AQA GCSE Biology: Respiration
A clear GCSE Biology revision guide to respiration: aerobic and anaerobic respiration, their equations, how they compare, and fermentation in yeast.
Respiration is how cells release the energy stored in glucose, and it happens in every living cell all the time. This guide explains what respiration is and what the energy is used for, the difference between aerobic and anaerobic respiration, and why fermentation in yeast is so useful. The contrast between aerobic and anaerobic respiration runs through the whole guide.
What respiration is
Cellular respiration is a series of exothermic reactions that release energy from glucose. It happens continuously in all living cells, whether plant, animal, fungal or microbial.
The energy released is used in several ways:
- to build larger molecules from smaller ones (metabolic reactions);
- to keep a constant body temperature in birds and mammals;
- for movement, such as muscle contraction;
- for the active transport of substances across cell membranes.
Two points about the wording here are worth getting right, because they are where easy marks go.
Exothermic means the reactions transfer energy to the surroundings, which is why you warm up when you exercise. And respiration releases energy that was already stored in glucose. It does not make, produce or create energy. Writing that respiration "produces energy" is marked wrong, so train yourself to write "releases energy from glucose" every time.
The other common mix-up is respiration and breathing. Breathing moves air in and out of the lungs. Respiration is a chemical reaction inside cells, and it happens in organisms with no lungs at all, including plants and bacteria. A plant respires every minute of its life, day and night.
Aerobic respiration
Aerobic respiration uses oxygen and takes place in the mitochondria. It releases a large amount of energy from each glucose molecule, because the glucose is fully broken down.
glucose + oxygen → carbon dioxide + water
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O
Because this is where aerobic respiration happens, cells with a high energy demand have large numbers of mitochondria. Muscle cells and sperm cells are the two examples worth quoting, and this is a favourite link back to specialised cells in Topic 1.
Anaerobic respiration
Anaerobic respiration happens without oxygen, in the cytoplasm. It releases only a small amount of energy, because the glucose is only partly broken down. The products depend on the organism.
- In animal cells (muscles): glucose → lactic acid.
- In plant and yeast cells: glucose → ethanol + carbon dioxide.
"Only partly broken down" is the reason for everything else in this section, so it is worth picturing. Think of a log on a fire. With plenty of air it burns right down to ash and gives out all its heat. Starve it of air and it smoulders, leaving charcoal behind. The charcoal still holds most of the fuel, so you got far less heat out of it. Lactic acid and ethanol are that leftover charcoal: they are molecules that still contain a great deal of unreleased energy, which is why incomplete breakdown means a smaller energy yield.
Notice that the animal reaction produces no carbon dioxide. Only the plant and yeast version does. Mixing these up is one of the most common errors in this topic.
Comparing aerobic and anaerobic respiration
| Feature | Aerobic | Anaerobic |
|---|---|---|
| Oxygen | Required | Not required |
| Breakdown of glucose | Complete | Incomplete |
| Energy released | Large amount | Small amount |
| Products | Carbon dioxide and water | Lactic acid (animals); ethanol and CO₂ (plants and yeast) |
| Where | Mitochondria | Cytoplasm |
If a question asks you to compare the two, answer in matched pairs. Say that aerobic respiration requires oxygen whereas anaerobic does not, rather than listing four facts about one and then four about the other. Comparison marks are usually awarded for the link, not for the separate facts.
Fermentation in yeast
Anaerobic respiration in yeast is called fermentation, and it is economically important.
- Bread making: the carbon dioxide produced forms bubbles in the dough, making it rise.
- Alcoholic drinks: the ethanol produced is the alcohol in beer, wine and spirits.
Both uses come from the same single reaction. Bread making uses the gas and loses the alcohol, because the ethanol evaporates in the heat of the oven. Brewing keeps the alcohol and lets the gas escape, except in drinks that are deliberately sealed so the carbon dioxide stays dissolved and makes them fizzy.
How this fits together
Respiration supplies the energy for everything a body does, and its rate rises sharply during exercise. How the body responds to that extra demand is the subject of the response to exercise guide.