At GCSE
At GCSE you need the aerobic and anaerobic equations, the location of respiration (mitochondria for aerobic), and to compare ATP yield. Questions often link to exercise, oxygen debt and lactic acid in muscle.
GCSEA-LevelAQAEdexcelOCR
Respiration revision — GCSE & A-Level Biology
Respiration is how all living cells release energy from glucose to make ATP. It is a guaranteed topic at GCSE and a major one at A-Level Biology, where you must explain glycolysis, the link reaction, the Krebs cycle and oxidative phosphorylation in detail.
Exam questions reward students who use precise vocabulary: oxidation, reduction, decarboxylation, substrate-level vs oxidative phosphorylation. AQA, Edexcel and OCR all expect you to compare aerobic and anaerobic pathways and to explain why aerobic respiration releases so much more energy.
At A-Level
At A-Level you must explain all four stages — glycolysis, the link reaction, the Krebs cycle and oxidative phosphorylation — including the role of NAD, FAD, decarboxylation and chemiosmosis. Expect questions on respiratory inhibitors (cyanide, oligomycin), respiratory quotient (RQ) calculations and the use of respirometers.
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Example flashcards
Q: Where in the cell does glycolysis occur?
A: In the cytoplasm — it does not require oxygen and produces 2 ATP and 2 NADH per glucose.
Q: What is the final electron acceptor in aerobic respiration?
A: Oxygen, which combines with electrons and protons to form water.
Q: Why does anaerobic respiration release so much less ATP?
A: Because only glycolysis runs — the Krebs cycle and oxidative phosphorylation cannot occur without oxygen to accept electrons.
Q: Name the products of anaerobic respiration in human muscle cells.
A: Lactic acid (and a small amount of ATP).
Quick summary
Respiration — overview:
- Aerobic word equation: glucose + oxygen → carbon dioxide + water (+ ATP).
- Symbol: C6H12O6 + 6O2 → 6CO2 + 6H2O.
- Anaerobic in animals: glucose → lactic acid (small ATP yield, builds up an oxygen debt).
- Anaerobic in yeast (fermentation): glucose → ethanol + CO2.
- A-Level stages: 1) Glycolysis in the cytoplasm — glucose to 2 pyruvate, net 2 ATP and 2 NADH; 2) Link reaction in the mitochondrial matrix — pyruvate decarboxylated to acetyl-CoA; 3) Krebs cycle — produces NADH, FADH2 and 1 ATP per turn; 4) Oxidative phosphorylation on the inner mitochondrial membrane — electrons from NADH/FADH2 pass down the ETC, pumping H+ to create a gradient that drives ATP synthase. Final electron acceptor is oxygen, forming water.
- Total yield aerobic ≈ 30–32 ATP per glucose; anaerobic only 2.
Key terms
- ATP
- Glycolysis
- Krebs cycle
- Link reaction
- Oxidative phosphorylation
- Electron transport chain
- Lactic acid
- Anaerobic respiration
- Mitochondrion
Respiration FAQs
Is respiration the same as breathing?+
No. Breathing (ventilation) moves air in and out of the lungs. Respiration is the chemical release of energy from glucose inside every cell — it happens in plants too.
Why is ATP useful as an energy currency?+
ATP releases a small, manageable amount of energy when hydrolysed to ADP + Pi, and can be rapidly resynthesised. It can be made and used at the exact site where energy is needed.
What causes the oxygen debt after vigorous exercise?+
Anaerobic respiration in muscle produces lactic acid, which must later be oxidised back to pyruvate using oxygen — the extra oxygen needed is the oxygen debt.
Why does the inner mitochondrial membrane have so many folds (cristae)?+
Cristae increase the surface area available for the electron transport chain and ATP synthase, maximising the rate of oxidative phosphorylation.
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