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Good day everyone, Today we are going to discuss on metabolism;

Learning outcomes

After studying this section, you should be able to:

  • discuss general principles of metabolism, including anabolism, catabolism, units of energy and metabolic rate
  • compare and contrast the metabolic rates of the body’s main energy sources (carbohydrate, protein and fat)
  • describe in simple terms the central metabolic pathways; glycolysis, citric acid cycle and oxidat phosphorylation.
Diagram of metabolism
Diagram of metabolism

Metabolism constitutes all the chemical reactions that occur in the body, using nutrients to:

  • provide energy by chemical oxidation of nutrients
  • make new or replacement body substances.

Catabolism. Catabolic processes break down large ecules into smaller ones releasing chemical energy, which is stored as adenosine triphosphate (ATP), and heat. Heat generated maintains core body temperature at the optimum level for chemical activity (36.8°C). Excess heat is lost, mainly through the skin (Ch. 14).

Anabolism. This is building up, or synthesis, of large molecules from smaller ones and requires a source of energy, usually ATP.

Metabolic pathways

Anabolism and catabolism usually involve a series of chemical reactions, known as metabolic pathways. These consist of ‘small steps’ that permit controlled, efficient and gradual transfer of energy from ATP rather than large intracellular ‘explosions’. Metabolic pathways (see below) are switched on and off by hormones, provid- ing control of metabolism and meeting individual requirements.

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Both catabolic and anabolic processes occur continually in all cells. Very active tissues, such as muscle or liver, need a large energy supply to support their requirements.


The energy produced in the body may be measured and expressed in units of work (joules) or units of heat (kilocalories).

A kilocalorie (kcal) is the amount of heat required to raise the temperature of 1 litre of water by 1 degree Celsius (1°C). On a daily basis, the body’s collective metabolic processes generate a total of about 3 million kilocalories.

1 kcal = 4184 joules (J) = 4.184 kilojoules (kJ)

The nutritional value of carbohydrates, protein and fats eaten in the diet may be expressed in either kilojoules per gram or kcal per gram.

1 gram of carbohydrate provides 17 kilojoules (4 kcal)

1 gram of protein provides 17 kilojoules (4 kcal) 1 gram of fat provides 38 kilojoules (9 kcal)

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Energy balance

Energy balance is important as it determines changes in body weight. Body weight remains constant when energy intake is equal to energy use. When intake exceeds requirement, body weight increases, which may lead to obesity. Conversely, body weight decreases when nutrient intake does not meet energy requirements.

Metabolic rate

The metabolic rate is the rate at which energy is released from the fuel molecules inside cells. As most of the processes involved require oxygen and produce carbon dioxide as waste, the metabolic rate can be estimated by measuring oxygen uptake or carbon dioxide excretion.

The basal metabolic rate (BMR) is the rate of metabolism when the individual is at rest in a warm environment and is in the postabsorptive state, i.e. has not had a meal for at least 12 hours. In this state the release of energy is sufficient to meet only the essential needs of vital organs, such as the heart, lungs, nervous system and kidneys. The postabsorptive state is important because the intake of food, especially protein, increases metabolic rate.

Central metabolic pathways

Much of the metabolic effort of cells is concerned with energy production to fuel cellular activities. Certain common pathways are central to this function. Fuel molecules enter these central energy-producing pathways and in a series of steps, during which a series of interme- diate molecules are formed and energy is released, these fuel molecules are chemically broken down. The end results of these processes are production of energy and carbon dioxide and water (called metabolic water). Much of the energy is stored as ATP, although some is lost as heat. The carbon dioxide is excreted through the lungs and excess water excreted as urine.

The preferred fuel molecule is glucose, but alternatives should glucose be unavailable include amino acids, fatty acids, glycerol and occasionally nucleic acids. Each of these may enter the central energy-producing pathways and be converted to energy, carbon dioxide and water. There are three central metabolic pathways :

  • glycolysis
  • the citric acid (Krebs) cycle
  • oxidative phosphorylation.

Products from glycolysis enter the citric acid cycle, and products from the citric acid cycle proceed to oxidative phosphorylation. The fates of the different fuel molecules entering the central metabolic pathways are discussed in the following sections.

Carbohydrate metabolism

Erythrocytes and neurones can use only glucose for fuel and therefore maintenance of blood glucose levels is needed to provide a constant energy source to these cells Most other cells can also use other sources of fuel.

Digested carbohydrate, mainly glucose, is absorbed into blood capillaries of the villi of the small intestine. It is transported by the portal circulation to the liver, where it is dealt with in several ways:

  • glucose may be oxidised to provide the chemical energy, in the form of ATP, necessary for the considerable metabolic activity which takes place in the liver itself
  • some glucose may remain in the circulating blood to maintain the normal blood glucose of about 3.5-8 millimoles per litre (mmol/L) (63-144 mg/100 mL).

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