Carrier proteins have a specific binding site for a solute. The binding of the solute causes a series of conformational changes in the shape of the protein which results in the solute being transported across the membrane and released.
Many carrier proteins work passively, by facilitated diffusion. This is because they are transporting the solute down its concentration gradient (downhill). However, some carrier proteins transport molecules against their concentraion gradient (uphill) and therefore work by active transport.
There are three main ways in which this active transport is achieved :
- Coupled transporters (also referred to as secondary active transport) couple the uphill transport of the target solute with the downhill transport of another solute. (See symporter and antiporter)
- ATP-ase pumps (also referred to as primary active transport) use the hydrolysis of ATP to drive the uphill transport of the solute.
- Light-driven pumps are found in bacteria and archaea. They use an input of light energy to drive the uphill transport of the solute.