Leaf Gas Exchange: Photosynthesis vs. Respiration

Steps:

• In light, leaf performs photosynthesis, releasing oxygen into surrounding air at X.
• This elevates oxygen content above normal atmospheric levels (about 21% O2).
• In dark, photosynthesis stops; leaf undergoes respiration, consuming oxygen.
• This reduces oxygen content below normal atmospheric levels.

Why A is correct:

• Photosynthesis equation (6CO2 + 6H2O → C6H12O6 + 6O2) produces O2 in light; respiration (C6H12O6 + 6O2 → 6CO2 + 6H2O) consumes it in dark.

Why the others are wrong:

• B: Ignores O2 consumption by respiration in dark.
• C: Reverses effects—light increases O2, dark decreases it.
• D: Fails to account for O2 production in light.

Final answer: A