Anaerobic Respiration

When oxygen is not available, organisms can undergo anaerobic respiration.

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. The lactic acid build-up in the muscles can cause the cramps that people feel during intense exercise.

When there’s insufficient blood flow to deliver oxygen, anaerobic respiration occurs. 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.

This is evident during vigorous exercise when our muscle tissues can’t receive enough oxygen for aerobic respiration. As oxygen levels decrease, the body switches 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 + 2CO2

This aids in the manufacture of bread and alcoholic drinks.

Fermentation produces carbon dioxide, forming bubbles in the dough. This causes the dough to rise, resulting in light and fluffy bread. It also produces ethanol, which is the alcohol found in alcoholic drinks, such as beer and wine.