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Stage one

Glucose in the cytoplasm of cells is broken down into two molecules of pyruvate. This releases enough energy to produce two molecules of ATP for every molecule of glucose that is broken down.

The breakdown reactions that occur are catalysed by specific enzymes. If these enzymes are not present or functional then the reactions will not occur, so the breakdown of glucose is controlled by enzymes.

This stage does not require oxygen. If oxygen is available to the cell then the pyruvate molecules can be further broken down in stage two to release more energy and produce a greater quantity of ATP.

Stage two

A pyruvate molecule enters the mitochondria and is broken down in the presence of oxygen to produce carbon dioxide and water. Enough energy is released from each pyruvate molecule to produce a large number of ATP molecules.

As in stage one, the breakdown reactions in this process are catalysed by specific enzymes.

Oxygen does not react directly with molecules of pyruvate. It is involved in the final step of the breakdown.

If oxygen is not available then only two ATP are produced.

Cells that have a high-energy demand such as nerve cells and muscle cells contain a large number of mitochondria to maximise ATP production by aerobic respiration.

A muscle cell contains myofibrils, proteins that make the cell contract, and high numbers of mitochondria to produce the large quantity of ATP molecules required.

In this video science presenter Jon Chase describes aerobic respiration as the release of energy from glucose in the presence of oxygen.