Biology Practice Test 2026: Energy, Enzymes, and Metabolism Essentials

Cellular respiration relies on three linked ideas: oxygen acts as the final electron acceptor in the electron transport chain, the chain operates in the mitochondria, and electrons move through a series of membrane-bound proteins to transfer energy and pump protons for ATP synthesis. Oxygen is needed for maximal ATP production because the electron transport chain only keeps running as long as oxygen can accept electrons; without it, NADH and FADH2 can’t be reoxidized, the chain stalls, and ATP yield drops dramatically. The mitochondria are the primary site where these steps occur in eukaryotic cells, with glycolysis in the cytoplasm and oxidative phosphorylation taking place across the inner mitochondrial membrane. The electrons move through the membrane-bound proteins of the electron transport chain, releasing energy that powers proton pumping and the synthesis of ATP via ATP synthase. Put together, all of these statements are true, so the correct conclusion is that all of the statements apply.