What respiration is: and the verb that matters

Define respiration as the chemical reactions in cells that break down glucose (nutrient molecules) to release energy. The key word is release: energy is released from glucose, never 'produced', 'made' or 'created'. This is the most common single mark lost in the whole topic.

Distinguish respiration (a chemical reaction in cells releasing energy) from breathing/ventilation (moving air in and out of the lungs) and from gas exchange (diffusion of gases). Examiners regularly test whether you confuse them. The released energy is used for muscle contraction, growth, active transport, maintaining body temperature in mammals, and making large molecules from smaller ones. Respiration happens in all living cells, all the time, mainly in the mitochondria for the aerobic stage. Linking back to cell structure.

Aerobic respiration: the equation in full

Aerobic respiration is the release of a relatively large amount of energy by the breakdown of glucose using oxygen. Learn the word equation: glucose + oxygen → carbon dioxide + water (+ energy released).

(Extended only) The balanced symbol equation is C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O. Note 'energy' is written as 'energy released' beside the equation, not as a product molecule. Examiners may accept it written after the arrow but never want it shown as a chemical formula. Aerobic respiration releases far more energy per glucose molecule than anaerobic because the glucose is fully broken down. Most respiration in human cells is aerobic; it occurs in the mitochondria. The carbon dioxide produced is carried in the plasma and removed at the lungs, connecting to transport in humans.

Anaerobic respiration: humans vs yeast

Anaerobic respiration releases energy from glucose without oxygen, and releases much less energy because the glucose is not fully broken down. The products differ by organism. A guaranteed exam distinction:

  • In humans (muscles during hard exercise): glucose → lactic acid (+ a little energy).
  • In yeast and plants: glucose → alcohol (ethanol) + carbon dioxide (+ a little energy). This is fermentation.

(Extended only) The balanced equation for yeast: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂. The most common error is giving the yeast products (alcohol + CO₂) for human muscle, or vice versa. State clearly that anaerobic respiration releases less energy than aerobic because glucose is only partially broken down. Yeast fermentation links to bread-making (CO₂ makes dough rise) and brewing (alcohol). Frequent application contexts.

Oxygen debt and recovery (Extended)

(Extended only) During vigorous exercise, muscles cannot get enough oxygen, so they respire anaerobically, producing lactic acid. The lactic acid builds up and must be removed afterwards. This creates an oxygen debt: the extra oxygen needed after exercise to break down the accumulated lactic acid.

The mark-scheme reasoning: after exercise you continue to breathe deeply and rapidly to take in extra oxygen; this oxygen is used to oxidise/break down the lactic acid (transported in the blood to the liver) into carbon dioxide and water. This is why your heart rate and breathing stay raised after stopping. A common slip is saying the body 'removes lactic acid' without explaining that oxygen is needed to break it down, or forgetting that lactic acid is transported to the liver. Build-up of lactic acid also causes muscle fatigue and cramp.

Investigating respiration (practical marks)

Two classic practicals appear. First, showing germinating seeds or small organisms respire: they are kept in a sealed flask with an indicator. Hydrogencarbonate indicator turns from red towards yellow as carbon dioxide is produced (the CO₂ makes the solution more acidic). Limewater turns milky/cloudy with carbon dioxide. Examiners want you to include a control (e.g. Boiled/dead seeds or glass beads) to show the change is due to living, respiring organisms.

Second, measuring rate using a respirometer. Soda lime/potassium hydroxide absorbs the CO₂ so any volume change reflects oxygen used. State control variables (temperature, mass of organisms, time). The temperature dependence links to enzymes, since respiration is enzyme-controlled. Forgetting the dead-organism control is a frequent loss. To practise these stepwise answers, book a trial class.

Key terms to connect with this topic

Use these definitions inside explanations and questions rather than memorising them as isolated sentences.

  • Aerobic: A process that requires oxygen. Aerobic respiration uses oxygen to completely break down glucose into carbon dioxide and water, releasing a large amount of energy. The equation is: glucose + oxygen produces carbon dioxide + water (plus energy released).
  • Aerobic respiration: The chemical reactions in cells that break down glucose using oxygen to release energy. Word equation: glucose + oxygen → carbon dioxide + water.
  • Anaerobic: A process that occurs without oxygen. In anaerobic respiration, glucose is only partially broken down, releasing less energy than aerobic respiration. In animals, glucose produces lactic acid. In yeast, glucose produces ethanol and carbon dioxide (fermentation).
  • Anaerobic respiration: The chemical reactions in cells that break down glucose without oxygen, releasing much less energy than aerobic respiration. In muscles: glucose → lactic acid. In yeast: glucose → alcohol + carbon dioxide.
  • Ethanol: One of the products of anaerobic respiration in yeast and some plant cells. The equation for anaerobic respiration in yeast is: glucose produces ethanol plus carbon dioxide. This process is used in brewing and baking.
  • Lactic acid: The waste product of anaerobic respiration in animals. When muscles do not receive enough oxygen during intense exercise, glucose is partially broken down to lactic acid, which accumulates in muscles causing fatigue and cramp.
  • Oxygen debt: The amount of extra oxygen needed after vigorous exercise to break down the lactic acid that accumulated during anaerobic respiration. This is why breathing rate and heart rate remain elevated after exercise stops.

Browse the full IGCSE Biology glossary when a related term needs checking.

Apply the topic in practical work

These practical guides show how the Biology appears in methods, variables, measurements, graphs and evaluation.

  • How to Plan a Biology Investigation for Paper 6: A strong investigation plan states a range of independent-variable values, explains exactly how the dependent variable is measured, controls key conditions, repeats each value, calculates a mean and says how results will answer the question.
  • Investigating Conditions Needed for Germination: Seeds need three conditions to germinate: water (to activate enzymes and expand cells), oxygen (for aerobic respiration to provide energy), and a suitable temperature (for enzyme activity). Light is NOT required for germination.
  • Investigating the Effect of Exercise on Heart Rate: Exercise increases heart rate because muscles need more oxygen and glucose for increased aerobic respiration. Heart rate rises quickly during exercise and gradually returns to resting rate during recovery: fitter individuals recover faster.
  • Investigating the Rate of Respiration: The rate of aerobic respiration can be measured using a respirometer, which tracks oxygen uptake. As organisms respire, they absorb O₂ and release CO₂. A CO₂ absorber (e.g. Soda lime or KOH) removes the CO₂, so any volume change in the gas represents oxygen consumed.