Which organelle is the site of cellular respiration?

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

Answer: (A)

Cellular respiration—the way cells extract usable energy from nutrients—takes place mainly in the mitochondria, the cell’s energy hubs. Inside these organelles, the major energy-producing steps occur: the citric acid cycle and oxidative phosphorylation (the electron transport chain and chemiosmosis). The inner membrane with its folds, called cristae, provides a large surface area for the electron transport chain, which ultimately drives the production of most ATP powered by a flow of electrons and a proton gradient. Glycolysis happens in the cytoplasm, producing pyruvate that enters mitochondria to fuel these later stages. Oxygen serves as the final electron acceptor in the chain, making aerobic respiration efficient.

Nuclei house genetic material and manage transcription and DNA-related processes, not energy production. Chloroplasts carry out photosynthesis in plants and some algae, converting light energy into chemical energy, while mitochondria then convert that chemical energy into ATP during respiration. Ribosomes synthesize proteins, not ATP directly.