The Plasma membrane of a cell is made up of a phospholipid bilayer which has a hydrophobic core and hence is impermeable to water soluble or polar organic and inorganic molecules.
However, a cell can maintain proper homeostsis and undergo metabolism only when it is able to exchange different hydrophilic and polar organic and inorganic chemical components present inside the cell with those present outside the cell.
This necessitates the development of a "path" that could connect the cell interior to the cell exterior.
The path is developed by integral membrane protein that spans across the lipid bilayer opening on either side of the membrane, thereby connecting the cell interior with the cell exterior.
These integral membrane proteins are amphipathic in nature, that is, they are composed of both polar and non-polar amino acid residues. The amino acid residues are arranged such that the non polar amino acid residues face and undergo hydrophobic interactions with the water avoiding lipid molecules in the plasma membrane. The polar amino acid residues face towards the lumen of the opening in between the cell interior and exterior such that they can form hydrogen bonding with the water soluble polar organic or inorganic molecules and help them traverse the plasma membrane.
These integral membrane proteins are of two types: Channel proteins and Carrier proteins.
The Channel proteins:
Form a direct and continuous opening across the plasma membrane. They can either remain always open or can be gated, that is, they open only on associating with specific signalling molecules.
They transport solutes from a region of high electrical charge or concentration to a region of low electrical charge or concentration, that is, down the electrochemical or concentration gradient.
They usually allow the easy and quick transport of water molecules and small ions across the membrane.
The Carrier proteins:
They undergo a change in their conformation to translocate solute molecules across the plasma membrane and they do not form a continuous opening.
When they open on one side of the membrane they are closed on the other side. They cannot remain, simultaneously, open on both sides of the membrane.
They are required mainly for the translocation of small polar organic molecules across the plasma membrane down the electrochemical or concentration gradient.
