2.8 Anaerobic respiration

Summary: During a fast race, athletes have very little time to breathe. This means that the amount of oxygen they take in is not very high. Respiration must provide your body with energy all the time, even… Tags: science oxford-science student-book-7 biology Created: 2026-05-19 Last Updated: 2026-05-19

After this topic, you will be able to:

• compare the processes of aerobic and anaerobic Respiration
• give the word equation for anaerobic respiration
• give the word equation for fermentation.

Think back

1. What is the word equation for aerobic respiration?
2. Which part of the blood carries glucose?
3. Why do muscles need Energy?

Key idea Anaerobic respiration does not use oxygen. It transfers energy from glucose for short periods of time. However, anaerobic respiration produces lactic acid. Lactic acid sometimes causes muscle cramps after exercise.

Key words anaerobic respiration, oxygen debt, microorganism, fermentation

During a fast race, athletes have very little time to breathe. This means that the amount of oxygen they take in is not very high. Respiration must provide your body with energy all the time, even when you are unable to breathe enough.

How do you respire without oxygen?

Anaerobic respiration is a type of respiration that does not use oxygen. Your body uses this type of respiration to transfer energy from glucose when there is not enough oxygen for aerobic respiration to take place.

Anaerobic respiration often happens during heavy exercise because the body needs extra energy to be transferred quickly (see Figure 1). The body can transfer this extra energy for short periods of time without oxygen. The word equation for anaerobic respiration is:

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A Describe the conditions in which anaerobic respiration happens.

Figure 1After heavy exercise, you breathe heavily to break down lactic acid in your muscles.

There are two reasons why the body normally respires aerobically.

1. Aerobic respiration transfers more energy per glucose molecule than anaerobic respiration.
2. The lactic acid produced from anaerobic respiration can cause pain in your muscles. This is called muscle cramps.

When you have finished exercising, you keep on breathing heavily.

The extra oxygen you inhale (breathe in) breaks down the lactic acid. The oxygen needed for this process is called the oxygen debt.

B Name the type of respiration that transfers more energy.

Do other organisms carry out anaerobic respiration?

Other animals also use anaerobic respiration when they need a lot of energy quickly. For example, when a fox chases a rabbit, the Cells in both organisms are likely to respire anaerobically.

Anaerobic respiration also takes place in plants and in some microorganisms when there is no oxygen available. For example, the roots of plants in soils with too much water respire anaerobically.

Fermentation

Anaerobic respiration in some microorganisms produces ethanol and carbon dioxide, instead of lactic acid. This process is called fermentation. Fermentation is a type of anaerobic respiration because the microorganism respires without oxygen.

The word equation for fermentation is:

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C Describe the difference between anaerobic respiration in animals and fermentation.

Yeast is an important microorganism in food production (see Figure 2). For example, it is needed to make bread. Bread is made using fermentation.

Figure 2Yeast is a type of fungus.

Literacy skills Write a paragraph explaining how anaerobic respiration makes a useful product. Use the information on this double page to help you.

How do you make bread?

1. Flour, water, and yeast are mixed to make dough.
2. The yeast ferments carbohydrates in the flour into ethanol and carbon dioxide.
3. Carbon dioxide gas is trapped inside the dough. This makes it rise.
4. The ethanol evaporates as the dough is baked.
5. Once baked, bread is produced.

Stretch zone

1. Copy and complete the sentences. Aerobic/Anaerobic respiration is a type of respiration that does not use glucose/oxygen. Anaerobic respiration in humans causes energy/minerals to be transferred from glucose. Hydrochloric acid/Lactic acid is produced as a waste product, which can build up in microorganisms/muscles and cause pain.
2. Describe the main differences between aerobic and anaerobic respiration.

Stretch zone

1. Explain why cells in the body normally respire aerobically.