Question

1. Considering the process of carbon fixation in C3 vs. C4 plants, develop a plausible explanation for why C4 plants fix more 13C than C3 plants.

Answer 1

C4 plants fixes more 13C than C3 plants because in C4 plants there is an additional conversion step due to the presence of PEP carboxylase in C4 plants. CO2 get diffused into the stomata and get converted into bicarbonate by which the heavy isotope of carbon i.e. 13C get concentrated. The PEP carboxylase enzyme helps in the fixation of this 13C and that's why it is less depleted in C4 plant comparative to C3 plants. Some portion of the fixed 13C get leaked out of the bundle sheath cells which limits the quantity of available carbon to RuBisCO. This step make the fractionation effect slightly lower. Stil C4 efficiently fixes more 13C comparative to C3.

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