The correct statement is: B. NAD+ is reduced to NADH during glycolysis, pyruvate oxidation, and the citric acid cycle.
NAD+ plays a crucial role in cellular respiration, acting as an electron carrier. During the process of glycolysis, when glucose is broken down, NAD+ accepts electrons and is reduced to NADH. This reduction also occurs during pyruvate oxidation and the citric acid cycle, where more NAD+ is converted to NADH as it helps in capturing energy from nutrient breakdown. The other statements are not correct because:
- A. NAD+ has more chemical energy than NADH: This is false; NADH is the reduced form and holds more energy.
- C. NAD+ is oxidized by the action of hydrogenases: This is incorrect; NAD+ is reduced, not oxidized.
- D. NAD: This statement is incomplete and lacks context.
In summary, NAD+ is primarily significant for its reduction to NADH during key metabolic processes, allowing cells to efficiently harness energy.