Spontaneous Formation of Cell Membrane's Lipid Bilayer

Phospholipids

Lipids are fats.Lipids are discussed in detail in an earlier post of biochemistry. Phospholipid has a backbone derived from the 3-carbon polyalcohol glycerol. Attached to this backbone are two fatty acids, long chains of carbon atoms ending in a carboxyl (–COOH) group. A fat molecule has three such chains, one attached to each carbon in the backbone. A phospholipid, by contrast, has only two fatty acid chains attached to its backbone. The third carbon of the glycerol carries a phosphate group, thus the name phospholipid. An additional polar organic molecule is often added to the phosphate group as well.

                          By varying the polar organic group, and the fatty acid chains, a large variety of lipids can be constructed on this simple molecular framework. Mammalian membranes, for example, contain hundreds of chemically distinct phospholipids.

The Lipid Bilayer Forms Spontaneously

The phosphate groups of these lipids are charged, and other molecules attached to them are also charged or polar. This creates a huge change in the molecule’s physical properties compared with a triglyceride:The strongly polar phosphate end is hydrophilic, or “water-loving,” while the fatty acid end is strongly nonpolar and hydrophobic, or “water-hating.” The two nonpolar fatty acids extend in same direction, roughly parallel to each other, and the polar phosphate group points in the opposite  direction. To represent this, phospholipids are often diagrammed as a polar head with two dangling nonpolar tails.see figure

What happens when a collection of phospholipid molecules is placed in water? The polar water molecules repel the long, nonpolar tails of the phospholipids while seeking partners for hydrogen bonding. Because of the polar nature of the water molecules, the nonpolar tails of the phospholipids end up packed closely together, sequestered as far as possible from the water. When two layers form with the tails facing each other, no tails ever come in contact with water. The resulting structure is the phospholipid bilayer (figure below).        

Phospholipid bilayers form spontaneously, driven by the tendency of water molecules to form the maximum  number of hydrogen bonds.The nonpolar interior of a lipid bilayer impedes the passage of any water-soluble polar or    charged substances through the bilayer, just as a layer of oil impedes the passage of a drop of water. 

This barrier to water-soluble substances is the key biological property of the lipid bilayer.

Fluid Nature of Phospholipid Bilayer

A lipid bilayer is stable, because water’s affinity for hydrogen bonding never stops. Just as surface tension holds a soap bubble together, even though it is made of a liquid, so the hydrogen bonding of water holds a membrane together. Although water continually drives phospholipid molecules into the bilayer configuration, it does not have any effect on the mobility of phospholipids relative to their lipid and nonlipid neighbors in the bilayer. Because phospholipids interact relatively weakly with one another, individual phospholipids and unanchored proteins are comparatively free to move about within the membrane, like ships floating on a lake.The degree of fluidity of the plasma membrane can be altered by changing the fatty acid composition. Unsaturated fats make the membrane more fluid—the “kinks” introduced by the double bonds keep them from packing tightly.

             In animal cells, cholesterol may make up as much as 50% of membrane lipids in the outer leaflet. The cholesterol can fill gaps left by unsaturated fatty acids. This has the effect of decreasing membrane fluidity, but it increases the strength of the membrane. Overall this leads to a plasma membrane with intermediate fluidity that is more durable and less permeable.

              Changes in the environment can have drastic effects on the membranes of single-celled organisms such as bacteria. Increasing temperature makes a membrane more fluid, and decreasing temperature makes it less fluid. Bacteria have evolved mechanisms to maintain a constant membrane fluidity despite fluctuating temperatures. Some bacteria contain enzymes called fatty acid desaturases, which can introduce double bonds into membrane fatty acids. Genetic studies, involving either the inactivation of these enzymes or the introduction of them into cells that normally lack them, indicate that the action of these enzymes confers cold tolerance. At colder temperatures, the double bonds introduced by fatty acid desaturase make the membrane more fluid, counteracting the environmental effect of reduced temperature.