Energy is the capacity to do work and overcome resistance.  Molecules tend to seek the least organized, least energetic state or highest enthalpy, H.

Equilibrium in a biological system is death - i.e. organisms require a regular input of energy.  Energy in biological systems is supplied by certain forms of chemical energy (see review) such as highly reduced molecules or high-energy structures.  Chemical energy has traditionally been measured in calories, abbreviated cal, but the SI unit, the joule, is now recommended.

Gibbs free energy, G, (named for J. Willard Gibbs, one of the founders of thermodynamics) = H - TS where T is the temperature (°K) and S is the entropy.

• DG is that portion of total energy that is available to do work and can be transferred to another system.
• energy is liberated as a reaction proceeds toward equilibrium under constant conditions of temperature and pressure.
• at equilibrium DG = 0.

R = the gas constant 1.987 cal/mol.
T = temperature in degrees Kelvin, 273.15 + °C

Standard free energy is designated DG°

• compares reaction with a standard
• reactants and products are kept at 1 M, T=25°C & pressure = 1 atmosphere

DG° = -RTlnK_eq

with a strongly -DG° at equilibrium there will be a higher molar concentration of C + D than A + B.

For the following reaction series the change in free energy is given for each step

The reaction can be coupled to an exergonic reaction

Kinase has two substrates glucose and ATP and binds to both.  The Gibbs Free Energy for the overall reaction is DG° = -4 (+3.3 - 7.3).  Therefore this reaction would tend to go toward G-6-P and ADP.

Another example:

DG° = 7.7 - 7.6 - 8.0 = ^-7.9