respiration

Process that occurs inside cells in which carbohydrate, particularly glucose, is broken down to release energy that the cell can use. This energy is used for many different processes, but in all of them energy transfer occurs. The processes range from muscle contraction to the manufacture of protein for new cells. Respiration is a key feature of life and is carried out by all living cells. There are two kinds of respiration in organisms – aerobic and anaerobic respiration. Aerobic respiration is a complex process of chemical reactions in which oxygen is used to break down glucose into carbon dioxide and water. This releases energy in the form of energy-carrying molecules (ATP). Respiration sometimes occurs without oxygen, and this is called anaerobic respiration. In this case, glucose is only partially broken down, and the end products are energy and either lactic acid or ethanol (alcohol) and carbon dioxide; this process is termed fermentation.

The word ‘respiration’ should not be used to refer to the air movements in the air passageways. These air movements are called ventilation (breathing).

The starting and finishing points of aerobic respiration would be the same if glucose was burned. During burning the energy that is released is all in the form of heat. In aerobic respiration, however, the energy is released in a controlled way and less is released as heat. Most of the released energy is used to drive various processes in the cell, such as growth or movement.

In humans, anaerobic respiration can only carry on for a short time. The muscles producing lactic acid will stop working as it builds up. However, many micro-organisms can respire anaerobically for long periods of time or all the time. Yeast respires aerobically if oxygen is present, but, if there is no oxygen, it respires anaerobically. In anaerobic respiration it produces alcohol.