Lipids and carbohydrates: Higher Level only: This statement is Higher Level only. It compares the two main fuels of respiration (SL 3.2.4 names them).
Practise this as you read
- Distinguish between lipids and carbohydrates as substrates.
- Explain why lipids yield more energy per gram.
Free preview
This is the free notes preview
You're reading the free notes. Aimnova Pro unlocks the full study experience — and you can try it with your first topic free to keep:
- FlashcardsLock in vocabulary and key terms with spaced repetition.
- Practice questionsAnswer exam-style questions and get instant AI marking.
- Mock exams & past-paper vaultSit full mocks and see exactly how examiners award marks.
- Personalised study planA daily plan built around your exam date and weak areas.
Start Studying Free Full access to Aimnova Pro · cancel anytime
The big idea: Both lipids and carbohydrates can be respired, but differently. Glucose goes through glycolysis, so it can be respired with or without oxygen. Fatty acids are cut into two-carbon acetyl groups that enter the Krebs cycle, so they need oxygen. Per gram, lipids release about twice the energy.
The points to remember
- Carbohydrate (glucose): glycolysis → pyruvate; can be respired anaerobically.
- Lipid: fatty acids → two-carbon acetyl groups → acetyl coenzyme A → Krebs cycle.
- Fatty acids miss glycolysis, so they can be respired only aerobically.
- Energy per gram: lipid ≈ 37 kJ, carbohydrate ≈ 17 kJ.
- Lipids have more hydrogen, less oxygen per carbon: more reduced NAD, more ATP.
Remember it as: Fat: twice the energy, but only with oxygen.
Real example: marathon runners and sprinters. In a marathon, muscles burn a high proportion of fat aerobically; in a sprint, they depend on glucose, which glycolysis can use quickly even without enough oxygen.
Trap: Lipids give more energy per gram, but carbohydrates are quicker to use and can give ATP without oxygen.
Never wonder what to study next
Get a personalized daily plan based on your exam date, progress, and weak areas. We'll tell you exactly what to review each day.
Try Free Study PlanYour first topic is free to keep • No credit card required
How it is asked: Expect a Distinguish or Compare question on lipids and carbohydrates as respiratory substrates.
IB-style questionDistinguish[3 marks]
Distinguish between lipids and carbohydrates as respiratory substrates.
Model answer plan
See the answer broken into clear points, with guidance on how each point earns marks.