If oxygen is not available then pyruvate cannot be completely broken down. This could happen in your muscle cells if you are doing strenuous exercise.

Your muscles are using up oxygen faster than it can be supplied to them by your circulatory system and your lungs. Glucose can still be broken down in the absence of oxygen in order to meet the cells' energy requirements.

If oxygen is not available to animal cells then pyruvate is converted into lactate (sometimes referred to as lactic acid). In plant and yeast cells pyruvate is converted into carbon dioxide and a type of alcohol called ethanol . This process is called fermentation and yields only two molecules of ATP per glucose molecule broken down.

The following is the summary word equation for fermentation pathway in animal cells.

\[glucose\to{lactate}+energy\]

This process is reversible. When oxygen is available to the cell again the lactate can be converted back to pyruvate.

The following is the word equation for fermentation pathway in plant and yeast cells.

\[glucose\to{carbon~dioxide}+ethanol+energy\]

This process is irreversible as carbon dioxide diffuses away.