Question

In $C_4$ plants, where does the Calvin cycle take place?

• A. Bundle sheath
• B. Mesophyll cells
• C. Phloem
• D. Epidermal cells

Answer 1

The correct option is A Bundle sheath

In $C_4$ plants, the Calvin cycle takes place in the bundle sheath, whereas in $C_3$ plants it takes place in mesophyll cells.

In $C_4$ plants, phosphoenolpyruvate (PEP) accepts $C{O}_2$ in the mesophyll cells. The mesophyll cells lack the RuBisCO enzyme. On accepting $C{O}_2$, PEP gets converted to oxaloacetic acid in the presence of the enzyme phosphoenolpyruvate carboxylase (PEPcase). Oxaloacetic acid is then converted to malic acid. This malic acid is transported to the bundle sheath. The plasmodesmata between the bundle sheath and the mesophyll cells allows malic acid to pass through. The bundle sheath cells are rich in the RuBisCO enzyme but lack PEPcase. The malic acid undergoes decarboxylation in the bundle sheath and releases $C{O}_2$. The $C{O}_2$ that is released then goes into the $C_3$ cycle.