C₄ pathway is a more efficient photosynthetic pathway than C3, because of

The correct answer is lower photorespiration rate.

To understand why the C₄ pathway is more efficient than the C₃ pathway, let us examine the biochemical differences between them:

1. The Problem of Photorespiration in C₃ Plants:
In C₃ plants, the primary carbon-fixing enzyme is RuBisCO (Ribulose-1,5-bisphosphate carboxylase-oxygenase). RuBisCO has a dual affinity for both carbon dioxide (CO₂) and oxygen (O₂). Under conditions of high temperature, drought, or low CO₂ concentration relative to O₂, RuBisCO binds with O₂ instead of CO₂. This initiates a process called photorespiration (C₂ cycle), which wastes energy (ATP and NADPH) and releases previously fixed CO₂ without producing any sugars.

2. The C₄ Mechanism to Minimize Photorespiration:
C₄ plants have evolved a structural and biochemical mechanism to overcome photorespiration. They exhibit a specialized leaf anatomy known as Kranz anatomy, which separates the initial carbon fixation and the Calvin cycle in space:
- Mesophyll Cells: The initial fixation of CO₂ occurs here, catalyzed by the enzyme PEP carboxylase (Phosphoenolpyruvate carboxylase). PEP carboxylase has a very high affinity for CO₂ and no affinity for O₂. It fixes CO₂ into a 4-carbon compound (oxaloacetate).
- Bundle Sheath Cells: The 4-carbon compound is transported to bundle sheath cells, where it is decarboxylated to release a high concentration of CO₂ directly around RuBisCO. This high local concentration of CO₂ ensures that RuBisCO functions almost exclusively as a carboxylase and not as an oxygenase.

3. Conclusion:
By virtually eliminating photorespiration, C₄ plants avoid the energy loss associated with the oxygenase activity of RuBisCO. This makes the C₄ pathway highly efficient at fixing carbon under conditions of high temperature, intense light, and dryness, compared to the C₃ pathway.