Facilitated diffusion is the type of transport that occurs when membrane proteins bind to a molecule on one side of the cell membrane and then releases it on another side. An integral membrane protein provides continuous protein lined pathway through bilayer. There are two classes of proteins transport membrane one which carries specific molecules across and two is channel proteins which form narrow pore where ions can pass.
Channel proteins also carry out passive transport where ions level spontaneously down their gradients membranes which transport proteins are important pharmacologically with two reasons. One is some drugs exploit endogenous membrane transport process to enter or exit cells. Two membrane transport proteins are major drug targets.