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Respiration is examined in every AQA Biology series, both as standalone questions and in combination with exercise physiology and homeostasis. It bridges Unit 3 (Bioenergetics), Unit 6 (Homeostasis), and applied contexts in sport science.

• Equation recall (1-2 marks): You must know both aerobic and anaerobic equations in word form. The aerobic word equation is: glucose + oxygen → carbon dioxide + water. Anaerobic (animals): glucose → lactic acid. Anaerobic (yeast/plants): glucose → ethanol + carbon dioxide. Confusing which organisms produce which products is a very common error.
• Explain oxygen debt (3-4 marks): One of the most frequently examined application questions. Structure your answer: anaerobic respiration during exercise → lactic acid accumulates → need extra oxygen after exercise → lactic acid transported to liver → converted back to glucose (aerobic process). Do not just say "to pay back the debt" — explain the chemical process.
• Compare aerobic and anaerobic (4 marks): Exam tables or structured questions testing your ability to compare: oxygen requirement, products, energy yield, and location (mitochondria vs cytoplasm).
• 6-mark extended response: Often asks you to explain what happens to the body during and after vigorous exercise. Link respiration type to oxygen supply, lactic acid build-up, oxygen debt, and recovery breathing rate.
• Fermentation applications: Brewing and baking questions ask you to explain the role of yeast anaerobic respiration. Know that CO2 makes bread rise and ethanol makes alcoholic drinks. The yeast is killed by the ethanol if concentrations get too high.

Common mark losses: Writing "anaerobic respiration produces CO2" in animals (it produces lactic acid). Saying "respiration is breathing." Not mentioning the liver in oxygen debt answers (it is where lactic acid is processed). Confusing exothermic (respiration) with endothermic (photosynthesis).