At a glance
- Cambridge section
- 0610 topic 12 (12.1 to 12.3)
- Edexcel section
- 4BI1 statements 2.34 to 2.39
- Aerobic (balanced)
- C6H12O6 + 6O2 -> 6CO2 + 6H2O
- Yeast (balanced, Cambridge Supplement)
- C6H12O6 -> 2C2H5OH + 2CO2
- Where
- Aerobic respiration mainly in mitochondria
Aerobic and anaerobic respiration compared
| Feature | Aerobic | Anaerobic (muscles) | Anaerobic (yeast, plants) |
|---|---|---|---|
| Oxygen needed | Yes | No | No |
| Products | Carbon dioxide and water | Lactic acid | Ethanol and carbon dioxide |
| Energy released per glucose | Much more | Much less | Much less |
| When it happens | All the time in most cells | Vigorous exercise when oxygen supply cannot keep up | Yeast in low oxygen; plant roots in waterlogged soil |
| Use to people | Bread making (carbon dioxide), brewing (ethanol) |
Glucose is only partly broken down in anaerobic respiration, which is why less energy is released.
Key ideas board by board
Cambridge 0610 begins with the uses of energy (12.1.1): muscle contraction, protein synthesis, cell division, active transport, growth, the passage of nerve impulses and keeping a constant body temperature. Core candidates give the word equations; Supplement candidates add the balanced equations for aerobic respiration and for anaerobic respiration in yeast, and explain how the oxygen debt is removed after exercise: the heart rate stays fast to carry lactic acid in the blood to the liver, breathing stays deep and fast to supply oxygen, and the liver respires the lactic acid aerobically.
Edexcel 4BI1 states that respiration produces ATP and that ATP provides energy for cells (2.34, 2.35). Students compare aerobic and anaerobic respiration, give the word and balanced equations for aerobic respiration, and the word equations for anaerobic respiration in plants and animals. The required practical (2.39) shows carbon dioxide and heat released by respiring seeds or other organisms. Edexcel also lists yeast practicals under biotechnology (5.6).
Cambridge Core 12.1.2 asks students to investigate the effect of temperature on respiration in yeast, usually by measuring the gas produced.
Worked example: rate of respiration in yeast
Yeast in sugar solution produces 12 cm^3 of carbon dioxide in 5 minutes at 25 °C and 30 cm^3 in 5 minutes at 35 °C. Rate at 25 °C = 12 ÷ 5 = 2.4 cm^3 per minute. Rate at 35 °C = 30 ÷ 5 = 6.0 cm^3 per minute.
Percentage increase = (6.0 - 2.4) / 2.4 x 100 = 150%. Explanation: the enzymes controlling respiration have more kinetic energy at 35 °C, so there are more frequent effective collisions with substrate. At much higher temperatures, the enzymes denature and the rate falls.
Germinating seeds practical: a flask of germinating seeds and a control flask of boiled (dead) seeds, both rinsed in disinfectant to kill microorganisms, each with a thermometer and plugged with cotton wool. The live seeds warm up; the dead seeds do not. Limewater or hydrogencarbonate indicator shows carbon dioxide released.
Common mistakes
- Confusing respiration with breathing (ventilation). Breathing moves air; respiration is a chemical reaction in every living cell.
- Writing that respiration "produces" or "creates" energy. It releases energy.
- Saying plants only respire at night.
- Putting carbon dioxide in the muscle anaerobic equation. Lactic acid is the only product.
- Writing oxygen debt as "lack of oxygen". It is the extra oxygen needed after exercise to break down the lactic acid.
- Forgetting the control flask, or not killing microorganisms on the seeds, in the heat practical.
Exam technique
Equations are easy marks. Write the reactants and products in words, then symbols, and check the atoms balance: C6H12O6 + 6O2 -> 6CO2 + 6H2O has 6 carbon, 12 hydrogen and 18 oxygen atoms on each side. For "explain why muscles get tired" questions, chain the points: oxygen cannot be supplied fast enough, so anaerobic respiration takes place, lactic acid builds up, causing fatigue and an oxygen debt.
When data compares two organisms or temperatures, quote numbers with units and calculate a difference or percentage if it helps the comparison. Examiners reward the processed figure, not just the raw readings.
How one-to-one lessons help
Respiration overlaps with enzymes, photosynthesis, breathing and exercise physiology, so students often blur the boundaries. A tutor separates the ideas with short diagnostic questions, drills the equations until they are automatic, and works through practical data questions on the whiteboard, marking against real mark schemes from the student's board.
Self-check
- Give the word and balanced equations for aerobic respiration.
- Give the word equations for anaerobic respiration in muscles and in yeast.
- List five uses of energy in the body.
- Explain oxygen debt and how it is removed.
- Describe a practical showing heat released by respiring seeds, with a control.
- Calculate a rate and a percentage change from gas volumes.
Common questions
Is ATP on the Cambridge syllabus?
The Cambridge 0610 respiration statements do not name ATP; they describe respiration as releasing energy. Edexcel 4BI1 statements 2.34 and 2.35 do require ATP.
Do I need the balanced equation for anaerobic respiration?
Cambridge Supplement candidates need C6H12O6 -> 2C2H5OH + 2CO2 for yeast. Edexcel requires only word equations for anaerobic respiration.
Is anaerobic respiration in plants the same as in yeast?
Yes for IGCSE purposes: glucose -> ethanol + carbon dioxide. Edexcel asks for anaerobic respiration in plants and in animals.
How much do LiveTutor biology lessons cost?
$15 a lesson, with a weekly plan of 1 to 5 lessons billed monthly. Lessons are one to one, 60 minutes, online, and the first lesson is free.
Why does respiration release less energy without oxygen?
Glucose is only partly broken down. Much of its chemical energy stays in the lactic acid or ethanol.
Sources
Dates and figures on this page come from these official and published sources. Always confirm deadlines on the official page before acting on them.