEP), a 3-carbon compound. The enzyme responsible for this fixation is PEP carboxylase (PEPcase).
This reaction yields oxaloacetic acid (OAA), a 4-carbon organic acid, which is why the pathway is called the C₄ pathway.

$\mathrm{PEP}+{\mathrm{CO}}_2\to \mathrm{Oxaloacetate\; (OAA)}$

Since this initial pathway of fixation and formation of C₄ acid takes place in the mesophyll cells, the C₄ pathway begins and takes place there before the C₄ acids are transported to the bundle sheath cells for decarboxylation and entry into the Calvin (C₃) cycle.