Question

Photosynthesis in $C_4$ plant is relatively less limited by atmospheric ${\mathrm{CO}}_2$ levels because

• A. there is effective pumping of ${\mathrm{CO}}_2$ into bundle sheath cells
• B. RuBisCO in $C_4$ plants has higher affinity for ${\mathrm{CO}}_2$
• C. six carbon acids are the primary initial ${\mathrm{CO}}_2$ fixation products
• D. the primary fixation of ${\mathrm{CO}}_2$ is mediated via PEP carboxylase

Answer 1

The correct option is D the primary fixation of ${\mathrm{CO}}_2$ is mediated via PEP carboxylase

C4 plants approximately have 50% higher photosynthesis efficiency than C3 plants as they are relatively less limited by atmospheric ${\mathrm{CO}}_2$.

This is because of the different mechanism of carbon fixation by the two types of photosynthesis.
C3 photosynthesis uses the Calvin cycle for fixing CO₂ catalyzed by ribulose-1,5-bisphosphate carboxylase (Rubisco), which takes place inside of the chloroplast in mesophyll cell.
For C4 plants, photosynthetic activities are partitioned between mesophyll and bundle sheath cells that are anatomically and biochemically distinct. The initial carbon fixation is catalyzed by phosphoenolpyruvate carboxylase (PEPC) forming oxaloacetate (OAA) from CO₂ and phosphoenolpyruvate (PEP).