Fermentation
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 lactic acidA toxic chemical produced during anaerobic respiration. (sometimes referred to as lactic acid). In plant and yeast cells pyruvate is converted into carbon dioxide and a type of alcohol called ethanolThe alcohol which is produced as a result of fermentation of sugars by yeast.. 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 yeastA unicellular fungus used in the brewing and baking industries. cells.
\(glucose\to{carbon~dioxide}+ethanol+energy\)
This process is irreversible as carbon dioxide diffuses away.
Comparison of fermentation and aerobic respiration
| Fermentation | Aerobic Respiration | |
| Oxygen required? | No | Yes |
| Location in cell | Cytoplasm | Stage 1 in cytoplasm and stage 2 in mitochondria |
| Breakdown of glucose | Partial | Complete |
| Energy produced | small amount (from initial 2 ATP molecules) | large amount |
| End products: animal cells | Lactate (lactic acid) | Carbon dioxide and water |
| End products: plant and yeast cells | Carbon dioxide and ethanol | Carbon dioxide and water |
| Oxygen required? | |
| Fermentation | No |
| Aerobic Respiration | Yes |
| Location in cell | |
| Fermentation | Cytoplasm |
| Aerobic Respiration | Stage 1 in cytoplasm and stage 2 in mitochondria |
| Breakdown of glucose | |
| Fermentation | Partial |
| Aerobic Respiration | Complete |
| Energy produced | |
| Fermentation | small amount (from initial 2 ATP molecules) |
| Aerobic Respiration | large amount |
| End products: animal cells | |
| Fermentation | Lactate (lactic acid) |
| Aerobic Respiration | Carbon dioxide and water |
| End products: plant and yeast cells | |
| Fermentation | Carbon dioxide and ethanol |
| Aerobic Respiration | Carbon dioxide and water |