Which of the following statements about the electron transport chain is true? 1 Water is the last electron acceptor. 2 The electron transport chain is the first step in cellular respiration. 3 NADH and FADH2 donate their electrons to the chain. 4 Electrons gain energy as they move down the chain.

The correct statement is:

3. NADH and FADH2 donate their electrons to the chain.

Explanation:

• Statement 1 (Incorrect): Oxygen (not water) is the last electron acceptor in the electron transport chain. Oxygen accepts electrons and combines with protons to form water.
• Statement 2 (Incorrect): The electron transport chain is not the first step in cellular respiration. It comes after glycolysis, pyruvate oxidation, and the citric acid cycle.
• Statement 3 (Correct): NADH and FADH2_22​ donate their high-energy electrons to the electron transport chain, which are then passed through a series of protein complexes to generate a proton gradient.
• Statement 4 (Incorrect): Electrons lose energy as they move down the electron transport chain. This energy is used to pump protons across the inner mitochondrial membrane, creating a proton gradient.