Respiration releases CO2 in the carbon cycle

Steps:

• Recall the carbon cycle: CO2 moves between atmosphere, plants, animals, and soil via key processes.
• Identify inputs/outputs: Photosynthesis absorbs CO2; respiration, decomposition, and burning release CO2.
• Locate process X on diagram: It shows organisms exhaling CO2, distinct from plant uptake or fuel combustion.
• Match to options: X aligns with biological CO2 release from living things.

Why D is correct:

• Respiration is the process where organisms break down glucose (C6H12O6 + 6O2 → 6CO2 + 6H2O + energy), releasing CO2 back to the atmosphere.

Why the others are wrong:

• A: Burning involves combustion of fossil fuels or organic matter, not biological activity in the cycle's organism phase.
• B: Decomposition breaks down dead matter via microbes, releasing CO2 but not from living respiration.
• C: Photosynthesis fixes CO2 into glucose (6CO2 + 6H2O → C6H12O6 + 6O2), absorbing rather than releasing it.

Final answer: D