Which statement reflects a basic thermodynamics rule?

• A. Energy cannot be created or destroyed
• B. Free energy is always conserved
• C. Entropy is always decreasing
• D. Enthalpy equals total energy exactly

Answer: (A)

Energy cannot be created or destroyed; it can only be transformed or transferred as heat or work. This fundamental rule means that, even though biological processes flip energy between forms—chemical energy in nutrients to kinetic energy, heat, or stored chemical bonds—the total amount of energy in a system and its surroundings stays constant. The other statements mix up how energy and entropy behave: free energy is not conserved—it represents the usable energy a system can do work and typically decreases as reactions move toward equilibrium. Enthalpy is not total energy; it’s defined as H = U + pV and reflects heat content at constant pressure, not the entire energy content. And entropy is not always decreasing; the second law states that the entropy of an isolated system tends to increase, though local decreases can occur if they are offset by greater increases elsewhere.