metabolism According to the Shorter Oxford English Dictionary, the term metabolism is defined as ‘the chemical processes by which nutritive material is built up into living matter, or by which complex molecules are broken down into simpler substances during the performance of special functions’. The various reactions which involve the synthesis of complex molecules can be grouped under the heading of anabolism, whereas the breakdown of complex molecules is known as catabolism. As might be expected, both anabolic and catabolic processes include a vast number of different chemical reactions, but there are a number of common features. Most of the metabolic processes occur inside the cells of the body, mainly in the cytoplasm, but also inside intracellular organelles such as the mitochondria. Anabolic and catabolic reactions involve the action of enzymes and the utilization of energy. In some cases the metabolic processes are regulated locally, i.e. by the cell itself, but often the metabolism of the whole body is controlled in an integrated fashion by the action of hormones and/or the nervous system.

Anabolic processes

These mainly involve the use of the carbohydrates, fats, proteins, and minerals consumed in the diet to synthesize complex molecules — such as the structural material of the skeleton, connective tissue, and cell membranes; nutrient stores for later use; and hormones and proteins which are secreted from cells into the blood or into the digestive tract. In order for these anabolic processes to proceed efficiently, it is essential that the cells are provided with the correct raw materials (and are able to extract them from the blood) and that the appropriate enzymes are present within the cells. Obviously, these enzymes will have been synthesized within the cells, as a result of activation of the appropriate genes in the cell nucleus.

Catabolic processes

These can be classified into a variety of categories, including the breakdown of energy-containing components of the diet (or their storage forms) to make energy available for the cells; the removal and breakdown of potentially toxic substances in the bloodstream; and the breakdown of damaged cells and tissues with the re-use of many of the components. These catabolic processes require the presence of the appropriate enzymes; many also require oxygen to be available, and the waste products to be removed from the tissues by the blood.

In many cases the processes of anabolism and catabolism occur coincidentally. A good example relates to the protein in the body, which is in a constant state of flux. Every day some of the body protein undergoes catabolism and is replaced by new material. Thus, there is a constant turnover of protein in the body, which requires a continuous supply of protein in the diet, and which also uses a substantial amount of energy.

Control of metabolism

For the body to function efficiently, there has to be an effective means of controlling and integrating the metabolic processes occurring in all the cells, tissues, and organs. This integration and control is mainly achieved by circulating hormones, with their release being regulated in turn partly by the nervous system and partly by direct effects of substances in the blood on the endocrine glands. An example of this integrated control of metabolism is the way in which blood glucose concentration is regulated to ensure an adequate supply of glucose to the brain. After meals, the hormone insulin acts to promote storage of glucose in the form of glycogen in the liver. The brain continuously extracts glucose from the blood to use as a fuel for its metabolic processes. In the periods between meals, this continued use of blood glucose causes the concentration to fall, which could impair brain function. However, a fall in blood glucose is detected in the pancreas and leads to the release of the hormone glucagon, which acts on the liver to cause breakdown of glycogen and release of glucose into the blood. In addition, if blood glucose falls sufficiently to affect brain metabolism, the sympathetic nervous system is activated, causing the adrenal gland to release adrenaline, which also stimulates the release of glucose from the liver; also the individual feels hungry and is prompted to eat.

Energy metabolism

A fundamental feature of both anabolic and catabolic processes is the utilization of energy. Almost all of the chemical reactions in the body require the expenditure of energy, which is made available mainly by the catabolism of the ‘macronutrients’: fats and carbohydrates (particularly glucose), and (to a small extent) proteins. This utilization of energy can be compared with the use of fuel for cooking or for generating electricity. In these two cases, the combustion of a fuel (coal, gas, or oil) produces carbon dioxide and water and releases heat which is used to warm the food (often causing chemical changes in it) or to generate steam to drive turbines. In the body's metabolism, the energy released from the oxidation of the macronutrients is used for a series of chemical reactions, instead of being released only as heat.

The main way in which the energy contained in the macronutrients is used in metabolism is via the substance adenosine triphosphate (ATP). Cells require energy for their metabolic processes, so they contain the enzymes and organelles needed to produce ATP from the catabolism of fats, carbohydrates, and/or proteins. In most cases, the production of ATP occurs in association with the oxidation, so that the final products are ATP, carbon dioxide, and water, as illustrated below for the oxidation of glucose (C₆H₁₂O₆):C₆H₁₂O₆ + 6O₂ = 6CO₂ + 6H₂O + ATPThis is an example of aerobic metabolism, requiring the supply of oxygen and the removal of carbon dioxide from the cells by the circulating blood. Thus, in order for this predominant type of metabolism to proceed effectively in the whole body, there needs to be integration of the respiration, circulation, and supply of nutrients.

In some situations, anaerobic metabolism can occur — ATP is produced without the use of oxygen — but the energy-releasing capacity of these systems is very small compared with that of aerobic metabolism, and the anaerobic reactions lead to the production of waste products such as lactic acid which impair cell function if they are present in high concentrations.

ATP is the single most important molecule for the metabolism of almost all the cells of the body. It is used to release the energy needed for muscles to contract, for chemical bonds to be made during the synthesis of complex molecules, and for other bonds to be broken during catabolic processes. Cells do not store large quantities of ATP, but rather produce it when it is needed. Thus, most cells of the body need to regulate the concentration of ATP within them. This occurs via the effects of ATP, and its immediate breakdown product ADP (adenosine diphosphate), on the enzymes responsible for synthesizing ATP: when more ATP is used, its concentration falls, and that of ADP rises, leading to the activation of the enzyme which synthesizes more ATP. This in turn requires more oxygen to be used, and nutrients to be broken down.

An example of the complex integration of metabolism is provided by considering the processes involved in muscle contraction during exercise. This involves the brain and other parts of the nervous system in the initiation of voluntary muscle contraction and movement. Contraction can occur only if ATP is available within the muscle cells. As the ATP already present is used, so the concentration of ADP will rise, which stimulates more ATP production. At the same time the contraction of the muscles stimulates the breakdown of the intramuscular glycogen, and may also stimulate the uptake of glucose and fatty acids from the blood. The increased availability of these fuels is accompanied by stimulation of their oxidation, so the ATP concentration is maintained, and muscle contraction continues, supported by an increase in aerobic energy metabolism. For this to be possible, it is also necessary for the supply of blood to the muscles to increase, in order to deliver more oxygen and carry away more carbon dioxide and heat; the action of chemical products of local metabolism, which dilate local blood vessels, effectively links flow to requirement.

The above examples illustrate the complexity of metabolism in the human body, and show that for normal function it is essential that local processes are co-ordinated and integrated throughout the body.

I. A. Macdonald

metabolism The processes of interconversion of chemical compounds in the body. Anabolism is the process of forming larger and more complex compounds, commonly linked to the utilization of metabolic energy. Catabolism is the process of breaking down larger molecules to smaller ones, commonly oxidation reactions linked to release of energy. There is approximately a 30% variation in the underlying metabolic rate (basal metabolic rate) between different individuals, determined in part by the activity of the thyroid gland. See also energy.