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Cellular respiration, explained simply
How cells turn the glucose in food into the energy that powers everything they do.
Cellular respiration is how cells release energy from glucose and store it in ATP. Aerobic respiration uses oxygen and produces carbon dioxide and water. In eukaryotic cells most of it happens in the mitochondria.
Covers GCSE, AP and A-level. Exam boards differ slightly, so check your course specification.
C6H12O6 + 6O2 → 6CO2 + 6H2O
Aerobic respiration: glucose + oxygen → carbon dioxide + water. Released energy is used to make ATP from ADP and phosphate.The four stages
For AP, IB and A-level you need these four stages in order. Glycolysis happens in the cytoplasm. The other three happen in the mitochondria.
- 01
Glycolysis
CytoplasmGlucose (6 carbons) is split into two pyruvate molecules (3 carbons each). No oxygen is needed.
Net gain: 2 ATP and 2 NADH
- 02
Link reaction
Mitochondrial matrixEach pyruvate is converted to acetyl-CoA. A carbon is lost as carbon dioxide, and NADH is made.
Makes NADH and CO2
- 03
Krebs cycle
Mitochondrial matrixAcetyl-CoA is broken down in a cycle of reactions. Carbon dioxide is released, and electron carriers are loaded up.
Per glucose: 2 ATP equivalents, plus NADH and FADH2
- 04
Oxidative phosphorylation
Inner mitochondrial membraneNADH and FADH2 pass electrons along a chain of proteins. The energy pumps H+ across the membrane, and ATP synthase uses the flow back to make ATP. Oxygen is the final electron acceptor and forms water.
Most of the ATP
A common estimate for eukaryotic cells is about 30 to 32 ATP per glucose. The actual yield varies with the cell and conditions. Some courses use 36 to 38, so check your course specification.
The GCSE version
Many GCSE courses focus on the overall process rather than all four stages. Respiration releases energy for cell processes. Glycolysis happens in the cytoplasm; most aerobic stages in eukaryotic cells happen in the mitochondria. The word equation is:
glucose + oxygen → carbon dioxide + water (energy is released)
You also need anaerobic respiration, which happens without oxygen (for example in muscles during hard exercise). In humans, glucose is broken down to lactate, usually called lactic acid in GCSE courses. In yeast, it produces ethanol and carbon dioxide. Anaerobic respiration releases much less energy than aerobic.
Aerobic vs anaerobic respiration
This table uses the GCSE meaning of anaerobic respiration. At AP and university level, the muscle and yeast processes are called fermentation. True anaerobic respiration uses an electron transport chain with a final acceptor other than oxygen.
| Feature | Aerobic | Anaerobic |
|---|---|---|
| Oxygen needed? | Yes | No |
| Products | Carbon dioxide and water | Lactate, often called lactic acid (animals), or ethanol and carbon dioxide (yeast) |
| Energy released | A lot (about 30 to 32 ATP per glucose) | Much less (2 ATP per glucose) |
| Where | Cytoplasm and mitochondria | Cytoplasm only |
Key terms
- ATP
- The cell's energy currency. Energy is transferred to ATP, then used for processes like muscle contraction and active transport.
- Mitochondrion
- The organelle where most aerobic respiration happens. Cells with high energy demands, like muscle cells, have many.
- Aerobic
- Needing oxygen.
- Anaerobic
- Without oxygen.
- NADH
- An electron carrier that carries energy from earlier stages to the electron transport chain.
- ATP synthase
- The enzyme that makes ATP using the flow of H+ back across the inner mitochondrial membrane.
Common mistakes
Respiration is not breathing. Breathing moves air in and out of the lungs. Respiration releases energy from glucose inside cells.
Plants respire too. They respire all the time, not only at night. In the light they also photosynthesise, so photosynthesis may hide it.
Energy isn't “made”. It's transferred from glucose to ATP (and some is lost as heat). Energy can't be created.
Oxygen doesn't “burn” glucose directly. It's the final electron acceptor at the end of the electron transport chain.
A worked example
This is the kind of answer SnapBio gives you when you snap a question: the answer first, then the steps.
Why do muscle cells contain many mitochondria?
How to get there
- 1
Muscle cells contract, which needs a lot of energy in the form of ATP.
- 2
Most ATP is made in the mitochondria by aerobic respiration.
- 3
So cells with a high energy demand contain more mitochondria.
Check yourself
Where does glycolysis happen?
Glycolysis happens in the cytoplasm and doesn't need oxygen.
What is the final electron acceptor in the electron transport chain?
Oxygen accepts electrons at the end of the chain and combines with H+ to form water.
What does anaerobic respiration make in human muscle cells?
Human muscle cells make lactate, called lactic acid in many GCSE courses. Yeast makes ethanol and carbon dioxide. The net 2 ATP comes from glycolysis.
FAQ
What is the equation for cellular respiration?
C6H12O6 + 6O2 → 6CO2 + 6H2O. Energy released by these reactions is used to make ATP from ADP and phosphate.
Where does cellular respiration happen?
Glycolysis happens in the cytoplasm. The link reaction, Krebs cycle and electron transport chain happen in the mitochondria (in eukaryotic cells).
How much ATP does cellular respiration make?
About 30 to 32 ATP per glucose is a common estimate for aerobic respiration in eukaryotic cells, but the yield varies. Some courses use 36 to 38. Glycolysis followed by fermentation gives a net 2 ATP per glucose, all from glycolysis.
What is the difference between respiration and photosynthesis?
Respiration releases energy from glucose and produces carbon dioxide and water. Photosynthesis stores energy in glucose using carbon dioxide, water and light. They are roughly opposite processes. See photosynthesis explained.
Sources and further reading
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