In a biological membrane, the phospholipids are organized into a bilayer, which serves as the framework in which the other components of the membrane are embedded. The figure to the left shows the arrangement of the phospholipids in a cross section of a membrane.
Observe how the phospholipids are organized into two layers, with the fatty acid chains in each layer pointing towards the center of the membrane. Phospholipids naturally tend to assemble in this way, because the arrangement tends to satisfy the opposing tendencies of these amphipathic molecules. The nonpolar regions of the molecules, which are referred to as hydrophobic, face the interior of the bilayer, where they are shielded from water. On the other hand, the polar regions of the molecules, termed hydrophilic, face outward where they interact with the water inside and outside of the cell.
Phospholipid bilayers have been studied with computer simulations that follow the interactions of phospholipid molecules with each other and with water. (H. Heller, M. Shaefer, K. Schulten, J. Physical Chem.97:8343-8360,1993) The following set of figures are based on simulations of a bilayer. The researcher's hope was that such calculations would help understand the molecular environment in which membrane proteins function.
The bilayer was composed of a single type of phospholipid and no cholesterol was present. In the first set of figures below, the bilayer was assembled, but no interactions between the phospholipid molecules were computed. These figures are best for observing the general organization of the bilayer.
In the bilayer before molecular interactions were computed, observe:
Water molecules also were included on both sides of phospholipid bilayer. Observe the locations of the water molecules:
Once the bilayer was assembled, interactions between molecules were calculated on supercomputer to provide a glimpse into the molecular environment of a phospholipid bilayer. (The initial simulation was for 100 picoseconds.)
The figure to the left steps through several configurations adopted by the phospholipid molecules at the end of this simulation. Notice how much movement is actually occurring at the molecular level. The environment is much more dynamic than the impression that static images convey. (Images via Rasmol)
Actual biological membranes contain cholesterol as well as phospholipids. The figure to the right shows how cholesterol molecules are aligned with the phospholipid molecules on both sides of the bilayer. The hydroxyl group is positioned where it can interact with the lower oxygens on the phospholipids. Next, the ringed portion of cholesterol, which is rigid and flat, lies next to the first portions of the fatty acid chains. The cholesterol's linear tail, which is quite flexible, lies among the lower portions of the fatty acid chains.
Cholesterol influences the physical properties of the membrane, especially its fluidity. For example, positioned between the phospholipids, the cholesterol prevents their interaction and possible crystallization. Also, the rigid, planar portion of the molecule tends to block many of the motions of the first portions of the fatty acid chains. This makes the intermediate portion of the bilayer more rigid and less permeable to small, polar molecules. Lower down, near the center the of the bilayer, the flexible tail of the cholesterol molecule allows more movement, making the central part of the bilayer the most fluid.