Topic 12 of IGCSE Biology (Cambridge 0610) is respiration. You need to state the word and symbol equations for aerobic and anaerobic respiration, say where each type occurs (mitochondria vs cytoplasm), compare aerobic and anaerobic (oxygen, products, amount of energy), and state uses of energy and what is meant by oxygen debt. This revision guide takes you through each of these with clear explanations and exam-style tips for full marks.

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What is respiration?

Respiration is the chemical process that releases energy from food (usually glucose) inside cells. The energy is used for life processes such as muscle contraction, growth, cell division, active transport, and maintaining body temperature. Respiration is not the same as breathing. Breathing (ventilation) is the movement of air in and out of the lungs (gas exchange). Respiration is the release of energy from glucose in cells. In exams, do not use “respiration” to mean breathing; use “gas exchange” or “breathing” for that.

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Aerobic respiration: equation and site

Aerobic respiration is respiration that uses oxygen. It releases more energy per molecule of glucose than anaerobic respiration. The word equation is: glucose + oxygen → carbon dioxide + water (+ energy). The symbol equation is: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O (+ energy). Aerobic respiration takes place in the mitochondria of the cell. Mitochondria have a folded inner membrane that provides a large surface area for the reactions. You may be asked to state the word or symbol equation and to state where aerobic respiration occurs (mitochondria). Do not write the equation the wrong way round: respiration uses glucose and oxygen and produces carbon dioxide and water (the opposite of photosynthesis).

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Anaerobic respiration: in humans and in yeast

Anaerobic respiration is respiration without oxygen. It releases less energy per molecule of glucose than aerobic respiration because glucose is not fully broken down. In humans (e.g. during vigorous exercise when not enough oxygen can be supplied), the word equation is: glucose → lactic acid (+ energy). Lactic acid builds up in muscles and can cause fatigue. In yeast (and some other microorganisms), the word equation is: glucose → ethanol + carbon dioxide (+ energy). This process is fermentation and is used in making bread (CO₂ makes dough rise) and alcoholic drinks (ethanol). Anaerobic respiration in both humans and yeast takes place in the cytoplasm (not in mitochondria). In exams you may be asked to state the word equation for anaerobic respiration in humans and in yeast; always name ethanol and carbon dioxide for yeast, not just “alcohol”.

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Comparing aerobic and anaerobic respiration

Feature	Aerobic	Anaerobic (humans)	Anaerobic (yeast)
Oxygen	Yes	No	No
Products	CO₂ + H₂O	Lactic acid	Ethanol + CO₂
Energy released	More per glucose molecule	Less	Less
Where	Mitochondria	Cytoplasm	Cytoplasm

You may be asked to compare aerobic and anaerobic respiration in terms of oxygen, products, amount of energy, and where they occur.

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Uses of energy from respiration

The energy released in respiration is used for: muscle contraction (movement), growth (building new cells), cell division, active transport (moving substances against a concentration gradient), maintaining body temperature (in mammals and birds), and building large molecules (e.g. proteins from amino acids). You may be asked to state three uses of energy in the body.

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Oxygen debt

After vigorous exercise, when anaerobic respiration has occurred in muscles, lactic acid has built up. The body needs extra oxygen to react with the lactic acid (to break it down or convert it back). This extra oxygen is called the oxygen debt. Breathing rate and heart rate stay high for a while after exercise so that more oxygen can be supplied to pay back this “debt” and remove or break down the lactic acid. You may be asked to explain what is meant by oxygen debt (extra oxygen needed after exercise to deal with lactic acid).

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Exam tips and command words

• State the equation: Word or symbol for aerobic; word for anaerobic (human and yeast).
• Compare: Aerobic vs anaerobic (oxygen, products, amount of energy, where it occurs).
• Explain: Why anaerobic releases less energy (glucose not fully broken down); why mitochondria are needed for aerobic.
• Describe: Where respiration occurs (aerobic in mitochondria; anaerobic in cytoplasm).

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Common mistakes to avoid

• Confusing respiration (release of energy from food) with breathing (gas exchange).
• Writing the respiration equation as photosynthesis (respiration uses glucose and O₂; produces CO₂ and H₂O).
• Saying anaerobic in yeast produces “alcohol” without naming ethanol and carbon dioxide.
• Saying anaerobic respiration occurs in mitochondria (it occurs in the cytoplasm).

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Revision checklist

• Write the word equation for aerobic respiration and state where it occurs (mitochondria).
• Write the word equations for anaerobic respiration in humans (lactic acid) and in yeast (ethanol + CO₂).
• Compare aerobic and anaerobic respiration (oxygen, products, energy, where).
• State three uses of energy in the body and explain “oxygen debt”.

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Next steps

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