Carrier proteins are proteins involved in the movement of ions , small molecules , or macromolecules , such as another protein, across a biological membrane .  Carrier proteins are integral membrane proteins; that is, they exist within and span the membrane across which they transport substances. The proteins may assist in the movement of substances by facilitated diffusion (., passive transport) or active transport . These mechanisms of movement are known as carrier-mediated transport .  Each carrier protein is designed to recognize only one substance or one group of very similar substances. Research has correlated defects in specific carrier proteins with specific diseases.  A membrane transport protein (or simply transporter ) is a membrane protein  that acts as such a carrier.