Peripheral proteins , which are not embedded but attached to the membrane's surface. On cytoplasmic side, may be held by filaments of cytoskeleton. Membranes are bifacial.
Membrane proteins have distinct directional orientation.
When present, carbohydrates are restricted to the membrane's exterior. Side of the membrane facing the lumen of the ER, Golgi and vesicles is topologically the same as the plasma membrane's outside face. The ability of a cell to determine if other cells it encounters are alike or different from itself. Cell-cell recognition is crucial in the functioning of an organism. Rejection of foreign cells by the immune system.