Question

Explain the $C_4$ cycle of photosynthesis.

Answer 1

This pathway mostly takes place in monocot plants and in some dicot plants. In this pathway, the first product of CO22fixation is a 4 carbon compound, oxaloacetic acid, hence it is called C44 pathway.
The anatomy of leaves of C44 plants is different from leaves of C33 plants (Calvin cycle). Palisade cells in the mesophyll tissues are absent. There is bundle sheath around the vascular bundles. In the bundle sheath cells, large chloroplasts without grana or less developed grana are present, whereas, in mesophyll cells, there are chloroplasts with well-developed grana. This type of anatomy is called Kranz anatomy.
In C44 pathway, CO22 taken from the atmosphere is accepted by phosphoenol pyruvic acid present in the chloroplast of mesophyll cells, leading to the formation of a 4 carbon compound oxaloacetic acid. This is converted into malic acid (4 carbon compound) which enters into the chloroplast of bundle sheath cells. Here the malic acid dissociates into pyruvic acid and CO22. Pyruvic acid re-enters with RuBP gives rise to sugars and other carbohydrates in bundle sheath cells. There is no photorespiration in these plants. Example, Maize, Sorghum.