Photosynthesis in C4 plants is relatively less limited by atmospheric CO2 levels because
- effective pumping of CO2 into bundle sheath cells
- RuBisCo in C4 plants has higher affinity for CO2
- four carbon acids are the primary initial CO2 fixation products
- the primary fixation of CO2 is mediated via PEP carboxylase.
Answer
(4) the primary fixation of CO2 is mediated via PEP carboxylase.
Solution
As per NCERT, in C4 plants the primary CO2 acceptor is PEP in the mesophyll cells, and the reaction is catalysed by PEP carboxylase (PEPcase).
PEP + CO2 PEPcase OAA
∵ PEP carboxylase has a very high affinity for CO2 (and no oxygenase activity), it fixes CO2 efficiently even at low atmospheric CO2 levels.
⇒ The C4 acid then releases CO2 in the bundle sheath cells, keeping CO2 high around RuBisCO.
∴ Photosynthesis is less limited by atmospheric CO2 because primary fixation is mediated via PEP carboxylase.
Note: RuBisCO of C4 plants is the same enzyme as in C3 plants; it does not have a higher affinity for CO2.
Answer: (4)