In cells respiration can take place in two ways: aerobically or anaerobically. The energy produced is then utilised in a number for different functions. During exercise the human body reacts in a particular way in order to deal with the need for my energy.

By the end of this section you should understand:

• how to interpret data which looks at how the human body is effected by exercise

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Aerobic respiration

Respiration is the process used by cells in order to release energy from glucose. For aerobic respiration oxygen must be present. This can be seen more clearly in the equation below:

glucose + oxygen ? carbon dioxide + water (+ energy)

The reason why energy is shown inside brackets is because it’s not a substance like the other parts of the equation.

The two main points that can be noted are:

• glucose (a sugar) and oxygen are used
• energy is released

Both animals and plants are capable of respiring aerobically and they do so continuously. The majority of the process happens in the mitochondria which can be found in the cytoplasm of cells. This reaction, as with all chemical reactions that occur in cells, is controlled by enzymes.

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Energy uses

The energy released during respiration can be used by an organism in a number of ways:

• to form larger molecules using small one
• to enable muscle contractions in animals
• to ensure that a steady body temperature is maintained when the surroundings are cold in birds and mammals
• to create amino acids from nitrates, sugars and other nutrients which are then used to create proteins in plants

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The effect of exercise

During exercise muscle cells need to respire at a faster rate than when they’re resting. Due to this, changes take place in the body including:

• an increase in heart rate
• an increase in the rate and depth of breathing

Both of these changes work to increase the amount of blood and therefore oxygen and sugar reaching the muscle cells. The heart beating faster means blood can flow around the body quicker. While an increase in breathing rate increases the rate at which gaseous exchange (of oxygen and carbon dioxide) occurs in the lungs. This also helps to increase the rate of removal of the waste product carbon dioxide.

To help increase the amount of sugar available to the cells, the muscle stores glycogen which can be converted back into glucose when required during exercise.

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Anaerobic respiration

The main difference between aerobic and anaerobic respiration is that anaerobic respiration doesn’t require oxygen to be present.

What tends to happen during exercise is that, no matter how fast the heart and breathing rate get, not enough oxygen can reach the muscles. So, they begin to respire anaerobically in order to obtain the energy they need.

Another difference inanaerobic respiration is that glucose isn’t broken down completely. The end result of this is that far less energy is released (only 5% per glucose molecule of what aerobic respiration can achieve) and the waste product lactic acid is produced.

glucose ? lactic acid (+ little energy)

Lactic acid

If muscles are made to exercise for a long period then then become tired or fatigued. This means that they’re unable to contract properly anymore. One of the reasons why this happens is due to a build-up of lactic acid. This is due to the fact that the body simply can’t remove it fast enough. However, once the muscles are at rest, the blood is able to remove the lactic acid from the muscles.

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HIGHER TIER

• Oxygen debt:

The reason why less energy is released during anaerobic respiration compared to aerobic is that the glucose is not broken down completely.

Oxygen debt is caused by anaerobic respiration. It refers to the additional oxygen needed by the body after exercise in order to oxidise the waste product, lactic acid. The lactic acid needs to be broken down into carbon dioxide and water. This explains why, after a lot of exercise, you still need to breathe deeply for a while afterwards.