In which organelle does photosynthesis occur, and what are its two main stages?

• A. Chloroplasts; light-dependent reactions and the Calvin cycle.
• B. Mitochondria; glycolysis and the Krebs cycle.
• C. Nucleus; transcription and translation.
• D. Endoplasmic reticulum; rough and smooth.

Answer: (A)

Photosynthesis happens in chloroplasts, the green organelles in plant cells that capture light energy. It has two main stages: light-dependent reactions and the Calvin cycle. The light-dependent reactions occur in the thylakoid membranes and use light to convert water into oxygen, while producing ATP and NADPH. The Calvin cycle takes place in the stroma and uses that ATP and NADPH to fix carbon dioxide into sugars, building glucose. So, chloroplasts with these two stages—light-driven energy capture and carbon fixation to sugars—are the key. (Other organelles listed perform respiration, genetic information processing, or protein/lipid synthesis, not photosynthesis.)