Membrane Transport - Thermodynamics

Thermodynamics

A physiological process can only take place if it complies with basic thermodynamic principles. Membrane transport obeys physical laws that define its capabilities and therefore its biological utility.
A general principle of thermodynamics that governs the transfer of substances through membranes and other surfaces is that the exchange of free energy, ΔG, for the transport of a mole of a substance of concentration C₁ in a compartment to another compartment where it is present at C₂ is:

Where C₂ is less than C₁ ΔG is negative, and the process is thermodynamically favorable. As the energy is transferred from one compartment to another, except where other factors intervene, an equilibrium will be reached where C₂=C₁, and where G=0. However, there are three circumstances under which this equilibrium will not be reached, circumstances which are vital for the in vivo functioning of biological membranes:

• The macromolecules on one side of the membrane can bond preferentially to a certain component of the membrane or chemically modify it. In this way, although the concentration of the solute may actually be different on both sides of the membrane, the availability of the solute is reduced in one of the compartments to such an extent that, for practical purposes, no gradient exists to drive transport.
• A membrane electrical potential can exist which can influence ion distribution. For example, for the transport of ions from the exterior to the interior, it is possible that:

Where F is Faraday's constant and ΔP the membrane potential in volts. If ΔP is negative and Z is positive, the contribution of the term ZFΔP to ΔG will be negative, that is, it will favor the transport of cations from the interior of the cell. So, if the potential difference is maintained, the equilibrium state ΔG=0 will not correspond to a equimolar concentration of ions on both sides of the membrane.

• If a process with a negative ΔG is coupled to the transport process then the global ΔG will be modified. This situation is common in active transport and is described thus:

Where ΔGb corresponds to a favorable thermodynamic reaction, such as the hydrolysis of ATP, or the co-transport of a compound that is moved in the direction of its gradient.