Dynamic Equilibrium

∞ generated and posted on 2016.08.20 ∞

A system in which change is constantly occurring but, without input of energy, over time change to any net degree does not occur.

Dynamic equilibrium is a situation in which a system that otherwise is prone to changing in fact is displaying no overall change. This lack of overall change is a consequence of a balance between forces that otherwise would result in a change to the system and those forces that instead serve to restore the system. Such lack of change also can occur in situations in which there is not just no net change but no change whatsoever. That is, an equilibrium can either by dynamic or instead static (where no change whatsoever represents a static equilibrium).

The concept of dynamic equilibrium is important for understanding biology, and particularly so for understanding the chemistry of aqueous solutions. In biology one sees this concept especially in terms of substance movement across membranes and in terms of ligand or regulatory molecule binding to proteins. In the case of movement across membranes, rates of crossing in both directions come to balance each other at dynamic equilibria such as the movement of water into and out of cells.

In the case of small-molecule binding to proteins, this is equivalent to a saturated salt solution in which ions are constantly entering into and the leaving the fluid phase (dissolved) from the solid phase (not dissolved). The higher the concentration in the fluid phase, the more likely the solute is to exit the fluid phase for the solid phase. So too, the more solid phase that is in contact with the fluid (solvent) then the faster the rate of movement to the fluid phase (dissolving).

At dynamic equilibrium, the rates of solid → fluid movement (dissociation) equals the rate of fluidsolid movement (association). With ligand-protein interactions, the more ligand that is in solution, the more ligand-to-protein binding that will be occurring at a given moment. As this binding is reversible, if ligand is removed from solution, then the ligand-protein dissociation process will come to dominate. At dynamic equilibrium, binding and dissociation exactly balance with, at a given moment, (1) some fraction existing in solution, (2) some fraction bound, (3) some fraction dissociating, and (4) some fraction associating. That is, (3) and (4) exactly balance each other. (1) and (2), by contrast, typically are not equal to each other.

Dynamic equilibrium

Figure legend: At dynamic equilibrium, ligand and protein – across the ligand and protein population – are associating just as fast as they are dissociating. Adding more ligand will increase rates of association until dynamic equilibrium is reestablished. Removing ligand will decrease rats of association until dynamic equilibrium again is reestablished.

If nothing is changing, but something is happening (without input of energy), then the system is at dynamic equilibrium whereas if nothing is changing, and nothing is happening, then the system is at static equilibrium. If energy is being pumped into the system, causing change but no net change, then the system instead can be said to be displaying a steady state.

The following video shows calcium carbonate decomposition and reformation, including showing what happens when you perturb an equilibrium:

The following video Illustrates equilibrium and its establishment: