What is the role of protein channels in the cell membrane?
The phospholipid bilayer, shown below, that makes up the cell membrane is partially permeable . This means that it selectively prevents large, polar molecules and certain ions from passing into or out of the cell. Transport proteins are therefore used to move them, essentially by-passing the membrane.
There are two types of transport proteins: carrier and channel. Channel proteins are water-filled pores that enable charged substances (like ions) to diffuse through the membrane into or out of the cell. In essence, they provide a tunnel for such polar molecules to move through the non-polar or hydrophobic interior of the bilayer.
The molecule moves through the channel protein down its concentration gradient or, in other words, from an area of its higher concentration to an area of its lower concentration. This process is called facilitated diffusion.
Most channel proteins are gated, which means that a part of the protein molecule on the inside of the membrane can move to close the pore. This allows more control over ion exchange .