Anaerobic respiration

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Key points

  • Respiration is a chemical reaction which takes place in all livings cells and releases energy from glucose.
  • Anaerobic respiration occurs without oxygen and releases less energy but more quickly than aerobic respiration.
  • Anaerobic respiration in microorganisms is called fermentation.

Types of respiration

is a chemical reaction which occurs in every one of the cells in the human body and all cells on the entire planet. Remember, respiration is not the same as breathing. The scientific name for breathing is ''.

Respiration releases energy stored in glucose and without it these cells would die. There are two types of respiration:

  • Aerobic respiration occurs in the presence of oxygen and in most cells most of the time.
  • Anaerobic respiration occurs without oxygen and much less frequently than aerobic respiration.

Human bodies use both types of respiration. Humans do aerobic respiration unless they are short of oxygen when they switch to anaerobic respiration.

Respiration occurs in which are found in the . Cells which need more energy eg sperm cells which swim or muscle cells which contract and relax have more mitochondria.

Other organisms such as bacteria and yeast do a different version of anaerobic respiration called .

Video - When does anaerobic respiration take place?

A case study video on how an athlete uses anaerobic respiration during exercise

Can you answer these questions based on the video?

1. What is missing when anaerobic respiration occurs?

2. What is the product of anaerobic respiration in humans?

  1. Oxygen.

  2. Lactic acid.

Anaerobic respiration in humans

During vigorous exercise your body cells may not have enough oxygen for aerobic respiration to take place and anaerobic respiration occurs instead.

The equation for this is:

glucose → lactic acid

Anaerobic respiration releases less energy than aerobic respiration but it does this more quickly. The product of this reaction is lactic acid. This builds up in muscles causing pain and tiredness, which can lead to cramp.

After you finish vigorous exercise you continue to breathe deeply and quickly for a short period. This is called excess post-exercise oxygen consumption or EPOC. It used to be called ‘oxygen debt.’ During this time the lactic acid reacts with oxygen to form carbon dioxide and water, and releases the rest of the energy originally in the glucose.

Anaerobic respiration in bacteria and fungi

Some bacteria and fungi such as yeast complete their own version of anaerobic respiration called fermentation. This is the equation:

glucose → ethanol + carbon dioxide

Yeast undergoes fermentation when bread and beer is made. Ethanol is the alcohol produced. This is evaporated away when bread is baked. Carbon dioxide gas trapped in bread makes it rise and gives beer its bubbles.

Yeast is used in bread production

Video - Fermentation

Can you answer these questions based on the video?

1. Give an example of a food that uses the fermentation process when it's being made.

2. What is produced in fermentation?

  1. Bread, yoghurt, beer, wine.

  2. Ethanol and carbon dioxide.

Test your knowledge


Test questions

Write a paragraph to answer the following question. Tap 'Show answer' to see five points you could have included.

Explain why beer contains alcohol but bread doesn't.

  • Both bread and beer are made by anaerobic respiration in microorganisms.
  • This is also called fermentation.
  • The formula for this is glucose → ethanol + carbon dioxide.
  • Bubbles in bread and beer are made by the carbon dioxide.
  • Unlike beer, bread is baked which kills the yeast and evaporates the alcohol.