Energy Metabolism
What this domain tests
Energy metabolism converts food energy into ATP and heat. The exam tests energy values, the stages of cellular respiration, respiratory quotient and energy balance.
Core rules
- Glycolysis breaks glucose to pyruvate in the cytoplasm, yielding a small amount of ATP.
- Pyruvate enters the mitochondria and is oxidised through the Krebs cycle, producing NADH and FADH2.
- The electron transport chain uses oxygen to generate most ATP, with water as a product.
- Respiratory quotient (RQ) is CO2 produced divided by O2 consumed; it is about 1.0 for carbohydrate, 0.8 for protein and 0.7 for fat.
- Energy balance is intake minus expenditure; positive balance leads to weight gain and negative balance to weight loss.
Worked example
A meal supplies 60 g carbohydrate, 15 g protein and 15 g fat. Energy = (60 × 4) + (15 × 4) + (15 × 9) = 240 + 60 + 135 = 435 kcal. Reasoning: calculate each macronutrient separately, then add. If the person expends 2,000 kcal and eats 2,435 kcal, the surplus is 435 kcal, which favours weight gain over time.
Traps that cost marks
- Saying the Krebs cycle occurs in the cytoplasm rather than the mitochondrial matrix.
- Reversing the RQ values for fat and carbohydrate.
- Ignoring oxygen as the final electron acceptor.
- Treating energy balance as a one-day event rather than a trend.