Living systems increase the entropy of their surroundings, as predicted by thermodynamic law. We can trace the ancestry of the plant kingdom to much simpler organisms called green algae.  However, this increase in organization over time in no way violates the second law.  The entropy of a particular system, such as an organism, may actually decrease, so long as the total entropy of the universe – the system plus the surroundings – increases.  Thus, organisms are islands of low entropy in an increasingly random universe.

Free-Energy Change, dG:

The universe is equivalent to “the system” plus “the surroundings”.

Free Energy: Measures the proportion of a system’s energy that can perform work when temperature and pressure are uniform throughout the system, as in a living cell.


dH symbolizes the change in the system’s enthalpy (in biological systems, equivalent to total energy); dS is the change in the system’s entropy; and T is the absolute temperature in Kelvin (K).

Once we know the value of dG for a process, we can use it to predict whether the process will be spontaneous (that is whether it will run without an outside input of energy). Negative dG are spontaneous.  For a process to occur spontaneously, therefore, the system must either give up enthalpy (H must decrease), give up order (TS must increase), or both: When the changes in H and TS are tallied, dG must have a negative value (dG<0).  This means that every spontaneous process decreases the system’s free energy.  Processes that have a positive of zero dG are never spontaneous.

Free Energy, Stability, and Equilibrium:

When a process occurs spontaneously in a system, we can be sure that dG is negative.  Another way to think of dG is to realize that it represents the difference between the free energy of the final state and the free energy of the final state and the free energy of the initial state.

dG=Gfinal state – Ginitial state

Thus, dG can only be negative when the process involves a loss of free energy during the change from initial state to final state.  Because it has less free energy, the system in its final state is less likely to change and is therefore more stable than it was previously.

·Unstable systems (higher G) tend to change in such a way that they become more stable (lower G).  A diver on top of a platform is less stable than when floating in the water, a drop of concentrated dye is less stable than when the dye is spread randomly through the liquid, and a sugar molecule is less stable than the simpler molecules into which it can be broken.

Another term for a state of maximum stability is equilibrium.  There is an important relationship between free energy and equilibrium, including chemical equilibrium.  Most chemical reactions are reversible and proceed to a point at which the forward and backward reactions occur at the same rate.  The reaction is at chemical equilibrium if there is no further net change in the relative concentration of products and reactants.

As a reaction proceeds toward equilibrium, the free energy of the mixture of reactants and products decreases.

·For a system at equilibrium, G is at its lowest possible value in that system.  Any small change from the equilibrium position will have a positive dG and will not be spontaneous.  Systems are never spontaneously move away from equilibrium.  Because a system at equilibrium cannot spontaneously change, it can do no work.  A process is spontaneous and can perform work only when it is moving toward equilibrium.


  • ·More free energy (higher G) > less stability > greater work capacity
  • ·In a spontaneous change
  • -The free energy of the system decreases (dG<0)
  • -The system becomes more stable
  • -The released free energy can be harnessed to do work  > less free energy (lower G) > more stable > less work capacity
  • Gravitational Motion: Objects move spontaneously from a higher altitude to a lower one.
  • Diffusion: Molecules in a drop of dye diffuse until they are randomly dispersed.
  • Chemical Reaction: In a cell, a sugar molecule is broken down into simpler molecules.

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