A series of chemical reactions in cells that break down glucose to release energy as ATP. Learn it step by step →
Energy, Respiration & Photosynthesis is a core part of A-Level Biology. Revise the key concepts and common mistakes below, then lock them in with the free games.
Key concepts
GlycolysisSplits 1 glucose into 2 pyruvate in the cytoplasm — net 2 ATP, 2 NADH
PhosphorylationAdding a phosphate (often from ATP) to make a molecule more reactive
Pyruvate3-carbon product of glycolysis
Link reactionPyruvate → acetyl CoA + CO₂ + NADH in mitochondrial matrix
Coenzyme ACarrier that hands the 2-carbon acetyl group to the Krebs cycle
Substrate-level phosphorylationDirect ATP synthesis from a phosphorylated substrate
Anaerobic respirationRespiration without oxygen — much lower ATP yield
Lactate fermentationPyruvate + NADH → lactate + NAD (mammalian muscle)
Ethanol fermentationPyruvate → CO2 + ethanal → ethanol + NAD (yeast)
Oxygen debtLactate later transported to liver, oxidised back to pyruvate
Respiratory quotientRQ = CO2 produced ÷ O2 consumed — varies with substrate
Lipids as fuelMore ATP per gram than carbs, but slower to release
Krebs cycleMatrix cycle starting with citrate (6C) — produces NADH, FADH2, ATP, CO2
Electron transport chainSeries of carriers on the inner mitochondrial membrane
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Learn Energy, Respiration & Photosynthesis step by step
Free interactive mini-lessons: short screens, worked examples and quick checks. Pick your exam board to open its course map, then tap L on this topic:
Questions where students often pick the tempting wrong answer — make sure you know the right one:
What is cellular respiration?✗ Respiration means breathing — inhaling oxygen and exhaling carbon dioxide.✓ A series of chemical reactions in cells that break down glucose to release energy as ATP.
Why is ATP described as an 'energy currency' rather than energy storage?✗ Cells store large amounts of ATP for emergency use.✓ ATP is constantly made and broken down — it transfers energy between reactions rather than storing large amounts for later.
What is produced in anaerobic respiration in human muscle cells?✗ Anaerobic respiration produces ethanol and CO2 in humans.✓ Lactic acid (lactate) and a small amount of ATP — no CO2 or water in this pathway.
Why is pyruvate reduced to lactate in muscles during anaerobic respiration?✗ To produce more ATP✓ To regenerate NAD so glycolysis continues
Practise Energy, Respiration & Photosynthesis — free games
Test yourself with these quick revision games for this topic:
