[orgo814]

I'm a bit confused on the definition of antibiotic ionophores. My class notes say that it dissipate ion gradients but I thought it increased the permeability of an ion to a membrane (so would facilitate the gradient?). Please help me clear this up

[Yggdrasil]

A gradient is when the concentration of a particular ion is higher on one side of a membrane relative to the other.  If the ions are free to diffuse across the membrane, in what direction will the net flow of ions be?  Will this help to increase the concentration difference between the two sides or decrease the concentration difference between the two sides?

[orgo814]

They would flow towards lower concentration to establish an equilibrium. So how does this relate to antibiotic ionophores?

[Yggdrasil]

If you allow ions to flow towards lower concentration to establish an equilibrium, you are by definition dissipating ion gradients.

[orgo814]

Aren't ion gradients by themselves trying to establish equilibrium?

[Yggdrasil]

An ion gradient represents a system out of equilibrium because at equilibrium, the concentration of ions would be equal on both sides of the membrane.  Establishing a gradient requires energy as it is energetically unfavorable to move a molecule from an area of lower concentration to an area of higher concentration.  Energy can be recovered from gradients because the movement of ions from an area of high concentration to an area of lower concentration occurs spontaneously.  Thus, many processes establish gradients as a way to store energy, which can then be recovered later to do work (e.g. chemiosmosis in the production of ATP).

Extracting energy from a gradient requires that the ions move through a protein that is able to couple the movement of ions across the membrane to some chemical process (e.g. ATP synthase).  Although ionophores allow the movement of ions down their concentration gradient, they do not couple the ions to any other chemical process, so the energy stored in those ions are lost as heat.

A good analogy here might be a hydroelectric dam.  Dams can produce energy because they couple the flow of water across the dam to a turbine.  However, if one were to punch a hole in the dam, the water would flow through that hole without going through the turbine, so it will drain the reservoir without producing any electricity.  Ionophores are essentially acting like a hole in the metaphorical dam.