Plasma membrane proteins. Coloured atomic force micrograph (AFM) showing the intracellular surface of a plasma membrane. The membrane consists of a lipid bilayer with many proteins (peaks) attached to it. Some of the proteins span the whole membrane (transmembrane proteins), whilst other sit on the surface (peripheral proteins). The proteins have a number of roles. Transmembrane proteins include channels, which allow ions and molecules to passively pass through them, transporters, which use energy to carry molecules into the cell, and structural proteins. Peripheral proteins include receptors, which allow cellular recognition and enzymes, which trigger cellular biochemical processes.