Question

explain the relationship between the Law of Diffusion and the 2nd Law of Thermodynamics. What process(s) are available to cells to reverse the effects of the 2nd Law of Thermodynamics when it comes to molecules entering and exiting cells? Explain.

Answer 1

Diffusion is driven by entropy. There are more ways for molecules of a substance to be spread over the whole system than just one part, so diffusion of the solute represents an increase in entropy. Entropy tends to increase with time and so the solute will tend to spread out in time owing to random thermal motion. This is diffusion.

If there is more of a substance one side of the membrane than the other, there will tend to be a net flow of this substance from the side of low concentration to the side of high concentration due to the fact that this increases the entropy.

In a biological context, things are usually more complicated because there are forces between the solvent, solute, and membrane. Biological membranes often have proteins that act as channels that allow some solutes to pass but not others. However, the Second Law of Thermodynamics is still king. When you have energies of interactions between the constituents of the system, the maximization of the total entropy of the system and its environment (S_tot) means minimization of the free energy of the system, which is a balance between maximizing the entropy of the system (S_sys) and minimizing its internal energy (U_sys). The Second Law of Thermodynamics can’t tell us how fast diffusion will happen, but does tell us the direction everything flows and when it will stop flowing. Under these conditions, diffusion is described by two terms: (1) the drift current, which is due to free energy of interaction between the components of the system and (2) the diffusion current, which is purely entropic and causes flow from regions of high concentration to regions of low concentration.