Key Stage 3 science · Biology – Material cycles and energy

KS3 aerobic and anaerobic respiration: a guide for parents

Respiration is the chemical process that releases energy from food in every living cell. In Key Stage 3 children learn the difference between aerobic respiration, which uses oxygen, and anaerobic respiration, which does not, including what happens in tired muscles and in yeast. They write both as word equations and compare what goes in, what comes out and what that means for the organism.

Aerobic and anaerobic respiration, explained

The first thing teachers do is separate respiration from breathing. Breathing is moving air in and out of the lungs. Respiration is a chemical reaction that happens inside cells, in which glucose is broken down to release energy. In plant and animal cells, aerobic respiration happens mostly in tiny structures called mitochondria. That energy powers everything a cell does: moving muscles, building new molecules, keeping warm, sending nerve signals and growing. Plants, animals, fungi and bacteria all respire, and they do it all the time, not just when they exercise.

Aerobic respiration needs oxygen. Glucose reacts with oxygen to give carbon dioxide and water, and energy is released. The word equation is: glucose + oxygen → carbon dioxide + water. The oxygen arrives through breathing and the blood, and the carbon dioxide is carried back to the lungs and breathed out. Children find it helpful to notice that this is almost the reverse of the photosynthesis equation, though the reactions are quite different and the energy is released rather than captured. Aerobic respiration is how most organisms get most of their energy, because it releases a lot of energy from each molecule of glucose.

Anaerobic respiration happens without oxygen and releases much less energy for each molecule of glucose. In humans, muscle cells use it during short bursts of hard exercise, when oxygen cannot be delivered fast enough. The word equation is: glucose → lactic acid. The lactic acid builds up and contributes to tiredness in muscles. Afterwards, we keep breathing heavily so that extra oxygen can help break the lactic acid down. Pupils usually notice that their breathing and heart rate stay raised for a while after stopping running.

Micro-organisms such as yeast also respire without oxygen, and the process is called fermentation. The word equation is: glucose → ethanol + carbon dioxide. Fermentation is used to make bread rise, since the carbon dioxide makes bubbles in the dough, and to make drinks that contain alcohol. Many schools have pupils set up a simple yeast experiment and measure how the amount of gas produced changes with temperature or sugar concentration.

To summarise the differences, children compare reactants, products and implications. Aerobic needs glucose and oxygen and gives carbon dioxide and water and much more energy. Anaerobic needs only glucose, gives either lactic acid (in humans) or ethanol and carbon dioxide (in yeast) and releases less energy. The implication for an organism is that it can keep going with aerobic respiration for a long time, but relying on anaerobic respiration is short-term and in muscles leads to fatigue.

Words your child will hear

Respiration: The chemical reaction in cells that releases energy from glucose. It is not the same as breathing.
Aerobic: Using oxygen. Aerobic respiration releases the most energy from glucose.
Anaerobic: Without oxygen. Anaerobic respiration releases less energy and makes different products.
Mitochondria: Tiny structures in cells where most aerobic respiration takes place. One is a mitochondrion.
Lactic acid: The product of anaerobic respiration in human muscle cells.
Fermentation: Anaerobic respiration in yeast and some other micro-organisms, making ethanol and carbon dioxide.
Glucose: The sugar that cells use as their main fuel.

What the National Curriculum says

The programme of study for Key Stage 3 says:

Pupils should be taught about aerobic and anaerobic respiration in living organisms, including the breakdown of organic molecules to enable all the other chemical processes necessary for life
Pupils should be taught about a word summary for aerobic respiration
Pupils should be taught about the process of anaerobic respiration in humans and micro-organisms, including fermentation, and a word summary for anaerobic respiration
Pupils should be taught about the differences between aerobic and anaerobic respiration in terms of the reactants, the products formed and the implications for the organism

Objectives from the National Curriculum in England, © Crown copyright, used under the Open Government Licence v3.0.

Try it: explaining a sprint

A girl sprints for 100 m as fast as she can. At the end her legs ache and she is breathing hard for several minutes. Use ideas about respiration to explain both.

Start with what muscle cells normally do. At rest and in gentle exercise they use aerobic respiration: glucose + oxygen → carbon dioxide + water, releasing energy.
Say what changes in a sprint. The muscles need energy very fast, but breathing and blood flow cannot deliver oxygen quickly enough to keep up.
Name the extra process. The muscle cells also use anaerobic respiration: glucose → lactic acid, releasing energy without oxygen but only a small amount for each glucose molecule.
Explain the aching legs. Lactic acid builds up in the muscles during a hard sprint and contributes to tiredness, so she cannot sprint for long.
Explain the heavy breathing. Afterwards she keeps taking in extra oxygen so that the lactic acid can be broken down, which is why she breathes hard and her heart beats fast until she has recovered.

In a sprint oxygen cannot reach the muscles fast enough, so the cells use anaerobic respiration (glucose → lactic acid) as well as aerobic respiration. The build-up of lactic acid contributes to tired, aching muscles, and the heavy breathing afterwards brings in extra oxygen to help break it down.

Common mistakes and how to help

Using “respiration” and “breathing” to mean the same thing

In everyday speech the words overlap, and adults say “respiratory system” for the lungs and airways.

What helps: Breathing brings oxygen in and carbon dioxide out. Respiration is the reaction in cells that uses that oxygen. One supports the other, but they are different. A good test: does a potato breathe? It respires.

Believing plants respire only at night, or only photosynthesise

Photosynthesis gets a lot of attention, and the summary “plants take in carbon dioxide and give out oxygen” leaves respiration out.

What helps: Plant cells respire all day and night. In bright light they photosynthesise faster than they respire, so more oxygen is released than used. In the dark only respiration continues.

Saying respiration “makes” energy

Everyday talk of “producing energy” and “using up energy” sounds as if energy is made and then vanishes.

What helps: Energy is transferred, not created. The energy stored in glucose is released for the cell to use, and some is released as heat. Ask where the energy was before the reaction.

Thinking anaerobic respiration is only for athletes, or is “better” because it is quicker

The sprint example sticks in memory, and the fast release of energy sounds like an advantage.

What helps: Yeast and many bacteria use it when oxygen is short, and human muscles use it briefly, but it gives far less energy per glucose than the aerobic type. It is a short-term back-up in muscles, not a better method.

How to help at home

Check pulse and breathing. Find your pulse at the wrist, count beats for 30 seconds and double it. Do 60 seconds of star jumps in the garden or living room, then check again and keep checking every minute until it returns to normal. Ask what the muscles needed more of.
Try a yeast-and-balloon test with an adult. Mix a teaspoon of dried yeast and a teaspoon of sugar into a cup of lukewarm (not hot) water in a clean plastic bottle, then stretch a balloon over the neck. Leave it somewhere warm and watch the balloon inflate. Ask what gas is filling it and which process made it.
Spot fermentation in the kitchen. Bread dough, yoghurt and sourdough all depend on micro-organisms respiring without oxygen. If you bake at home, point out the bubbles in the dough, or look at the holes in a slice of bread, and ask what made them.
Compare tired muscles. In the garden or a park, compare five minutes of brisk walking with 20 seconds of running on the spot as fast as possible. Which could your child keep going for longer, and which left their legs feeling heavy? Talk about the difference using the words aerobic and anaerobic.