In photosynthesis, which gas is fixed and released by plants, and in which organelle do the light-dependent reactions occur?

• A. Oxygen; mitochondria
• B. Carbon dioxide; chloroplasts
• C. Nitrogen; nucleus
• D. Hydrogen; ribosome

Answer: (B)

Photosynthesis fixes carbon dioxide from the air into organic molecules, using energy from light. The gas fixed by plants is carbon dioxide, and oxygen is released as a byproduct when water is split during the light-dependent steps. The light-dependent reactions occur in chloroplasts, specifically in the thylakoid membranes that house the photosystems and the electron transport chain. This is why chloroplasts, not mitochondria or the nucleus/ribosome, are the correct location, and carbon dioxide is the correct gas associated with fixation.