The rate of photorespiration in most C3 plants rises at higher temperatures. Some plants mitigate this by a metabolic strategy that concentrates CO2 around Rubisco. What is this pathway called?

Introduction / Context:
Photorespiration increases when temperature rises and when internal CO2 levels drop, causing Rubisco to act more as an oxygenase than a carboxylase. Some plants evolved anatomical and biochemical innovations to suppress photorespiration by elevating CO2 near Rubisco. This question asks you to recognize that adaptation.

Given Data / Assumptions:

• Rubisco can react with O2, causing wasteful photorespiration.
• High temperature and low CO2 favor oxygenation reactions.
• Plants have evolved CO2-concentrating mechanisms.

Concept / Approach:
The C4 pathway uses mesophyll phosphoenolpyruvate carboxylase (PEPC) to fix HCO3- into a 4-carbon acid (e.g., malate), which is transported to bundle sheath cells. There, CO2 is released near Rubisco, raising the CO2:O2 ratio and suppressing photorespiration. This spatial separation (Kranz anatomy) helps maintain high photosynthetic efficiency in hot, bright environments.

Step-by-Step Solution:

Verification / Alternative check:
Gas exchange and isotope studies show lower photorespiratory flux in C4 plants (e.g., maize, sugarcane) compared with C3 plants at high temperatures; anatomical Kranz pattern correlates with function.

Why Other Options Are Wrong:

• ETS and Photosystem II: components of light reactions; they do not constitute a CO2-concentrating mechanism.
• Calvin cycle: the standard carbon fixation pathway common to C3, C4, and CAM; it does not by itself concentrate CO2.
• CAM: another CO2-concentrating mechanism but temporally separated (night/day), not what the question highlights; the widely taught answer here is the C4 pathway responding to heat-induced photorespiration.

Common Pitfalls:
Confusing C4 (spatial separation) with CAM (temporal separation). Both concentrate CO2, but the classic response to high-temperature photorespiration in many grasses is the C4 pathway.

Final Answer:
C4 pathway.