Practise Aerobic and anaerobic respiration. 14 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.
Grade by grade
What you need to be able to do, from the first marks up to the top grade.
3
State why organisms need energyFor chemical reactions that build larger molecules, for movement and for keeping warm.
3
Write the word equation for aerobic respirationglucose + oxygen → carbon dioxide + water.
4
Write the anaerobic equations for muscles and yeastIn muscles: glucose → lactic acid. In plant cells and yeast: glucose → ethanol + carbon dioxide.
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.
5
Describe fermentation and its usesAnaerobic respiration in yeast makes ethanol and carbon dioxide, which are used to make alcoholic drinks and bread.
6
Compare aerobic and anaerobic respirationCompare the need for oxygen, the products and the amount of energy transferred.
7
Explain why anaerobic respiration transfers less energyThe oxidation of glucose is incomplete, so much of the energy stays in the lactic acid or ethanol.
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
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.
Cheatsheet
Aerobic: glucose + oxygen → carbon dioxide + water
C6H12O6 + 6O2 → 6CO2 + 6H2O grade 6+
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
How to answer each type of question
Recall: complete a respiration equation
1 to 3 marks3
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 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 part5
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.
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(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)
Compare aerobic and anaerobic respiration
3 to 4 marks6
Make each point a direct comparison, using 'whereas' or 'but'.
Cover oxygen, products and the amount of energy transferred.
A similarity (both use glucose) can also count if the question says 'compare'.
Example. Compare aerobic respiration with anaerobic respiration in human muscle cells. [3 marks]
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Aerobic respiration uses oxygen, whereas anaerobic respiration does not (1). Aerobic respiration produces carbon dioxide and water, whereas anaerobic respiration produces lactic acid (1). Aerobic respiration transfers much more energy from each glucose molecule than anaerobic respiration (1).
Explain why anaerobic respiration transfers less energy
2 marks7
Use the key phrase: the oxidation of glucose is incomplete.
Say where the rest of the energy is: still stored in the lactic acid (or ethanol).
Example. Muscle cells transfer much less energy from each glucose molecule when they respire anaerobically. Explain why.
Show the model answer
Without oxygen, glucose is only partly oxidised / the oxidation of glucose is incomplete (1), so much of the energy is still stored in the lactic acid produced (1).
Shortcuts and memory tricks
Aerobic has 'aer' as in air (oxygen). 'An-' means without, so anaerobic means without oxygen.