A body can do work, or work can be done upon a body; a body of water can turn a turbine, or one may pedal a bike to move it. If work is done on a body, it will possess energy. When energy is possessed by a body, the body can do work.

An agent may do work when it possesses energy, i. e., the amount of work that an agent can do is the amount of energy it possesses. So a body may gain kinetic and potential energy or lose the gained energy by pro­ducing heat or converting it to other forms of work.

Kinetic energy is due to the motion of a body.

Potential energy is due to the position or status of a body.

Frictional or colligative motion energy is produced in a water­fall; heat evolves to overcome a frictional resistance or checks the motion of a body but sets useless motion to others (e. g., rolling of peb­bles in a stream or dust behind a vehicle). Mechanical friction causes a matchstick to ignite.

Units of energy are the same as those of work and are assigned equiv­alent quantities. Some important definitions and units are given in the appendix. Energy content of some common substances are provided in Table 1.1.

1.1.1 Thermodynamics

All three principles of thermodynamics are very much applicable in the area of biological energy and chemical changes related to it. It is worth­while to review a few fundamental points. Chemical reaction can take

place only if the energy status changes, i. e., A will be converted to B only if B has a free energy content less than that of a change in free energy AF that is easy and spontaneous; reactions may be written as

A = B + (-AF) or A = B — AF

or

-AF = Fa

The reaction is called exergonic, or energy is evolved or given out. If AF has a positive expression, the reaction is driven by the input of energy and called endergonic; such reactions are difficult to complete. At equi­librium, AF = 0 (±), a point which may be arrived at by the end of the reaction, or a reaction may be typically of that type (practically sluggish, the progress of the reaction will depend on the change in concentration of reactants, the change of temperature or pressure, etc.).

AF = AF0 + RT ln B/A, where B/Ais the ratio at equilibrium or equilib­rium constant, i. e., Ksq. Then, 0 = AF0 + RT ln B/A or AF0 = — RT ln B/A =

-1363 logio Keq at 25°C. Here, R = 1.987 cal/mol/K, T = (273 + 25) K = 298 K, and ln B/A = 2.303 log10 Keq. This expression can be very useful:

When A and B exist equimolar, then the expression AF = AF0 + RT ln 1 means AF = AF0, and the state is called a standard state.

Chemical conversions and change of state need some other consider­ation in the light of the third law of chemical thermodynamics:

AF = AH — ATS

AH is the change in heat content, T is the absolute temperature at which the reaction occurs, and AS is the change in entropy (change, GR), or degree of disorder in the system, understood as the heat gained isothermally and reversibly per unit rise of temperature at which it happens (unit being calories per kelvin). The absolute value of H and S of a system cannot be directly determined. “Heat content” is also known as “heat content at constant pressure” or “enthalpy.” The third law sug­gests chemical pathway of finding entropy values in absolute terms. The first law of thermodynamics deals with conservation of energy and the second law with the relation between heat and work.

1. Energy cannot be destroyed or created, i. e., the sum of all energies in an isolated system remains constant.

2. All systems tend to approach a state of equilibrium. This means that the entropy change of a system depends only on the initial and final stages of the system, expressed by R. Clausius.

a. The total amount of energy in nature is constant.

b. The total amount of entropy in nature is increasing.