Which term describes the complete set of metabolic pathways that transform glucose into ATP using oxygen?

• A. Cellular respiration
• B. Photosynthesis
• C. Fermentation
• D. Glycolysis

Answer: (A)

The concept being tested is how glucose is fully broken down to produce ATP when oxygen is present. This is cellular respiration, the whole suite of pathways that use oxygen to maximize ATP yield from glucose. It starts with glycolysis in the cytoplasm, where glucose is split into pyruvate and a small amount of ATP and NADH are produced. Then pyruvate is converted to acetyl-CoA and enters the citric acid cycle, generating more NADH and FADH2 and a little direct ATP. The final stage is oxidative phosphorylation, where electrons from NADH and FADH2 move through the electron transport chain and oxygen acts as the final electron acceptor, driving the production of a large amount of ATP.

Understanding why the other terms don’t fit helps reinforce the idea. Photosynthesis is the process by which light energy is captured to build sugars; it doesn’t primarily produce ATP from glucose in the presence of oxygen. Fermentation carries out glucose breakdown without oxygen, yielding far less ATP and not utilizing the full electron transport chain. Glycolysis is just the first step in glucose catabolism and can occur with or without oxygen, but it does not by itself describe the complete, oxygen-using series of pathways that maximizes ATP production.