Anaerobic Respiration

When oxygen is not present, organisms can respire anaerobically.

During anaerobic respiration, glucose is broken down in the absence of oxygen, so the breakdown of glucose is incomplete. This means that less energy is transferred compared to aerobic respiration.

Anaerobic Respiration in Muscles

Muscle cells need a lot of energy for contraction. However, at times the amount of oxygen available is limited.

The equation for anaerobic respiration is:

Glucose Lactic acid


C6H12O6 → 2C3H6O3

When organisms respire anaerobically, the muscles convert glucose to lactic acid. This causes the cramps that people feel when exercising.

There is not enough blood flowing through. Therefore, anaerobic respiration has to take place. More oxygen is needed in the body to get rid of the cramp. This happens so that the body can switch back to aerobic respiration.

For example, during vigorous exercise, when we can’t get enough oxygen to our muscle tissues to continue aerobic respiration. So, as the oxygen levels decrease, humans then switch to anaerobic respiration.

As humans exercise, they release more energy, so we benefit more from aerobic respiration.

Anaerobic Respiration in Yeast

Anaerobic respiration can also take place in yeast, which is called fermentation. In this case, glucose is broken down to create ethanol, carbon dioxide and energy.

Glucose Ethanol + Carbon Dioxide + Energy


C6H12O6 → 2C2H5OH + 2C02

This aids in the manufacture of bread and alcoholic drinks.

Fermentation produces carbon dioxide. This forms bubbles, which make the dough rise, so the bread appears light and fluffy. It also produces ethanol, which is the alcohol found in alcoholic drinks, such as beer and wine.