IGCSE Biology: Respiration Practice Questions
Respiration is the chemical reaction in all living cells that breaks down nutrient molecules to release energy. Aerobic respiration uses oxygen and releases much more energy per glucose molecule than anaerobic respiration, which does not.
Respiration is a guaranteed appearance on Cambridge IGCSE Biology 0610 papers, and it is one of the few topics where a single memorised equation reliably earns marks. The risk is the wording. Respiration is not breathing, and energy is released rather than produced. The four questions below test the equations, the comparison between aerobic and anaerobic pathways, and the standard practical.
What you need to know for Respiration
- Aerobic respirationThe release of energy from glucose using oxygen. Word equation: glucose + oxygen produces carbon dioxide + water. Balanced equation: C6H12O6 + 6O2 gives 6CO2 + 6H2O.
- Anaerobic respiration in muscleGlucose is broken down to lactic acid without oxygen. It releases far less energy per glucose molecule and builds an oxygen debt.
- Anaerobic respiration in yeastGlucose produces alcohol (ethanol) plus carbon dioxide. This is fermentation, used in brewing and bread making.
- Oxygen debtThe extra oxygen needed after exercise to break down the lactic acid that built up in muscles. This is why breathing rate stays high after you stop running.
- Site of respirationAerobic respiration takes place mainly in the mitochondria. Cells that need a lot of energy, such as muscle and sperm cells, contain many mitochondria.
- Uses of the energy releasedMuscle contraction, protein synthesis, cell division, active transport, growth, and maintaining a constant body temperature in mammals and birds.
IGCSE Biology Respiration questions and answers
4 exam-style questions written to the 0610 syllabus. Try each one on paper first, then open the worked answer to check your method against the marks.
Write the balanced chemical equation for aerobic respiration and state the part of the cell where most of it takes place.
Show the worked answer
- One molecule of glucose reacts with six molecules of oxygen.
- The products are six molecules of carbon dioxide and six molecules of water.
- Check the balance: 6 carbon, 12 hydrogen and 18 oxygen atoms on each side.
- Most aerobic respiration takes place in the mitochondria.
Explain why anaerobic respiration in muscle releases much less energy than aerobic respiration, and describe what happens to the lactic acid after exercise.
Show the worked answer
- In anaerobic respiration glucose is broken down incompletely into lactic acid.
- The lactic acid molecule still contains a large amount of chemical energy, so far less energy is released per glucose molecule than when glucose is fully broken down to carbon dioxide and water.
- Lactic acid builds up in the muscles and causes fatigue and cramp.
- After exercise the lactic acid is carried in the blood to the liver, where extra oxygen is used to break it down. This extra oxygen is the oxygen debt, which is why deep breathing continues after exercise stops.
A student places germinating peas in a vacuum flask with a thermometer, and boiled dead peas in an identical flask. After two days the flask with germinating peas is several degrees warmer. Explain this result and state why the second flask is needed.
Show the worked answer
- Germinating peas are alive and respiring rapidly to fuel growth.
- Respiration releases energy, and some of that energy is transferred to the surroundings as thermal energy, so the temperature inside the flask rises.
- The vacuum flask reduces heat loss so the rise can be measured.
- The boiled peas are dead and cannot respire, so any temperature change in that flask is not caused by respiration. It acts as a control and shows the rise is due to living tissue.
State two differences between aerobic and anaerobic respiration in humans.
Show the worked answer
- Difference 1: aerobic respiration requires oxygen, anaerobic respiration does not.
- Difference 2: aerobic respiration releases much more energy per glucose molecule.
- A third acceptable difference: the products differ. Aerobic produces carbon dioxide and water, anaerobic in muscle produces lactic acid.
Common mistakes in this topic
- Confusing respiration with breathing. Breathing is gas exchange and ventilation. Respiration is a chemical reaction inside cells.
- Saying respiration produces energy. Energy is released from glucose, not created.
- Writing lactic acid as a product of yeast. Yeast produces ethanol and carbon dioxide.
- Forgetting to balance the chemical equation. Unbalanced equations lose the balancing mark every time.
- Claiming anaerobic respiration produces no energy. It produces some, just far less.
Exam tips
- Learn all four equations cold: aerobic word and balanced, anaerobic in muscle, anaerobic in yeast.
- If a question gives you data from an exercise experiment, look for the delay between stopping exercise and breathing rate returning to normal. That is the oxygen debt.
- Any experiment on respiration rate needs a control with dead or boiled material. Say what the control proves.
- Link the number of mitochondria to energy demand whenever a cell structure is described.
- Watch the units in rate questions. Carbon dioxide produced per minute is not the same as total carbon dioxide produced.
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Respiration FAQs
What is the word equation for aerobic respiration?
Glucose plus oxygen produces carbon dioxide plus water, releasing energy. The balanced chemical equation is C6H12O6 + 6O2 giving 6CO2 + 6H2O. Energy is released by the reaction and is not written as a product in the balanced equation.
Is respiration the same as breathing?
No. Breathing, properly called ventilation, is the movement of air into and out of the lungs. Respiration is the chemical reaction inside every living cell that releases energy from glucose. Using the two words interchangeably is one of the most common reasons IGCSE answers lose marks.
What causes muscle cramp during hard exercise?
During intense exercise the muscles cannot get oxygen fast enough, so they respire anaerobically and produce lactic acid. Lactic acid accumulates in the muscle and causes fatigue and cramp. It is removed after exercise when extra oxygen is available to break it down in the liver.
Do plants respire?
Yes. Plants respire continuously, day and night, in every living cell. During daylight photosynthesis usually happens faster than respiration, so the plant takes in carbon dioxide overall, which is why students often assume plants only photosynthesise.
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Written to the published Cambridge IGCSE Biology (0610) syllabus. Check your school entry code and syllabus year, because Core and Extended candidates are assessed on different content. Last reviewed 2026-08-12.