Question

Where do c4 plants synthesize glucose?

Answer 1

Glucose is synthesized by a series of reactions of photosynthesis, collectively known as Calvin cycle.

Before starting the synthesis of glucose, CO2 needs to be fixed. CO2 fixation means that CO2 is converted into a particular compound that can be processed further to produce glucose. In C3 plants, this compound is 3-phosphoglycerate (it is a 3 carbon compound, thus the name C3 plants). In C4 plants, this compound is oxaloacetate (it is a 4 carbon compound, thus the name C4 plants).

In C3 plants, there is an issue. A process known as photorespiration occurs in C3 plants where the energy produced by photosynthesis is wasted. C4 plants have adapted themselves in a way they can minimize photorespiration.

In C3 plants, only Calvin cycle occurs where CO2 is fixed in the first step. This cycle occurs in the mesophyll cells of C3 plants.

However, in C4 plants the initial CO2 fixation and Calvin cycle occur in two different types of cells, thereby reducing photorespiration. Initial CO2 fixation occurs in mesophyll cells whereas Calvin cycle occurs in bundle sheath cells.

Therefore, C4 plants synthesize glucose in bundle sheath cells.

NOTE : CO2 fixation takes place twice in C4 plants → initial CO2 fixation in mesophyll cell + CO2 fixation that is part of the Calvin cycle in bundle sheath cells.