A few asides which are, nevertheless, important to electrophysiological principles 7. Both transporters and channels move solutes across the cell membrane but they differ in a number of fundamental characteristics. The energy for ion movements, per-se, is stored in the concentration gradients themselves. Channels do not saturate under physiological conditions. In fact, the amount of ion moved through the channel depends on the concentration gradient and the individual conductance of a given type of channel.
Channels allow ions to move down their concentration gradient primarily by diffusion.
The total number of ions moved across the channel is also restricted by the brief 'open' state of the channel. The specificity is dictated by the structure of the binding site for the solute. Channel specificity is defined within the channel proper by what is known as the 'specificity filter'. Although less specific that transporters, the channel structure conveys upon it sufficient specificity and the fact that no binding takes place allows for a very high speed of movement within the channel. Transporters under physiological conditions, transporters are designed for unidirectional movement of solutes.