AQA GCSE Biology Foundation (8461), Foundation tier · Bioenergetics › Respiration
Practise Aerobic and anaerobic respiration. 8 exam-style questions plus unlimited generated ones on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
Respiration is the exothermic reaction that transfers energy in every living cell, with oxygen (aerobic) or without it (anaerobic). Expect word equations, comparisons of aerobic and anaerobic respiration, fermentation by yeast, and data questions such as the carbon dioxide made by yeast or the temperature of germinating seeds.
Key facts
Aerobic: glucose + oxygen → carbon dioxide + water
Anaerobic in muscles: glucose → lactic acid
Anaerobic in plant cells and yeast: glucose → ethanol + carbon dioxide
Fermentation = anaerobic respiration in yeast (bread and alcoholic drinks)
Respiration is exothermic and happens continuously in all living cells
Energy is used for: building larger molecules, movement, keeping warm
Anaerobic: incomplete oxidation of glucose, so much less energy is transferred
Notes
What respiration is
Cellular respiration is an exothermic reaction that happens continuously in all living cells. It transfers the energy needed for all living processes.
Respiration is a chemical reaction in cells. It is not the same as breathing, which moves air into and out of the lungs.
Organisms need this energy for: chemical reactions that build larger molecules (such as proteins), movement (muscle contraction) and keeping warm.
Respiration can be aerobic (using oxygen) or anaerobic (without oxygen).
Aerobic respiration
Word equation: glucose + oxygen → carbon dioxide + water.
You need to recognise the formulae: C6H12O6, O2, CO2 and H2O.
Most aerobic respiration happens in the mitochondria. Glucose is completely oxidised, so a lot of energy is transferred.
Anaerobic respiration
Anaerobic respiration happens when there is not enough oxygen.
In muscle cells: glucose → lactic acid.
In plant cells and yeast cells: glucose → ethanol + carbon dioxide.
The oxidation of glucose is incomplete, so much less energy is transferred than in aerobic respiration.
Anaerobic respiration in yeast is called fermentation. It is used to make bread (carbon dioxide bubbles make the dough rise) and alcoholic drinks such as beer and wine (ethanol).
Comparing the two
diagram
Oxygen: aerobic respiration needs it; anaerobic respiration does not.
Products: aerobic makes carbon dioxide and water; anaerobic makes lactic acid (muscles) or ethanol and carbon dioxide (plants and yeast).
Energy: aerobic respiration transfers much more energy from each glucose molecule.
Both use glucose and both are exothermic.
The three ways a cell can respire glucose.
How to answer each type of question
Recall: complete a respiration equation
1 to 3 marksGrade 3
Check which type of respiration and which organism the question is about.
Muscles: lactic acid only. Yeast and plants: ethanol and carbon dioxide.
Aerobic, in any organism: carbon dioxide and water.
Example. (a) Complete the word equation for anaerobic respiration in yeast. glucose → .................... + .................... (b) Name the product of anaerobic respiration in human muscle cells.
Show the model answerHide the model answer
(a) ethanol (1); carbon dioxide (1), in either order (b) lactic acid (1)
Explain the use of yeast, with a rate calculation
2 to 3 marks per partGrade 5
Name the process: anaerobic respiration (fermentation) by yeast.
Name the product that matters: carbon dioxide for bread, ethanol for drinks.
For a rate, divide the amount of product by the time and give the unit.
Example. (a) A baker mixes yeast with flour, sugar and water and leaves the dough in a warm place. Explain why the dough rises. (b) In an experiment, some yeast produced 18 cm3 of carbon dioxide in 6 minutes. Calculate the mean rate of carbon dioxide production.
Show the model answerHide the model answer
(a) The yeast respires anaerobically / ferments the sugar (1), producing carbon dioxide (1). Bubbles of carbon dioxide are trapped in the dough, so it expands (1). (b) 18 ÷ 6 (1) = 3 cm3 per minute (1)
Don’t lose marks
Saying respiration 'makes' or 'produces' energy. Energy is transferred (released), never made.
Confusing respiration with breathing. Respiration is a chemical reaction inside cells.
Writing that anaerobic respiration in muscles produces carbon dioxide. It produces lactic acid only.
Saying anaerobic respiration releases no energy. It transfers much less energy than aerobic respiration, but not none.
Saying plants only photosynthesise. All living plant cells respire all the time.
More tips
Memory tricks
Aerobic has 'aer' as in air (oxygen). 'An-' means without, so anaerobic means without oxygen.
Legs make lactic acid; yeast makes the bread and beer products: carbon dioxide (bread rises) and ethanol (beer).
Three uses of the energy: build, move, keep warm.
Exam technique
In 'compare' questions, write paired statements: 'aerobic needs oxygen, whereas anaerobic does not'. Two separate descriptions may not score.
Always match anaerobic products to the organism: muscles (lactic acid) or plant cells and yeast (ethanol and carbon dioxide).
Use the scientific terms: 'transfers energy', 'incomplete oxidation of glucose', 'fermentation' and 'ethanol'.
Give word equations unless formulae are asked for; if you use formulae, check O2, CO2, H2O and C6H12O6.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 3
State why organisms need energyFor chemical reactions that build larger molecules, for movement and for keeping warm.
Grade 3
Write the word equation for aerobic respirationglucose + oxygen → carbon dioxide + water.
Grade 4
Write the anaerobic equations for muscles and yeastIn muscles: glucose → lactic acid. In plant cells and yeast: glucose → ethanol + carbon dioxide.
Grade 5
Describe respiration as a continuous exothermic reactionIt happens all the time in living cells and transfers energy to the surroundings, supplying the energy for living processes.
Grade 5
Describe fermentation and its usesAnaerobic respiration in yeast makes ethanol and carbon dioxide, which are used to make alcoholic drinks and bread.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
Complete the word equation for aerobic respiration. glucose + oxygen → .................... + ....................
glucose + oxygen → carbon dioxide + water
Complete the word equation for anaerobic respiration in muscles. glucose → ....................
lactic acid
Complete the word equation for anaerobic respiration in yeast. glucose → .................... + ....................
glucose → ethanol + carbon dioxide
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy5 marks
Respiration takes place continuously in all living cells.
(a) Complete the word equation for aerobic respiration. glucose + oxygen → .................... + ....................[2]
(b) In which part of a cell does most aerobic respiration take place? Tick (✓) one box.[1]
Chloroplast
Mitochondrion
Nucleus
Ribosome
(c) Respiration is an exothermic reaction. What does exothermic mean?[1]
(d) What name is given to respiration that takes place without oxygen?[1]
Show the answer and mark scheme
(a)Answer: glucose + oxygen → carbon dioxide + water
carbon dioxide
water
(b)Answer: Mitochondrion
(c)
energy is transferred to the surroundings
(d)Answer: Anaerobic respiration
anaerobic (respiration)
Question 2Medium6 marks
Human muscle cells can respire aerobically and anaerobically.
(a) Give two differences between aerobic respiration and anaerobic respiration in muscle cells.[2]
(b) Explain why anaerobic respiration transfers much less energy than aerobic respiration.[2]
(c) During a 100 m sprint, a lot of the respiration in the sprinter’s leg muscles is anaerobic. Suggest why.[2]
Show the answer and mark scheme
(a)
aerobic respiration uses oxygen; anaerobic respiration does not