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Key points

  • Respiration is a chemical reaction which occurs in all living cells, releasing energy from glucose.

  • Aerobic respiration occurs with oxygen and releases more energy but more slowly.

  • Anaerobic respiration occurs without oxygen and releases less energy but more quickly.

Understanding respiration


Can you answer these questions based on the video?

1. What does respiration release?

2. Where does the glucose used in respiration come from?

  1. Energy

  2. Food

Aerobic respiration

is not breathing. That is called . Respiration is a chemical reaction which occurs in every one of the cells in the human body. It releases energy stored in glucose and without it, these cells would die.

Aerobic respiration occurs in the presence of oxygen. The equation for aerobic respiration is:

glucose + oxygen → carbon dioxide + water

respiration slowly releases lots of energy stored in . It mostly occurs in tiny parts of your cells called mitochondria which are found in the . Cells which need more energy like sperm cells, which swim, or muscle cells which contract and relax, have more .

Did you know?

‘Aerobic’ does not mean ‘in air’ even though they sound the same. It means 'in the presence of oxygen.'

Anaerobic respiration

For a short period during vigorous exercise, the body’s cells may not have enough oxygen. This means aerobic respiration cannot occur and anaerobic respiration happens instead. The equation for anaerobic respiration is:

glucose → lactic acid

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

After vigorous exercise, people 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.

An illustration of two female joggers releasing energy. Lactic acid and oxygen, and carbon dioxide and water, are labelled underneath.
During exercise, energy is released

Comparing aerobic and anaerobic respiration

Reactant(s)Products(s)Rate of reactionEnergy released
Aerobic respirationGlucose, oxygenCarbon dioxide, waterSlowMore
Anaerobic respirationGlucoseLactic acidFastLess

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