Figure 1. Model of a biological membrane showing a phospholipid bilayer and intrinsic membrane proteins. The lipid bilayer consists of a hydrophobic core made up of the acyl chains of the lipids and interface regions made up of the polar headgroups of the lipids. Single-span or bitopic membrane proteins cross the membrane once and are of two types. Type I membrane proteins have their carboxyl-terminus on the cytosolic side of the membrane and their amino-terminus on the extracellular side, while type II membrane proteins have the opposite orientation. Multispan (polytopic) membrane proteins cross the membrane a number of times. The transmembrane segments (red boxes) are -helical in conformation. N, amino-terminus; C, carboxyl-terminus.

Figure 2. Structure of bacteriorhodopsin. The protein spans the membrane with seven helical segments that are bundled together to form a globular structure: (a) view from the inside of the cell, (b) side view from the lipid bilayer.

Figure 3. Structure of bacterial porin. Porin forms a barrel consisting of 16 antiparallel strands that form a transmembrane pore. Surface loops connecting the strands fold into the barrel: (a) view from the inside of the cell; (b) side view from the outer membrane.