An important role of ingested nutrients separate from roles in growth and other functions is to provide energy to the body. The provision of energy has many similarities to other energy using systems, such as were originally studied in machines. Consequently energy provision and utilisation in the body has been described in the concepts used in thermodynamics. This section gives a simple non-mathematical introduction to thermodynamics, the laws of which are obeyed by all biochemical processes. Through such systems are determined: (i) whether or not a reaction will proceed spontaneously; (ii) how the reaction is designed; and (iii) how complex structures are folded.

Organisms can utilise carbohydrates, fats and proteins to produce energy by oxidation. Metabolic oxidation during respiration consumes oxygen and produces energy. Energy is stored work or the capacity to do work. Thermodynamics describes the difference in energy between the reactants and products, but not the mechanism whereby the reaction takes place. The latter part of this chapter incorporates the concepts of thermodynamics into the processes of intermediary metabolism.

1. An important role of ingested nutrients is to provide energy to the body. Energy provision and utilisation in the body has been described in the concepts used in thermodynamics, which describe the difference in energy between the reactants and products, but not the mechanism whereby the reaction takes place.

2. The total amount of energy is constant and this is described in the first law of thermodynamics. While the overall energy remains constant, energy may flow from the system to the surroundings, or from the surroundings to the system.

3. The enthalpy of a compound is its internal energy.

4. The second law of thermodynamics describes a transition from an ordered to a disordered state (change in entropy).

5. Only a proportion of total potential energy is available for work. There is free or available energy and total energy in a system: Free energy = Enthalpy – absolute temperature × Entropy. Entropy measures the extent to which the total energy of the system is unavailable for the performance of useful work.

6. A reaction is only spontaneously possible if there is negative free energy. The molecules must, however, be in a reactive state. This reactivity is created by enzymatic activity in biological systems. Enzymes accelerate a reaction which is possible on energetic grounds.

7. When a substrate is bound to an enzyme, water molecules are displaced and this results in a positive entropy change. Another entropy effect between enzyme and substrate is called the chelation effect; this is important in enzyme kinetics.

8. Work that can be derived from biological systems: (i) mechanical work, which involves movements; (ii) osmotic and electrical work, changes in concentration, or movement of chemical compounds or ions against a concentration gradient; and (iii) synthetic work with changes in chemical bonds.

9. Some biological reactions are only possible by using high negative free energy of one reaction (exergonic reaction) to drive another reaction which has a low negative free energy (endergonic reaction).

10. The direction of reactions in living cells is dictated in response to metabolic needs, rather than to thermodynamic factors. The direction of such changes must be independent of changes in concentrations of metabolic intermediates. The amount of enzyme in a cell or compartment is regulated by protein synthesis and degradation.

11. Enzyme activity is regulated by non-covalent interactions with small molecular regulatory factors, or by reversible covalent reactions, e.g. phosphorylation or adenylation of an amino acid side chain. Enzymes which are directly regulated occupy key positions in metabolic pathways.