Which organelle in plant and algae cells is the site of photosynthesis?

• A. Mitochondrion
• B. Ribosome
• C. Chloroplast
• D. Nucleus

Answer: (C)

Photosynthesis takes place in chloroplasts, the green organelles found in plant and many algae cells. Chloroplasts contain chlorophyll, which captures light energy and drives the light-dependent reactions on the thylakoid membranes to produce ATP and NADPH. Those energy carriers power the Calvin cycle in the stroma, where carbon dioxide is fixed into sugars. That combination—light energy converted to chemical energy, then used to build glucose—happens only in chloroplasts, making them the site of photosynthesis. Mitochondria handle cellular respiration to release energy from sugars, ribosomes make proteins, and the nucleus houses genetic material, so they aren’t the sites of photosynthesis.