Transport proteins function in both active and passive transport to move molecules across the plasma membrane. Two main groups of transport proteins can be found within the plasma membrane, and each helps water-loving molecules pass across the plasma membrane.
Carrier proteins and channel proteins are the two major classes of membrane transport proteins.
Carrier proteins (also called pumps, permeases, or transporters) bind the specific solute to be transported and undergo a series of conformational changes to transfer the bound solute across the membrane. Cells carry out such active transport in three main ways.
- Coupled carriers couple the uphill transport of one solute across the membrane to the downhill transport of another.
- ATP-driven pumps couple uphill transport to the hydrolysis of ATP.
- Light-driven pumps couple uphill transport to an input of energy from light.
Channel proteins, in contrast, interact with the solute to be transported much more weakly. They form aqueous pores that extend across the lipid bilayer; when these pores are open, they allow specific solutes (usually inorganic ions of appropriate size and charge) to pass through them and thereby cross the membrane. Not surprisingly, transport through channel proteins occurs at a much faster rate than transport mediated by carrier proteins. More specifically, channel proteins help molecules across the membrane via passive transport, a process called facilitated diffusion. It’s important to remember that each channel protein can only bring in a specific molecule. For example, a calcium channel can only be used to transport calcium in and out of the cell. There are different types of channel proteins for different molecules, including ones for sodium, potassium, and chloride. In fact, there’s even a channel specifically for transporting water across the plasma membrane and are called aquaporins.