Energy In the Body, Tissues and Biochemical Processes

Normal life may be defined as the conversion of energy to perform meaningful work at an acceptable metabolic cost. Illness and injury may be defined as energy conversion, work requirements or metabolic costs that have new become excessive. Therefore death may be defined as the irreversible loss of the ability to use energy to perform sufficient work in one or more vital organs. 

The history of energy metabolism is a fascinating example of how progress depends alternatively on new concepts and, at other times, on new technology that allows elements of the problem to be revisited and objectively measured. 

There are two laws of thermodynamics that govern the behaviour of energy. The first states that Energy can neither be created nor destroyed. It can only be converted from one form into another. The second law can be stated in several ways but a simple one is Heat does not flow, spontaneously, from a cold object to a warmer one. The relevance of these two laws in health and disease may not 

The first law applies to the material presented in the first two sections and both laws apply to the third section. One factor common to all these discussions is that the same unit, the joule, is used in all these energy transformations (Table 2.1). 

Although details of the biochemistry of energy transformation have been established for over 50 years, it is only recently that this knowledge has been applied to key life processes in health and disease. This has been driven by several factors appreciation of the importance of provision of chemical energy (i.e. food) for patients, in hospital, provision of chemical energy for physical activity of all kinds and the need to balance energy intake and expenditure for prevention and treatment of obesity. In discussions of the last point, even the mass media refer to the first law of thermodynamics. 

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Part of this basal energy requirement is obvious — the heart beats to circulate the blood respiration continues and there is considerable electrical activity in nerves and muscles, whether they are working or not. These processes require a metabolic energy source. Less obviously, there is also a requirement for energy for the wide variety of biochemical reactions occurring all the time in the body laying down reserves of fat and carbohydrate (section 5.6) turnover of tissue proteins (section 9.2.3.3) transport of substrates into, and products out of, cells (section 3.2.2) and the production and secretion of hormones and neurotransmitters. 

If carbohydrates or proteins are ingested in excess of the amounts necessary to maintain optimal supplies of glycogen in tissues or protein, the excess is converted to fat. Conversion of excess carbohydrate and/or protein into fat is biochemically an irreversible process. As a result, the body conserves the compounds that it can interconvert, uses them to supply energy when needed, or converts them to fatty acids when it is more efficient to store the carbon in the form of fat. The body tends to conserve its protein reserves and to draw on fat reserves preferentially in time of energy demand. 

Calcium, potassium, and sodium are the three most abundant metals in the human body. Present in a much smaller amount, copper is nonetheless vitally important to human health. Copper-dependent enzymes known as cuproenzymes play a critical role in a host of biochemical processes, including cellular energy production, connective tissue formation. and reactions essential to normal functioning of the brain and central nervous system. Two hereditary diseases that involve errors of copper metabolism are Menkes disease and Wilson s disease. 

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