Lipids and carbohydrates as respiratory substratesSpec C1.2.17
In short
Lipids give a higher yield of energy per gram than carbohydrates because they contain less oxygen and more oxidisable hydrogen and carbon. Fatty acids are broken into 2C acetyl groups that enter the Krebs cycle as acetyl CoA. Glycolysis and anaerobic respiration occur only if carbohydrate is the substrate, so fatty acids can be respired only aerobically.
| Carbohydrate (e.g. glucose) | Lipid (fatty acids) | |
|---|---|---|
| Energy yield per gram | Lower | Higher, roughly twice as much |
| Why | More oxygen in the molecule; less oxidisable hydrogen and carbon | Less oxygen; more oxidisable hydrogen and carbon |
| Entry into respiration | Glycolysis → pyruvate → link reaction | Broken into 2C acetyl groups, entering as acetyl CoA |
| Anaerobic respiration possible | Yes | No |
Energy in respiration comes from oxidising carbon and hydrogen in the substrate. A fatty acid is mostly long hydrocarbon chain with very little oxygen, so more of its atoms can be oxidised than in glucose, where many carbons already carry oxygen. Each gram of lipid therefore yields more reduced NAD and more ATP.
Glycolysis and anaerobic respiration occur only if carbohydrate is the substrate. Fatty acids are broken down into 2C acetyl groups, which enter the pathway via acetyl CoA and are then oxidised in the Krebs cycle. As this needs oxygen to regenerate NAD through the electron transport chain, fatty acids cannot be used in anaerobic respiration. (Beyond the syllabus: the small glycerol part of a triglyceride can be converted to triose phosphate and enter glycolysis, but fatty acids supply nearly all the energy in a lipid.)
Linking question: in what forms is energy stored in living organisms? Fat stores more energy per gram than glycogen, but only glycogen can supply glucose for anaerobic respiration.
Quick check
What is the full name of ATP and what type of molecule is it?
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Adenosine triphosphate; a nucleotide.
Write the word equation for anaerobic respiration in humans.
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glucose → lactate
Why is soda lime used in a respirometer?
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It absorbs carbon dioxide, so the movement of the drop measures oxygen uptake.
HL only Name the two Krebs cycle intermediates you need to know and their number of carbons.
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Citrate (6C) and oxaloacetate (4C).
HL only What is the role of oxygen in aerobic respiration?
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It is the terminal electron acceptor: it accepts electrons and protons, forming water, so electrons keep flowing along the chain.
Written and checked against the IB Biology HL specification · Updated October 2026