A membrane protein is a protein molecule that is attached to, or associated with the membrane of a cell or an organelle. Integral membrane proteins are tightly bound to the plasma membrane and include the membrane channels. Peripheral membrane proteins are moe loosely associated with the plasma membrane.
Integral Membrane Proteins
Many of the proteins associated with the plasma membrane are tightly bound to it. Some are attached to lipids in the bilayer. In others - the transmembrane proteins - the polypeptide chain actually traverses the lipid bilayer. All G-protein-coupled receptors, e.g., receptors of peptide hormones, and odors each span the plasma membrane 7 times.
Some transmembrane proteins that span the bilayer several times forming a hydrophilic channel through which certain ions and molecules can enter (or leave) the cell.
Peripheral Membrane Proteins
These are more loosely associated with the membrane. They are usually attached noncovalently to the protruding portions of integral membrane proteins.
Membrane proteins are often restricted in their movements.
A lipid bilayer is really a film of oil. Thus we might expect that structures immersed in it would be relatively free to float about. For some membrane proteins, this is the case. For others, however, their mobility is limited:
Facilitated diffusion of ions takes place through proteins, or assemblies of proteins, embedded in the plasma membrane. These transmembrane proteins form a water-filled channel through which the ion can pass down its concentration gradient.
The transmembrane channels that permit facilitated diffusion can be opened or closed. They are said to be "gated".
Some types of gated ion channels:
Ligand-gated ion channels.
Many ion channels open or close in response to binding a small signaling molecule or "ligand". Some ion channels are gated by extracellular ligands; some by intracellular ligands. In both cases, the ligand is not the substance that is transported when the channel opens.
Mechanically-gated ion channels
Voltage-gated ion channels
In so-called "excitable" cells like neurons and muscle cells, some channels open or close in response to changes in the charge (measured in volts) across the plasma membrane.