Leaf Gas Exchange in Light and Dark

Steps:

• In light, photosynthesis in the leaf produces oxygen as a byproduct, raising O2 levels at X above atmospheric air.
• In dark, only respiration occurs, consuming oxygen and lowering O2 levels at X below atmospheric air.
• Atmospheric air maintains ~21% O2 unaffected by the leaf.
• Comparison shows elevated O2 in light and reduced O2 in dark relative to normal air.

Why A is correct:

• Photosynthesis releases O2 (6CO2 + 6H2O → C6H12O6 + 6O2), increasing local concentration; respiration consumes O2 (C6H12O6 + 6O2 → 6CO2 + 6H2O), decreasing it.

Why the others are wrong:

• B: In dark, respiration lowers O2, not keeps it the same.
• C: Light raises O2 via photosynthesis, not lowers it; dark lowers O2, not raises it.
• D: Light raises O2, not lowers it; dark lowers O2, not keeps it the same.

Final answer: A