Photosynthesis in `C_(4)` plants is relatively less limited by atmospheric `CO_(2)` levels because

Options

A. effective pumping of `CO_(2)` into bundle sheath cells
B. RuBisCo in `C_(4)` plants has higher affinity for `CO_(2)`
C. four carbon acids are the initial `CO_(2)` fixation products
D. the primary fixation of `CO_(2)` is mediated via PEP caroxylase

Answer

**Step-by-Step Solution:**

1. **Understanding C4 Photosynthesis**: C4 plants have a unique mechanism for photosynthesis that allows them to efficiently fix carbon dioxide (CO2) even when atmospheric CO2 levels are low.

2. **Role of Phosphoenolpyruvate (PEP)**: In C4 plants, the initial fixation of CO2 occurs in the mesophyll cells. This process involves the enzyme phosphoenolpyruvate carboxylase (PEP carboxylase), which catalyzes the reaction between CO2 and phosphoenolpyruvate (PEP) to form oxaloacetic acid (OAA).

3. **Affinity for CO2**: PEP has a high affinity for CO2, meaning it can effectively capture CO2 even when its concentration is low in the atmosphere. This high affinity allows C4 plants to maintain photosynthesis under conditions where C3 plants might struggle due to insufficient CO2.

4. **Final Fixation in Bundle Sheath Cells**: After the initial fixation in the mesophyll cells, the oxaloacetic acid is converted into malate, which is then transported to the bundle sheath cells. Here, the enzyme ribulose bisphosphate carboxylase/oxygenase (rubisco) facilitates the final fixation of CO2 in the Calvin cycle.

5. **Conclusion**: The combination of high CO2 affinity of PEP and the spatial separation of initial and final fixation processes in C4 plants allows them to be less limited by atmospheric CO2 levels compared to C3 plants.

**Final Answer**: Photosynthesis in C4 plants is relatively less limited by atmospheric CO2 levels because the initial fixation of CO2 is mediated by phosphoenolpyruvate (PEP), which has a high affinity for CO2, allowing effective carbon fixation even at low atmospheric concentrations.

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