12.1.4—Energy values of respiratory substrates

Carbohydrates, lipids and proteins can all act as respiratory substrates, but oxidation of the same mass does not release the same amount of energy. Their molecular composition changes how much reducing hydrogen and electron-transfer potential can enter respiration.

• Carbohydrates: Often provide the readily available respiratory starting substrate. Their oxidation releases energy for ATP production, but their energy value per unit mass is generally lower than that of lipids.
• Lipids: Fatty acids contain long hydrocarbon regions with a high proportion of hydrogen. More hydrogen can be transferred through carriers and contribute to a larger proton gradient, so lipids generally have the greatest energy value per unit mass.
• Proteins: Amino acids can be respired, but they are normally used after other available substrates because they are also needed for structural and functional proteins. Their energy value is intermediate rather than a reason to use them first.
• Boundary: “Higher energy value” means more energy released per unit mass under the stated respiratory conditions; it is not the same as an RQ value or a fixed ATP yield for every molecule and cell.

More available reducing hydrogen → more hydrogen carriers become reduced → more proton-gradient potential during aerobic respiration → more ATP can be synthesised. This composition-to-oxidation chain explains why lipid, carbohydrate and protein energy values differ.

Do not rank substrates only by when a cell uses them: protein may be energetically useful but is conserved for other cellular roles. Do not replace energy value with RQ; RQ is the separate gas-ratio objective that follows.