respiration

respiration The metabolic process in animals and plants in which organic substances are broken down to simpler products with the release of energy, which is incorporated into special energy-carrying molecules (see ATP) and subsequently used for other metabolic processes. In most plants and animals respiration requires oxygen, and carbon dioxide is an end product. The exchange of oxygen and carbon dioxide between the body tissues and the environment is called external respiration (see ventilation). In many animals the exchange of gases takes place at respiratory organs (e.g. lungs in air-breathing vertebrates) and is assisted by respiratory movements (e.g. breathing). In plants oxygen enters through pores on the plant surface and diffuses through the tissues via intercellular spaces or dissolved in tissue fluids.

Respiration at the cellular level is known as internal (cellular or tissue) respiration and can be divided into two stages. In the first, glycolysis, glucose is broken down to pyruvate. This does not require oxygen and is a form of anaerobic respiration. In the second stage, the Krebs cycle, pyruvate is broken down by a cyclic series of reactions to carbon dioxide and water. This is the main energy-yielding stage and requires oxygen. The processes of glycolysis and the Krebs cycle are common to all plants and animals that respire aerobically (see aerobic respiration).