Cellular respiration: releasing energy continuously

Delivering oxygen and glucose is useful because cells can release energy from glucose. Cellular respiration is a series of chemical reactions in living cells. It is not the same as breathing: breathing moves air into and out of lungs, while respiration is chemical change inside cells.

Respiration is exothermic, meaning that it releases energy. Some of the energy released from glucose becomes available for metabolic processes and some is transferred as thermal energy. Energy is transferred and released; it is not created by the cell.

Living cells respire continuously because their metabolic work continues even when an organism appears to be resting. The rate can change, but cells still require energy for processes such as:

• active transport across cell membranes;
• building larger molecules from smaller ones;
• movement, including muscle contraction;
• maintaining a stable body temperature in mammals.

Both aerobic and anaerobic respiration are exothermic ways of releasing energy from glucose. The key differences are whether oxygen is used, how completely glucose is broken down, the products formed and the amount of energy released.

Comparing aerobic and anaerobic respiration

Aerobic respiration

Aerobic respiration uses oxygen and completely breaks down glucose.

Because glucose is broken down more completely, aerobic respiration releases much more energy from each glucose molecule than anaerobic respiration. It occurs in plant and animal cells whenever oxygen and glucose are available.

Anaerobic respiration

Anaerobic respiration releases energy without using oxygen. Glucose is only partly broken down, so less energy is released from each glucose molecule.

In animal cells:

In yeast and plant cells:

The products differ between organisms, but the central comparison remains the same: anaerobic respiration does not require oxygen and releases less energy because glucose is not completely broken down.