Energy efficiency of catabolism: If only about 40% of the energy from complete oxidation of glucose is captured as ATP, what happens to the remaining ~60% under physiological conditions?

Introduction / Context:
Cellular energy transduction is not 100% efficient. The second law of thermodynamics predicts that some energy disperses as heat when chemical energy is converted to ATP and other forms of work.

Given Data / Assumptions:

• Complete aerobic oxidation of glucose releases a fixed amount of free energy.
• Only a portion is captured in ATP; the rest is dissipated.
• Organisms maintain temperature and enable endergonic processes using controlled inefficiencies.

Concept / Approach:
During oxidative phosphorylation and other steps, proton leakage, enzyme inefficiencies, and thermodynamic constraints release energy as heat. This heat contributes to maintaining body temperature in homeotherms and to accelerating reaction rates.

Step-by-Step Solution:
Recognize that ATP yield per glucose is finite; remaining energy cannot be fully stored. Consider thermodynamics: energy conversions invariably produce entropy (heat). Eliminate alternatives claiming perfect storage or exclusive diversion to other carriers. Select heat dissipation as the fate of the ~60% remainder.

Verification / Alternative check:
Calorimetry of metabolism demonstrates significant heat generation during respiration, consistent with partial conversion to ATP and partial heat loss.

Why Other Options Are Wrong:
NADPH generation is pathway-specific and does not account for most losses; not all energy remains in end products after complete oxidation; fat storage requires ATP and still loses heat; mechanical work is mainly ATP-driven in cells.

Common Pitfalls:
Assuming biological systems can achieve near 100% conversion efficiency.

Final Answer:
It is lost as heat.