A cell membrane is composed of many parts as opposed to one single component or piece. Think of it like a jigsaw puzzle with all of the pieces fitting tightly togtether. One of the main organic components is protein, the function of which is to help particles move in and out of cells. Protein molecules can be found around the small holes in a cell membrane. The movement of these organic protein molecules is very limited. They are not able to move around the cell but remain by the holes in the membrane, acting as receptors.