Cellular respiration is best described as

• A. Gas exchange between lungs and blood
• B. Gas exchange between blood and body tissues
• C. Transport of oxygen in blood
• D. Using oxygen to produce energy within cells

Answer: (D)

Cellular respiration refers to the series of metabolic steps by which cells harvest chemical energy from glucose (and other nutrients) to make ATP, the cell’s immediate energy currency. This process largely occurs in mitochondria and includes glycolysis (in the cytoplasm), the citric acid cycle, and oxidative phosphorylation (the electron transport chain). Oxygen serves as the final electron acceptor in the chain, allowing most of the ATP to be produced; water is formed as a byproduct. This focus on using oxygen to generate energy inside cells is what makes it the best description. The other ideas describe gas movement (breathing) or the delivery of oxygen in the blood, not the energy-producing reactions inside cells.