Aerobic and anaerobic respiration — AQA GCSE Biology
Test yourself on Aerobic and anaerobic respiration with AQA GCSE practice questions.
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Aerobic and anaerobic respiration explained
Cellular respiration is the set of enzyme-controlled reactions that transfer energy from glucose and other respiratory substrates to ATP and heat.
Read the full explanation
It is exothermic because the energy released when bonds form in the products is greater than the energy needed to break bonds in the reactants, so the overall reaction transfers energy to the surroundings. Respiration occurs continuously in all living cells, including plant cells, because the energy it transfers is needed for processes such as active transport, muscle contraction, protein synthesis and cell division. In aerobic respiration, glucose and oxygen are reactants and carbon dioxide and water are products. Anaerobic respiration occurs without oxygen and releases less energy per glucose molecule. The continuous nature means that even resting cells respire, and the rate increases when energy demand rises, for example during exercise.
The energy transferred supplies all the energy needed for living processes.
The energy transferred by respiration is essential for all living processes. It is a continuous process occurring in every living cell. In animals, this energy is used for muscle contraction, enabling movement. In mammals and birds, respiratory energy is used to maintain a steady body temperature in colder surroundings. Across all organisms, the energy transferred is used to build larger molecules from smaller ones, such as synthesising proteins from amino acids. In plants, it also powers the active transport of mineral ions, like nitrates, from the soil into root hair cells. Respiration does not 'create' energy; it transfers energy from chemical stores in glucose to power these vital cellular activities.
Respiration in cells can take place aerobically (using oxygen) or anaerobically (without oxygen), to transfer energy.
Cells can transfer energy from glucose by two pathways. Aerobic respiration takes place with oxygen, occurs mainly in mitochondria, and completely breaks down glucose to carbon dioxide and water, transferring a relatively large amount of energy per glucose molecule. Anaerobic respiration takes place without oxygen, occurs in the cytoplasm, and partially breaks down glucose. In animals and some other organisms, glucose is converted to lactic acid, which can cause muscle fatigue and an oxygen debt that must be repaid after exercise. In plants and yeast, anaerobic respiration produces ethanol and carbon dioxide; in yeast this is fermentation, used in brewing and bread-making. Both pathways transfer energy, but anaerobic respiration transfers less energy per glucose molecule because glucose is not fully oxidised.
Students should be able to compare the processes of aerobic and anaerobic respiration with regard to the need for oxygen, the differing products and the relative amounts of energy transferred.
Respiration transfers energy from glucose in cells. Aerobic respiration needs oxygen and occurs mainly in mitochondria; glucose and oxygen react to form carbon dioxide and water, transferring a relatively large amount of energy per glucose molecule. Anaerobic respiration occurs without oxygen, so it suits intense activity when oxygen supply is limited. In human muscle cells glucose is converted to lactic acid, transferring much less energy per glucose molecule. In plant and yeast cells glucose is converted to ethanol and carbon dioxide. Comparing the two processes means stating the oxygen requirement, naming the products, and judging energy transfer: aerobic releases more energy per glucose molecule than anaerobic. For example, a sprinter relies partly on anaerobic respiration, whereas steady jogging is mainly aerobic.
Organisms need energy for: • chemical reactions to build larger molecules • movement • keeping warm.
The energy transferred by respiration is not used only for movement. It powers chemical reactions that build larger molecules from smaller ones, such as joining amino acids into proteins or glucose units into starch and cellulose. It also enables movement, including muscle contraction, cilia beating and the movement of substances across membranes. In mammals and birds, some energy is transferred to the surroundings to keep the body at a constant warm temperature, which supports enzyme activity. For example, a growing plant uses energy to build cellulose for cell walls, a running mammal uses energy for muscle contraction, and a resting bird uses energy to maintain its body temperature. All three uses depend on respiration supplying energy in the form of ATP.
Anaerobic respiration in plant and yeast cells is represented by the equation: glucose ethanol + carbon dioxide Anaerobic respiration in yeast cells is called fermentation and has economic importance in the manufacture of bread and alcoholic drinks.
In plant and yeast cells, anaerobic respiration converts glucose into ethanol and carbon dioxide. The word equation is glucose → ethanol + carbon dioxide. In yeast cells this process is called fermentation. Fermentation has economic importance because it is used in baking and in making alcoholic drinks. In bread making, yeast ferments sugars in dough, and the carbon dioxide produced forms bubbles that make the dough rise; the ethanol evaporates during baking. In brewing and winemaking, yeast ferments sugars from grain, grapes or other plant material, and the ethanol produced is the alcohol in the drink, while carbon dioxide may be collected or released. The process transfers less energy per glucose molecule than aerobic respiration.
Your focus
- Define cellular respiration as an exothermic reaction occurring continuously in living cells.
- Explain why respiration is described as exothermic in terms of bond breaking and bond forming.
- Give examples of living processes that use the energy transferred by respiration.
Show all 18 objectives
- State that respiration transfers energy for all living processes and does not create energy.
- List three main uses of the energy transferred by respiration in animals and plants.
- Explain how mammals and birds use respiratory energy to maintain their body temperature.
- State the oxygen requirement, location and products of aerobic and anaerobic respiration.
- Compare the relative energy transferred by aerobic and anaerobic respiration.
- Explain the role of anaerobic respiration in muscle fatigue and in fermentation by yeast.
- State the oxygen requirement for aerobic and anaerobic respiration.
- Identify the products of aerobic respiration and of anaerobic respiration in muscle, plant and yeast cells.
- Compare the relative amounts of energy transferred by aerobic and anaerobic respiration per glucose molecule.
- List the three main uses of energy in organisms.
- Give a named example of a chemical reaction that builds a larger molecule.
- Explain why keeping warm is an energy-requiring process in mammals and birds.
- Write the word equation for anaerobic respiration in plant and yeast cells.
- Define fermentation as anaerobic respiration in yeast cells.
- Explain how fermentation is used in bread making and in the manufacture of alcoholic drinks.
Aerobic and anaerobic respiration exam tips
Marking Points
- Respiration is a series of enzyme-controlled reactions that release energy from glucose and other substrates.
- It is exothermic because the overall reaction transfers energy to the surroundings, often as heat.
- The energy transferred is used to make ATP, which supplies energy for cellular processes.
- Respiration occurs continuously in all living cells, not only in animals or only during exercise.
- Aerobic respiration uses oxygen and releases more energy per glucose molecule than anaerobic respiration.
- The word equation for aerobic respiration is glucose + oxygen → carbon dioxide + water, with energy transferred.
- Respiration transfers energy that is essential for all living processes in cells.
- Energy from respiration is used to build larger molecules from smaller ones, such as making proteins from amino acids.
- In animals, the energy transferred by respiration is required for muscle contraction to allow movement.
- In mammals and birds, respiratory energy is used to keep their bodies warm and maintain a constant internal temperature.
- In plants, energy from respiration is needed for the active transport of mineral ions into root hair cells.
- Aerobic respiration requires oxygen and occurs mainly in mitochondria.
- Aerobic respiration completely oxidises glucose to carbon dioxide and water, transferring a relatively large amount of energy.
- Anaerobic respiration occurs without oxygen and takes place in the cytoplasm.
- In animals, anaerobic respiration converts glucose to lactic acid and transfers less energy per glucose molecule.
- In plants and yeast, anaerobic respiration produces ethanol and carbon dioxide.
- Anaerobic respiration in muscle leads to oxygen debt, which is repaid by continued fast breathing and heart rate after exercise.
- Aerobic respiration requires oxygen; anaerobic respiration does not require oxygen.
- Aerobic respiration in cells uses glucose and oxygen and produces carbon dioxide and water.
- Anaerobic respiration in human muscle cells produces lactic acid from glucose.
- Anaerobic respiration in plant and yeast cells produces ethanol and carbon dioxide from glucose.
- Aerobic respiration transfers a relatively larger amount of energy per glucose molecule than anaerobic respiration.
- A valid comparison links all three factors: oxygen need, products formed and relative energy transferred.
- Energy from respiration is used in chemical reactions that build larger molecules from smaller ones.
- Examples of building larger molecules include making proteins from amino acids and making starch or cellulose from glucose.
- Energy is needed for movement, including muscle contraction and other cellular movement.
- Energy is transferred to keep warm in mammals and birds, maintaining a suitable body temperature.
- The three named uses are chemical reactions to build larger molecules, movement and keeping warm.
- Anaerobic respiration in plant and yeast cells produces ethanol and carbon dioxide from glucose.
- The word equation is glucose → ethanol + carbon dioxide.
- Anaerobic respiration in yeast cells is called fermentation.
- Fermentation is used in bread making, where carbon dioxide makes dough rise.
- Fermentation is used in the manufacture of alcoholic drinks, where ethanol is the alcohol produced.
- Fermentation has economic importance because it is used in food and drink production.
Examiner Tips
- 💡Use the term 'exothermic' and explain it in terms of energy transferred to the surroundings, not just 'gives out heat'.
- 💡When comparing aerobic and anaerobic respiration, state the oxygen requirement and the relative energy released.
- 💡Link continuous respiration to a specific process such as active transport in root hair cells to show understanding.
- 💡Always use the phrase 'transfers energy' rather than 'makes energy' or 'produces energy' when defining respiration.
- 💡Be prepared to list three specific uses of energy from respiration: building larger molecules, movement, and keeping warm.
- 💡Compare the two pathways using oxygen requirement, products, location and relative energy transferred.
- 💡Use the correct products for the named organism: lactic acid in animals, ethanol and carbon dioxide in plants and yeast.
- 💡When explaining oxygen debt, link lactic acid breakdown to the need for continued oxygen supply after exercise.
- 💡Use a table with rows for oxygen need, products and relative energy transferred to structure a comparison answer.
- 💡Name the cell type whenever you describe anaerobic respiration, because the products differ between muscle cells and plant or yeast cells.
- 💡Use comparative wording such as greater or smaller when judging energy transfer, rather than saying aerobic respiration makes energy.
- 💡Give a named example for each use, such as protein synthesis, muscle contraction and maintaining body temperature.
- 💡Link each use back to energy transferred by respiration rather than to food directly.
- 💡Use the phrase build larger molecules when describing synthesis, and support it with a specific molecule such as protein or cellulose.
- 💡Learn the word equation exactly as glucose → ethanol + carbon dioxide, including both products.
- 💡When explaining bread making, name carbon dioxide as the gas that makes dough rise.
- 💡When explaining alcoholic drinks, name ethanol as the alcohol produced by fermentation.
Common Mistakes
- Describing respiration as breathing; correction: breathing is ventilation, whereas respiration is the chemical process in cells.
- Saying respiration is endothermic because it needs glucose; correction: it is exothermic overall because it transfers energy to the surroundings.
- Believing respiration stops when a person is resting; correction: it is continuous in living cells, though the rate can change.
- Stating that respiration 'produces' or 'creates' energy; correction: energy cannot be created; respiration 'transfers' energy from glucose.
- Believing that only animals respire; correction: plants also respire continuously to transfer energy for processes like building molecules and active transport.
- Including A-level concepts like ATP or sliding filaments in GCSE answers; correction: stick to GCSE concepts such as building larger molecules, movement, and keeping warm.
- Writing that anaerobic respiration produces carbon dioxide and water in human muscle; correction: human muscle produces lactic acid, while plants and yeast produce ethanol and carbon dioxide.
- Stating that anaerobic respiration releases the same energy as aerobic respiration; correction: it transfers less energy per glucose molecule because glucose is only partially broken down.
- Placing anaerobic respiration in the mitochondria; correction: it occurs in the cytoplasm, whereas aerobic respiration occurs mainly in mitochondria.
- Saying anaerobic respiration produces no energy at all; correct this by stating that energy is still transferred, but less per glucose molecule than in aerobic respiration.
- Confusing the products of anaerobic respiration in muscle cells with those in yeast cells; correct this by linking muscle cells to lactic acid and yeast cells to ethanol and carbon dioxide.
- Writing that aerobic respiration produces lactic acid; correct this by stating that lactic acid is a product of anaerobic respiration in animal muscle cells.
- Thinking energy is used only for movement; correct this by naming building larger molecules and keeping warm as additional uses.
- Confusing building larger molecules with breaking down food; correct this by stating that respiration energy drives synthesis reactions such as protein formation.
- Assuming all organisms use energy to keep warm; correct this by linking keeping warm mainly to mammals and birds, while all organisms use energy for synthesis and movement.
- Writing that fermentation in yeast produces lactic acid; correct this by stating that yeast fermentation produces ethanol and carbon dioxide.
- Omitting carbon dioxide from the equation; correct this by writing glucose → ethanol + carbon dioxide.
- Confusing the role of ethanol and carbon dioxide in bread making; correct this by stating that carbon dioxide makes dough rise while ethanol evaporates during baking.