Carrier proteins permit, but limit, the diffusion of particles across a cell membrane that is impermeable to them. They are a major type of protein used in facilitated diffusion, with the other being channel proteins. Unlike channel proteins that remain open for the passage of particles, carrier proteins are open on only one side at a time, accepting a single particle, closing on that side and opening on the other.
Carrier proteins are very important molecules embedded in the membranes of cells. There are many types of molecules and ions, including large, polar or charged molecules, that cannot pass through the cell membrane. In facilitated diffusion, these particles are in a lower concentration at their intended destination than they are on their original side of the membrane, and so move spontaneously through the carrier proteins.
Carrier proteins are also used in active transport where particles are traveling to an area of higher concentration. In these cases, the carrier proteins require energy to move the intended particles. This energy usually comes from ATP, although it can also come from moving a second particle from higher to lower concentration or, in certain bacteria, by directly using solar energy. Both carrier proteins and channel proteins are generally designed to transport just one type of particle.