Final answer:
A carrier is a transmembrane protein that moves molecules across the membrane through conformational changes in its shape. This mechanism, vital for both passive and active transport, allows the selective movement of molecules that are too large for channel proteins.
Step-by-step explanation:
A carrier is a transmembrane protein that moves molecules across the membrane through conformational changes in its protein shape. This process is pivotal in both passive and active transport mechanisms of cells. Carrier proteins are specific to the molecules they transport, binding these molecules and altering their own structure to shuttle the substances across the cell membrane.
The specificity of these proteins plays a crucial role in the plasma membrane's selective permeability. Unlike channel proteins that provide a water-filled pore, carrier proteins undergo a series of shape changes to facilitate the movement of larger molecules which cannot pass through channels, such as amino acids and glucose. This transport can reach a saturation point where all carrier proteins are occupied, limiting the rate at which transport can occur.