Question

What is the benefit of separating the light and carbon reactions into different cells in C4 plants? Select one:

a. CO2 to O2 ratios are maintained in the bundle sheath cells, even with a large number of stoma closed, because the light reactions are generating oxygen in a different cell.

b. Since the light reactions occur closer to the leaf surface oxygen can escape more easily.

c. More ATP remains to power metabolism in the mesophyll cells because the carbon reactions are occurring in the bundle-sheath cells.

d. The enzyme required to make the 4-carbon compound (that C4 is named after) is only found in mesophyll cells and to have enough energy and room for the Carbon cycle these reactions must move to a different cell.

Answer 1

CO2 to O2 ratios are maintained in the bundle sheath cells, even with a large number of stoma closed, because the light reactions are generating oxygen in a different cell - In C3 plants due to the dual carboxylase and oxygenase activity of RuBisCo, some part of the substrate is oxidized rather than carboxylated, resulting in photorespiration. In C4 plants chloroplasts exist not only in the mesophyll cells but also in the bundle sheath cells and they have a mechanism to efficiently deliver CO2 to the RuBisCO enzyme. Instead of direct fixation to RuBisCO in the Calvin cycle like in C3 plants, CO2 is incorporated into a 4-carbon organic acid, which has the ability to regenerate CO2 in the chloroplasts of the bundle sheath cells, where the CO2 ratio is maintained and it is carboxylated effeciently.