PEP carboxylase and RuBisCO in C3 and C4 plants

[Davide U]

Hello there,

my questions are the following. If we agree that PEP carboxylase is more efficient than RuBisCO since it can't bind Oxygen, why isn't it present in C3 plants as well? Is it just a question of evolution and C3 plants are just "worse" with regards to efficiency, or is there any usefulness in having an enzyme like RuBisCO instead? (RuBisCO is present in C4 plants as well, right?)

Second question: PEP carboxylase doesn't bind Oxygen. In C4 plants, we have bundle sheeth cells that don't have grana. Why isn't it a good idea to let the Calvin cycle unfold in "normal" cells that have grana, if PEP carboxylase doesn't bind Oxygen anyway? Why are we better of with the Calvin cycle happening in the bundle sheeth cells for a good part? Or is RuBisCO still used even in C4 plants? I'd be grateful for any help, sorry if some english term is not correct.

[BabcockHall]

One thing to consider is that transporting CO₂ in C-4 plants costs two ATPs IIRC.

[CharonY]

RuBisCO is used in C4 plants. The trick is that it is a two-step process where a C4 body from Pep carboxylase is moved to deeper tissue where RuBisCO is better protected from oxygen. There, the C4 body is decarboxylated to release CO2 which is then used by RuBisCo.

[Davide U]

Thanks gotcha!