Respiration — AQA GCSE Combined Science: Trilogy
Aerobic and anaerobic respiration, how the body responds to exercise, oxygen debt, and metabolism as the sum of all reactions in a cell or body.
Revision notes
3 short notes, in the order of the specification. Each one in short:
Respiration is an exothermic reaction in living cells that transfers energy for living processes. Aerobic respiration uses oxygen: glucose + oxygen → carbon dioxide + water. Anaerobic respiration happens without oxygen and transfers much less energy, because glucose is only partly oxidised. In muscles it gives lactic acid; in plant and yeast cells it gives ethanol and carbon dioxide.
During exercise, heart rate, breathing rate and breath volume increase to supply the muscles with more oxygenated blood for respiration. If the oxygen supply is insufficient, muscles respire anaerobically. The incomplete oxidation of glucose causes a build-up of lactic acid and creates an oxygen debt. After long periods of vigorous activity, muscles become fatigued and stop contracting efficiently.
Metabolism is the sum of all the reactions in a cell or the body. It uses energy transferred by respiration for enzyme controlled processes that make new molecules. It includes converting glucose to starch, glycogen and cellulose, forming lipids from one glycerol and three fatty acids, making amino acids and proteins, respiration, and breaking down excess proteins to form urea.
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Quick check questions
Write the word equation for aerobic respiration.
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glucose + oxygen → carbon dioxide + water
What are the products of anaerobic respiration in yeast?
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Ethanol and carbon dioxide.
Why does anaerobic respiration transfer much less energy than aerobic respiration?
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Because the oxidation of glucose is incomplete.
Name the three things that increase during exercise to supply muscles with more oxygenated blood.
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Heart rate, breathing rate and breath volume.
What is metabolism?
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The sum of all the reactions in a cell or the body.
Frequently asked questions
Why is respiration an exothermic reaction?
Respiration is exothermic because energy is transferred from the reaction to the surroundings. It happens continuously in living cells, and the energy transferred supplies all the energy needed for living processes, such as chemical reactions to build larger molecules, movement and keeping warm. Both aerobic and anaerobic respiration transfer energy in this way.
How is respiration different from breathing?
Respiration is a chemical reaction that happens inside living cells to transfer energy from glucose. Breathing moves air into and out of the lungs, which supplies the oxygen used in aerobic respiration. Respiration happens all the time in every living cell, including plant cells, not only at night, and it can also happen anaerobically without oxygen.
What is anaerobic respiration in yeast?
Anaerobic respiration in yeast is respiration without oxygen, in which glucose is converted into ethanol and carbon dioxide. It is called fermentation, and it has economic importance in the manufacture of bread and alcoholic drinks. Much less energy is transferred than in aerobic respiration, because the oxidation of glucose is incomplete.
What is oxygen debt?
Higher tier Oxygen debt is the amount of extra oxygen the body needs after exercise to react with the accumulated lactic acid and remove it from the cells. It builds up when muscles respire anaerobically because the oxygen supply is insufficient. Blood carries the lactic acid to the liver, where it is converted back into glucose.
Why do you keep breathing hard after exercise?
Higher tier You keep breathing hard after exercise to repay the oxygen debt. During vigorous exercise, muscles respire anaerobically and lactic acid builds up. Afterwards, extra oxygen is needed to react with the lactic acid and remove it from the cells, so your breathing rate and heart rate stay high for a while after you stop.
Written and checked against the AQA GCSE Combined Science (8464) specification · Updated October 2026