Which compound would yield the most adenosine triphosphate (ATP) per mole when six different compounds with the same number of carbon atoms are completely catabolized by a cell under aerobic conditions?

ATP Yield from Complete Aerobic Catabolism

Among six compounds with equal carbon atoms, fatty acids yield the most ATP per mole under complete aerobic catabolization.

Biochemical Basis for ATP Production

The fundamental principle governing ATP yield relates to the oxidative efficiency and hydrogen content of different substrates 1:

• Glucose is most efficient per oxygen consumed, yielding 120 kcal per liter of oxygen, compared with 100 kcal from fat 1
• However, fatty acids yield more total ATP per mole because they contain significantly more carbon-hydrogen bonds available for oxidation
• Complete oxidation of glucose yields up to 33.45 ATP per glucose molecule with a maximum P/O ratio of 2.79 2

Why Fatty Acids Produce More ATP

The key determinant is the degree of reduction (hydrogen content) of the substrate 1:

• Fatty acids are highly reduced compounds with abundant C-H bonds
• Each carbon in a fatty acid chain undergoes beta-oxidation, generating FADH₂ and NADH
• These reducing equivalents feed into the electron transport chain for oxidative phosphorylation
• The ATP yield formula for even-numbered fatty acids is [(7C - 6 - 1.5D) - 2(D-2)], where C is carbon number and D is double bonds 3

Metabolic Pathway Considerations

Fat metabolism relies more heavily on FAD-dependent reactions, while glucose metabolism depends primarily on NAD-dependent pathways 1:

• This difference in cofactor utilization affects the respiratory quotient (RQ)
• Fat yields more calories per gram utilized, resulting in a lower RQ 1
• Under aerobic conditions, fatty acid oxidation can contribute 68-81% of ATP production in working tissues 4

Comparative ATP Yields

For compounds with the same carbon number:

• Saturated fatty acids produce the maximum ATP due to complete reduction state
• Carbohydrates (like glucose) produce less ATP per mole despite better oxygen efficiency 1, 2
• Amino acids vary widely but generally produce less than fatty acids
• Organic acids (like lactate) produce substantially less ATP 4

Important Caveats

The theoretical maximum ATP yields may not reflect actual cellular production 5:

• For O₂ respiration, theoretical ATP yield equals 27.9, but real values vary between 26-38 among organisms 5
• Mitochondrial P/O ratios are 2.73 for pyruvate plus malate oxidation and 1.64 for succinate oxidation 2
• Energy pathways serve multiple functions beyond ATP production, so evolution may not optimize purely for maximum yield 5