Membrane channel proteins. Illustration of the mechanism of action for membrane channel proteins (orange). These proteins, embedded in the cell membrane (phospholipid bilayer, blue), act as channels for molecules (orange spheres) moving into and out of a cell. The channel is closed at left and open at right. Biochemical signals prompt the opening and closing of the channels. Molecules diffuse down an electrochemical gradient to pass through an open channel. This differs from the action of membrane carrier proteins (see C023/8774). For this artwork with labels, see image C023/8771.