Aerobic and Anaerobic Respiration
Practise Aerobic and Anaerobic Respiration for GCSE Biology with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA, Edexcel and OCR. Respiration is exothermic; anaerobic respiration in muscle produces lactic acid, while in yeast it produces ethanol and carbon dioxide (fermentation).
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These worksheets and mark schemes are original, written for Virtus Academy and checked against the current AQA, Edexcel and OCR specifications. Every worksheet comes with a full mark scheme.
Topic overview
Respiration releases energy from glucose and happens continuously in every living cell. It is an exothermic reaction, transferring energy to the environment.
Aerobic respiration uses oxygen and releases much more energy per glucose molecule. The word equation is glucose + oxygen → carbon dioxide + water, and it takes place in the mitochondria.
Anaerobic respiration happens without oxygen and releases much less energy, because the glucose is only partially broken down. In muscle cells the product is lactic acid; in plants and yeast the products are ethanol and carbon dioxide, a process called fermentation used in brewing and breadmaking.
Revision notes
Aerobic respiration
Glucose + oxygen → carbon dioxide + water. In symbols: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O.
It takes place in the mitochondria and releases much more energy per glucose molecule than anaerobic respiration.
Anaerobic respiration in muscle
Glucose → lactic acid. No oxygen is used, and the glucose is only partially broken down, so far less energy is released.
Lactic acid builds up during hard exercise and causes muscle fatigue.
Fermentation
In plant cells and yeast: glucose → ethanol + carbon dioxide.
This is called fermentation and is economically important — used in brewing to produce alcohol and in breadmaking, where the carbon dioxide makes the dough rise.
Key points
- Respiration is exothermic and happens in all living cells.
- Aerobic respiration uses oxygen and releases more energy.
- Glucose + oxygen → carbon dioxide + water.
- Aerobic respiration occurs in the mitochondria.
- Anaerobic respiration in muscle produces lactic acid.
- Fermentation in yeast produces ethanol and carbon dioxide.
Worked examples
Example 1
Write the word equation for aerobic respiration. [2 marks]
Model answer
Glucose + oxygen → carbon dioxide + water. Correct reactants, glucose and oxygen (1 mark). Correct products, carbon dioxide and water (1 mark).
Example 2
Explain why anaerobic respiration releases less energy than aerobic respiration. [2 marks]
Model answer
In anaerobic respiration the glucose is only partially broken down (1 mark), so less of the energy stored in the glucose molecule is released (1 mark).
Example 3
Explain why yeast is used in breadmaking. [2 marks]
Model answer
Yeast respires anaerobically, producing carbon dioxide as well as ethanol (1 mark). The carbon dioxide forms bubbles in the dough which make it rise (1 mark).
Common mistakes
Saying respiration is breathing.
Respiration is a chemical reaction in cells; breathing is the movement of air into and out of the lungs.
Calling respiration endothermic.
It is exothermic — energy is transferred to the environment.
Giving lactic acid as the product in yeast.
Yeast produces ethanol and carbon dioxide; muscle cells produce lactic acid.
Saying anaerobic respiration produces no energy.
It produces much less, not none.
Exam tips
- Learn both anaerobic pathways and which organism uses each.
- Say exothermic and explain energy is transferred out.
- Never confuse respiration with breathing.
- Name the mitochondria as the site of aerobic respiration.
Key terms
- Aerobic respiration
- Respiration using oxygen, releasing the most energy.
- Anaerobic respiration
- Respiration without oxygen, releasing less energy.
- Lactic acid
- The product of anaerobic respiration in muscle cells.
- Fermentation
- Anaerobic respiration in yeast producing ethanol and carbon dioxide.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.