Terminology

Bioenergetics - The quantitative analysis of how organisms gain and use energy.

Chemical Potential - (also called partial molar free energy). In a system, the free energy that resides in a chemical component per mole of the component present. For example, in a system consisting of a moles of component A and b moles of component B, the total free energy G would be the sum of the free energy in the two components.

Coupled Reaction - A chemical reaction in which one of the products or reactants participates in another chemical reaction. The overall free energy change for the pair of reactions is the sum of the free energy change for each reaction.

Endergonic - In a nonisolated system, a process that is accompanied by a positive change in free energy (G) and therefore is thermodynamically not favored.

Enthalpy - A thermodynamic quantity (function of state) symbolized by H that is equal to the internal energy of a system plus the product of the pressure and volume: H = E + PV. It is equal to the heat change in constant-pressure reactions, such as most reactions in biological systems (Figure 3.3).

Entropy - A thermodynamic quantity (function of state) that expresses the degree of disorder or randomness in a system. According to the second law of thermodynamics, in an open system, this tends to increase unless energy is expended to keep the system orderly (Figure 3.2).

Exergonic - In a nonisolated system, a process that is accompanied by a negative change in free energy (G) and therefore is thermodynamically favored.

First Law of Thermodynamics - The law that states that energy cannot be created or destroyed and that it is therefore possible to account for any change in the internal energy of a system E by an exchange of heat ( q) and/or work ( w) with the surroundings. E = q - w.

Free Energy - A thermodynamic quantity (function of state), symbolized by G, that takes into account both enthalpy and entropy: G = H - TS where H is enthalpy, S is entropy, and T is absolute temperature. The change in this quantity (G) for a process, such as a chemical reaction, takes into account the changes in enthalpy and entropy and indicates whether the process will be thermodynamically favored at a given temperature (Figure 3.6).

Internal Energy - The energy contained in a system. For the purposes of biochemistry, the term encompasses all the types of energy that might be changed by chemical or nonnuclear physical processes, including the kinetic energy of motion and vibration of atoms and molecules and the energy stored in bonds and noncovalent interactions.

Second Law of Thermodynamics - The law that states that the entropy in a closed system never decreases. An alternative statement is that processes that are thermodynamically favored at constant temperature and pressure involve a decrease in free energy.

Standard State - A reference state, with respect to which thermodynamic quantities (such as chemical potentials) are defined. For substances in solution, standard state indicates 1 M concentration at 1 atm pressure and 25C.

State - A term defining the important parameters of a system. Thermodynamically, it is defined by prescribing the amounts of all substances present and any two of the following three variables: the temperature (T), the pressure on the system (P), and the volume of the system (V).

System - Any part of the universe chosen for study. It must have defined boundaries, but otherwise there are few restrictions.

Tautomerization - The process of forming structural isomers that differ in the location of their hydrogens and double bonds.