Anaerobic respiration

Cells can release energy from nutrient molecules without using oxygen. They do this through a series of chemical reactions called anaerobic respiration. The released energy can be used by the cell.

Anaerobic respiration releases much less energy per glucose molecule than aerobic respiration, which uses oxygen. The products of anaerobic respiration depend on the cells involved.

Anaerobic respiration in yeast

In yeast cells, glucose is broken down to produce alcohol and carbon dioxide:

Anaerobic respiration in muscles

During vigorous exercise, muscle cells can release energy from glucose by anaerobic respiration. In these cells, glucose is broken down to produce lactic acid:

Balanced equation for yeast anaerobic respiration

Yeast can break down glucose without oxygen, producing ethanol and carbon dioxide. The balanced chemical equation is:

The numbers in front of the formulae are coefficients. They show that one glucose molecule produces two ethanol molecules and two carbon dioxide molecules. A formula with no written coefficient has a coefficient of one.

The equation is balanced because the number of atoms of each element is the same on both sides. One glucose molecule contains six carbon atoms, twelve hydrogen atoms and six oxygen atoms. On the product side, the two ethanol molecules contain four carbon, twelve hydrogen and two oxygen atoms; the two carbon dioxide molecules contain two carbon and four oxygen atoms. Together, the products therefore also contain six carbon, twelve hydrogen and six oxygen atoms.

Oxygen debt after vigorous exercise

During vigorous exercise, muscles may not receive enough oxygen for all their respiration. They also respire anaerobically, producing lactic acid, which builds up in the muscles and blood. This creates an oxygen debt: extra oxygen is needed after exercise to deal with the lactic acid.

After exercise stops, the heart rate remains fast. This keeps blood moving quickly, transporting lactic acid from the muscles to the liver.

Breathing also remains faster and deeper, supplying the extra oxygen needed for the lactic acid to be respired aerobically in the liver. As the lactic acid is removed, the oxygen debt is repaid.