In cellular respiration, oxygen (O₂) plays a crucial role in the process of energy production within cells. It is primarily used in the electron transport chain, which is the final stage of cellular respiration.
Let's clarify each option given:
A. This option suggests that oxygen combines with the 4-carbon (4-C) starting material of the Krebs cycle to form a 6-carbon (6-C) molecule. This statement is incorrect because the Krebs cycle (also known as the citric acid cycle) involves the combination of acetyl-CoA (a 2-C molecule) with oxaloacetate (a 4-C molecule) to form citrate (a 6-C molecule), but oxygen is not directly involved in this part of the process.
B. This option states that oxygen combines with the depleted energy carrier NAD⁺ to form NADH. NAD⁺ is reduced to NADH during different stages of cellular respiration such as glycolysis and the Krebs cycle, but oxygen is not involved in this conversion. NADH is produced by enzymes that catalyze these specific substrate-level phosphorylation reactions.
C. This option indicates that oxygen accepts electrons (e⁻) at the end of the electron transport chain and forms water (H₂O). This is indeed correct. Oxygen serves as the final electron acceptor in the electron transport chain. When oxygen accepts electrons, it combines with protons (H⁺) to form water. This process is crucial because it allows for the continuation of the electron transport chain, which is essential for the production of ATP (adenosine triphosphate).
D. This option proposes that oxygen provides the energy needed to break chemical bonds in glucose. This is incorrect because the energy to break these bonds during glycolysis and other stages of cellular respiration comes from the breakdown of ATP molecules and not directly from oxygen.
Considering the explanations above, the correct answer to the question is:
c. To accept electrons (e⁻) at the end of an electron transport chain and cause water (H₂O) to form.
