Respiration

Respiration, or cellular respiration, is a series of chemical reactions in which food is broken down to release energy. In order to live, all living things must be able to extract needed energy from the organic compounds they take in. The chemical process of respiration may be either aerobic or anaerobic.

The living cells of every organism need a constant supply of energy, or fuel, which they use to stay alive. Therefore, all living things obtain their energy from the food they make themselves (as plants are able to use light and chemicals to make their own food) or which they capture from their environment (by eating plants or other animals). In an animal, once this food is broken down into simpler forms by its digestive system, it is absorbed into the bloodstream. There the circulatory system (a network that carries blood throughout an animal's body) transports these small bundles of nutrients to each cell. It is at this level that the process known as cellular respiration takes place. During this process, cells convert the stored energy into usable energy. To do this, these bundles of food, or nutrients, must have their bonds broken so that the chemicals necessary to sustain life are released.

There are two types of cellular respiration. One involves oxygen and is called aerobic respiration, and the other does not need oxygen and is known as anaerobic respiration. Most cells use aerobic respiration. It is by far the most common and efficient method of obtaining energy. During aerobic respiration, nutrients reach the cells in the form of glucose (a form of sugar). The cell breaks down the glucose by combining it with oxygen, thereby releasing usable energy. This occurs in four separate stages and is called oxidation. The process of oxidation (the combining of glucose and oxygen) releases a large amount of energy to the cell. Oxidation also gives off carbon dioxide (a major atmospheric gases) and water as waste products.

Anaerobic respiration, which does not use oxygen to break down glucose, is less common and does not result in the release of as much energy as aerobic respiration. It is performed mainly by bacteria and some fungi, some of which use other inorganic compounds like nitrates or sulfates in place of oxygen. Fermentation is a type of anaerobic respiration since it breaks down glucose without using oxygen. The term respiration is also used to describe the process of breathing, in which oxygen is taken into an animal's body. This type of respiration is called external respiration.

[See alsoBacteria; Blood; Diffusion; Fermentation; Metabolism; Respiratory System ]