Question

Q. Why the C4 plants use the C3 pathway or the Calvin cycle as the main biosynthetic pathway ?

Answer 1

C4 carbon fixation or the Hatch-Slack pathway is a photosynthetic process in some plants. It is the first step in extracting carbon from carbon dioxide to be able to use it in sugar and other biomolecules. It is one of three known processes for carbon fixation. The C4 in one of the names refers to the 4-carbon molecule that is the first product of this type of carbon fixation.

C4 fixation is an elaboration of the more common C3 carbon fixation and is believed to have evolved more recently. C4 overcomes the tendency of the enzyme RuBisCO to wastefully fix oxygen rather than carbon dioxide in the process of photorespiration. This is achieved by ensuring that RuBisCO works in an environment where there is a lot of carbon dioxide and very little oxygen. CO2 is shuttled via malate or aspartate from mesophyll cells to bundle-sheath cells. In these bundle-sheath cells CO2 is released by decarboxylation of the malate. C4 plants use PEP carboxylase to capture more CO2 in the mesophyll cells. PEP Carboxylase (3 carbons) binds to CO2 to make oxaloacetic acid (OAA). The OAA then makes malate (4 carbons). Malate enters bundle sheath cells and releases the CO2. These additional steps, however, require more energy in the form of ATP. Using this extra energy, C4 plants are able to more efficiently fix carbon in drought, high temperatures, and limitations of nitrogen or CO2. Since the more common C3 pathway does not require this extra energy, it is more efficient in the other conditions.