Ross & Wilson Anatomy and Physiology in Health and Illness (144 page)

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 oxidative phosphorylation.

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.

Two types of process are involved:

Catabolism

This process breaks down large molecules into smaller ones releasing
chemical energy
, which is stored as adenosine triphosphate (ATP), and
heat
. Heat is used to maintain 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.

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 are switched on and off by hormones, providing control of metabolism and meeting individual requirements.

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

Energy

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
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)

Chapter 11
provides examples of foods providing these nutrients.

Energy balance

Body weight remains constant when energy intake is equal to energy use. When intake exceeds requirement, body weight increases. 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. Some of the many different factors that affect metabolic rate are shown in
Table 12.3
.

Table 12.3 
Factors affecting metabolic rate

Most foods contain a mixture of different amounts of carbohydrate, protein, fat, minerals, vitamins, fibre (non-starch polysaccharide) and water. Carbohydrates, proteins and fats are the sources of energy and they are obtained from the variety of food eaten in the diet (see
Ch. 11
).

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 intermediate molecules are formed and energy is released, these fuel molecules are chemically broken down. The end results of these processes are energy production and carbon dioxide and water (called
metabolic water
) formation. Much of the energy is stored as ATP, although some is lost as heat. The carbon dioxide is excreted through the lungs.

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 (see
Fig. 12.44
):

•
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 the 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 (
Fig. 12.39
):

•
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 (
p. 301
).

•
Some glucose may remain in the circulating blood to maintain the normal blood glucose of about 3.5 to 8 millimoles per litre (mmol/l) (63 to 144 mg/100 ml).

•
Some glucose, if in excess of the above requirements, may be converted to the insoluble polysaccharide,
glycogen
, in the liver and in skeletal muscles. The hormone
insulin
is needed for this change to take place. The formation of glycogen inside cells is a means of storing carbohydrate without upsetting the osmotic equilibrium. Before it can be used to maintain blood levels or to provide ATP it must be broken down again into its constituent glucose units. Liver glycogen constitutes a store of glucose used for liver activity and to maintain the blood glucose level. Muscle glycogen stores provide the glucose requirement of muscle activity.
Glucagon
,
adrenaline
(
epinephrine
) and
thyroxine
are the main hormones associated with the breakdown of glycogen to glucose. These processes can be summarised: