Compare and contrast ion channels with passive transporters

Compare and contrast ion channels with passive transporters. Give a specific example of each as you give a detailed explanation of their characteristics.

Solution

Ans Channel Proteins concerned mainly with inorganic ion transport are called ion channels. Ion cahnnels are highly selective. Most of the ion channels are not permanently open. Some channels open in response to change in electric potential and are calleds voltage gated channel. The ion channels that open in response to the binding of ligands are called ligand gated channels.

Many animal and plant cells contain specialized water channel in their plasma membrane facilitate the water flow called aquaporins. Aquaporins assemle homotetramers in which each monomer consists of six membrane spanniing alha helical domains with cytoplasmically oriented amino and carboxy termini. Each monomer functions as an indepent pore.

Inonophores: these are small hydrophobic lipid soluble molecules that dissolve in lipid bilayers and increase their permeability to specific inorganic ions. Inophores sheild the charge of the ion to be transported, enabling it to penetrate the hydrophobic interior of the lipid bilayer. There are two classes of ionophores mobile ion carrier and channnel formers.

1) Examples: Valinomycin isolated from Streptomyces fulvissimus is an example of mobile ion carrier which transports K+.

2) Gramicidin A produced by Bacterium bacillus brevis is a fifteen residue peptide with alternating D and L amino acids. In membranes it forms a channel.