Where do photosynthesis and cellular respiration occur in cells?

• A. Photosynthesis occurs in chloroplasts; cellular respiration occurs in mitochondria.
• B. Photosynthesis occurs in mitochondria; cellular respiration occurs in chloroplasts.
• C. Photosynthesis occurs in nucleus; cellular respiration occurs in ribosomes.
• D. Photosynthesis occurs in lysosomes; cellular respiration occurs in peroxisomes.

Answer: (B)

Energy conversion in cells relies on specialized compartments. Photosynthesis happens in chloroplasts because they contain chlorophyll and the internal membranes (thylakoids) needed to capture light and drive the reactions that build sugars. The Calvin cycle then uses those sugars to store energy. Cellular respiration mainly happens in mitochondria. Glucose is first broken down in the cytosol (glycolysis), and the resulting pyruvate is processed in the mitochondria, where the Krebs cycle and the electron transport chain generate ATP. The two processes are connected: photosynthesis supplies the organic molecules that respiration uses for energy.

Other organelles mentioned don’t carry out the main energy-conversion roles described here: the nucleus houses genetic material, ribosomes build proteins, lysosomes digest wastes, and peroxisomes handle other metabolic tasks.