Answer: Can bind O 2 as well as CO 2 when CO 2 is low and O 2 is high.
- A Can bind O<sub>2</sub> as well as CO<sub>2</sub> when CO<sub>2</sub> is low and O<sub>2</sub> is high
- B Functions mainly at night, away from light reactions
- C Binds only CO<sub>2</sub>, regardless of ambient O<sub>2</sub> levels
- D Is largely absent from mesophyll cells in C<sub>3</sub> leaves
Correct answer: A. Can bind O<sub>2</sub> as well as CO<sub>2</sub> when CO<sub>2</sub> is low and O<sub>2</sub> is high
Explanation: Rubisco has both carboxylase and oxygenase activity; under high O<sub>2</sub>/low CO<sub>2</sub> conditions, it binds O<sub>2</sub> instead, initiating photorespiration, which consumes energy without fixing carbon or producing sugar.
Simplified photosynthesis diagram: light energy, CO2, and H2O enter the leaf and are converted in the chloroplast into glucose and O2.
Concept context
Light reactions, the Calvin cycle, C 3 /C 4 /CAM pathways, photorespiration, and factors affecting the rate of photosynthesis. One of the highest-yield NEET chapters.