Process: Metabolism

Thousands of chemical reactions and conversions take place continuously in body cells to keep the body alive and healthy and to generate energy. Metabolism is the collective term for all these chemical processes. The raw materials for metabolic processes are obtained from nutrients in food, which are broken down into simple molecules during digestion. These molecules are either recycled and built up into new complex molecules that can be used to repair or make new cells (anabolism) or are further broken down to release energy (catabolism).

Anabolism and catabolism

In anabolic processes, body cells are built up and repaired, or complex substances are constructed out of simpler ones. In catabolic processes, complex molecules are broken down into simple molecules, such as glucose and amino acids, and these simple molecules are broken down to supply the cells with energy and materials for renewing cell structures.

Metabolic activity

Anabolic and catabolic processes take place simultaneously in cells throughout the body to build complex molecules and to provide energy.

Basal metabolic rate

The amount of energy a person uses for essential functions, such as maintaining body heat, breathing, and heart rate, is called the basal metabolic rate (BMR). BMR decreases naturally with age, but it is raised in the short term by factors such as illness, pregnancy, breast-feeding, and menstruation. All forms of exercise increase the body’s use of energy above the BMR.

Basal metabolic rate (BMR)

After about the age of 10, basal metabolic rate decreases with age and tends to be lower in females than in males. BMR is measured at rest and is often expressed as kilojoules used per square metre of body surface per hour (1 kilojoule equals about 4 kilocalories).

How the body uses food

Every living body cell depends on essential nutrients in food. Carbohydrates, proteins, and fats are converted into glucose, amino acids, and fatty acids respectively during digestion. These molecules enter the lymphatic system and the bloodstream and are converted into a usable form (metabolized) in the liver and in all body cells. Glucose is used to produce energy, as are fatty acids when glucose is in short supply. Amino acids are used to build the complex proteins needed to make and repair cells.

Liver tissue

The liver has a rich blood supply. This magnified view of liver tissue shows a blood vessel surrounded by large liver cells (hepatocytes).


Each body cell contains many mitochondria. This magnified image of a mitochondrion, the cell “powerhouse”, shows the folds (cristae) where energy-producing reactions take place.


During digestion, complex carbohydrates are converted into simple sugars, such as glucose; fats and oils are broken down into fatty acids and glycerol; and proteins are broken down into amino acids. The bloodstream transports these nutrients to the liver, the body’s major metabolic site, and then to all body cells.

Layer of fat cells

Fats are deposited in oval storage cells known as fat cells or adipocytes. These cells form a layer, which varies in thickness, beneath the skin to insulate the body. A thin layer of adipose tissue surrounds the heart, kidneys, and other delicate internal organs.

From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.

The subjects, conditions and treatments covered in this encyclopaedia are for information only and may not be covered by your insurance product should you make a claim.

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