Energy from Respiration

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Aerobic Respiration

During aerobic respiration, glucose reacts with oxygen. This provides energy for all cells. Carbon dioxide and water are given off as waste products:


Aerobic respiration involves lots of chemical reactions, most of which take place in the mitochondria. Mitochondria have a folded inner membrane to provide a large surface area  for the enzymes involved.

The energy produced by aerobic respiration is used for a number of things:

  • living cells use it to build larger molecules from smaller ones
  • In animals, it is used to make muscles contract 
  • Mammals and birds keep their body at a constant temperature by using the energy.
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The effect of exercise on the body

Muscle tissue is made up of muscle fibres, they contract when they are supplied with energy from respiration and then relax to allow other muscles to contract. The muscles store glucose as the carbohydrate glycogen that can be rapidly turned back into glucose during exercise to be used in aerobic respiration to provide energy.

During exercise, some changes take place:

  • Heart rate increases and the arteries supplying blood to muscles dilate. This increases blood flow and the rate at which oxygen is supplied to the muscles and carbon dioxide is taken away.
  • Breathing rate increases so you bring in more air to the lungs each breath. This means more oxygen is picked up by the red blood cells and taken to the exercising muscles. This also mean more carbon dioxide can be removed from the blood in the lungs.

If you exercise more often, your heart and lungs will become larger and work more efficiently whether you are exercising or not.

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Anaerobic respiration

When you exercise hard your muscle cells can become short of oxygen. The cells can still get energy from glucose but without using oxygen, this is anaerobic respiration. this reaction produces lactic acid instead of carbon dioxide and water because the glucose is not broken down completely.


Muscle fatigue is when there is a build up of lactic acid in the muscles. This can cause the muscles to stop contracting efficiently. Blood flowing through the muscles removes the lactic acid.

The build up of excess lactic acid is a problem after exercise and in order to break it down properly into carbon dioxide and water, oxygen is needed. The amount of oxygen needed to get rid of all the lactic acid is known as an oxygen debt.

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