Which statement correctly describes the conditions that lead to photorespiration?

• A. O2 concentration is low and CO2 concentration is high, promoting efficient photosynthesis.
• B. O2 concentration is high and CO2 concentration is high, increasing photorespiration.
• C. O2 concentration is low and CO2 concentration is low, which has little effect on photorespiration.
• D. O2 concentration is high and CO2 concentration is low, and stomata closures cause this, making photorespiration counterproductive because CO2 gets used up.

Answer: (D)

Photorespiration occurs when Rubisco fixes oxygen instead of carbon dioxide. This becomes likely when oxygen is abundant inside the leaf and carbon dioxide is scarce, which happens when stomata close to conserve water. Closed stomata limit CO2 entry, while O2 produced by photosynthesis remains, increasing the O2/CO2 ratio in the leaf. Under these conditions Rubisco acts more as an oxygenase, starting photorespiration, which uses energy and releases CO2, reducing the efficiency of photosynthesis. So the scenario describing high O2 and low CO2 inside the leaf due to stomatal closures best explains when photorespiration occurs. The other scenarios don’t align with this key driver: high CO2 suppresses photorespiration, low O2 reduces it, and having both gases low or both high doesn’t match the conditions that promote the oxygenation reaction.