Unit Overview
Energy and Respiration (A2 Portion)
CIE A-Level BiologyΒ· 5 min read π n/a
1. Unit at a Glance
This unit follows on from the AS introduction to respiration, walking you through each stage of aerobic respiration in detail before exploring alternative pathways and practical assessment skills. You will build from understanding where each reaction occurs in the mitochondrion to calculating overall net ATP production from different food sources.
The learning sequence progresses logically: from the cyclical reactions of the Krebs cycle (which produce reduced coenzymes for energy production), through to ATP synthesis, then anaerobic respiration, energy from different substrates, and finally practical measurement of respiration rate.
This unit is split into 5 core sub-topics:
Krebs cycle
Detailed walkthrough of the reactions, products, and location of the Krebs cycle in aerobic respiration.
β β β β± 8 min
Oxidative phosphorylation
Explains the electron transport chain, chemiosmosis, and how most ATP is produced in aerobic respiration.
β β β β β± 10 min
Anaerobic respiration
Compares anaerobic pathways in animals, yeast and bacteria, and their net ATP yield compared to aerobic respiration.
β β β± 6 min
Respiratory substrates
Covers energy release from carbohydrates, lipids and proteins, and how to calculate RQ values.
β β β β± 7 min
Measurement of respiration rate
Explains how to use a respirometer to measure respiration rate and interpret experimental data.
β β β β± 7 min
2. Common Pitfalls
Wrong move:
Confusing Krebs cycle products per glucose vs per pyruvate
Why:
One glucose splits into two pyruvate molecules, so all Krebs cycle outputs are doubled for a single glucose molecule
Correct move:
Always multiply product counts by 2 when calculating total yield from one starting glucose molecule
Wrong move:
Overestimating ATP contribution from the Krebs cycle
Why:
Most ATP comes from oxidative phosphorylation, not substrate-level phosphorylation in earlier stages
Correct move:
Recall that ~30 of 32 total ATP per glucose comes from oxidative phosphorylation
Wrong move:
Mixing up RQ values for different respiratory substrates
Why:
RQ value depends on the oxygen content of the substrate, which varies between macronutrients
Correct move:
Memorize RQ = 1 for carbohydrates, 0.7 for lipids, and 0.9 for proteins
3. Quick Reference Cheatsheet
Concept | Key Fact / Value |
|---|---|
Location of Krebs cycle | Mitochondrial matrix |
Location of oxidative phosphorylation | Inner mitochondrial membrane |
Net ATP per aerobic glucose (eukaryotes) | 30-32 ATP |
Net ATP per anaerobic glucose | 2 ATP |
Respiratory Quotient formula | RQ = |
RQ for carbohydrates | 1.0 |
RQ for lipids | 0.7 |
RQ for proteins | 0.9 |
What's Next
Begin your study of this unit with the first sub-topic, which covers the Krebs cycle, the first core A2 stage of aerobic respiration after glycolysis. Work through each sub-topic in order to build your understanding sequentially, as later concepts depend on knowledge of earlier stages. Once you complete all sub-topics in this unit, you can move on to the next A2 unit to continue your syllabus progress.