The breakdown of glucose

The energy is released by chemical reactions between food materials and oxygen in a process called 'respiration'. The major food material, glucose, contains carbon, hydrogen and oxygen, and this is oxidised to form carbon dioxide and water. The oxygen reaches the tissues from the lungs via the bloodstream, and carbon dioxide is carried back to the lungs. Not all of the energy is released immediately or this would destroy the cells, the oxidation of glucose to carbon dioxide and water takes place via a number of steps giving a controlled release of energy.

Find out more about breathing and respiration

Chemical equation

Bacteria and viruses

Although organisms function very well independently, they are constantly under attack from tiny organisms called 'bacteria' and 'viruses'. These are extremely successful organisms and have existed throughout the history of life on Earth. The are very adaptable and survive in all regions of the world across a wide range of temperatures and conditions. Bacteria are a few micrometres across, whilst viruses are much smaller, typically 0.1 micrometres across.

Bacteria found in 'raw milk (left) and viruses found in plant leaves (right).

Once inside the body, bacteria and viruses cause problems to humans in two main ways. Some attack and destroy cells, whilst other produce poisonous substances which get into the bloodstream and are carried around the body. The body protects itself by producing 'antibodies'. These can react with the poisons produced and make them harmless, or attack the bacteria or viruses, bursting the cells or digesting them.

Bacteria may be engulfed and digested by special cells called 'phagocytes'.