The Structure and Function of the Cell Membrane

The cell membrane is a fluid mosaic of lipids, proteins, and carbohydrates. In this tutorial we will describe these three structures and how they function in the cell membrane. This topic provides another example of the relationship between structure and function.

The Structure of Lipids

Fats (triacylglycerols)

Although fats are not polymers, they are large molecules, and they are assembled from smaller molecules by dehydration reactions. A fat is constructed from two kinds of smaller molecules: glycerol and fatty acids.

Glycerol is an alcohol with three carbons, each bearing a hydroxyl group. A fatty acid has a long, linear (unbranched), carbon skeleton, usually 16 or 18 carbon atoms in length. At one end of the fatty acid is a "head" consisting of a carboxyl group. Attached to the carboxyl group is a long hydrocarbon "tail." The nonpolar C-H bonds in the tails of fatty acids are the reason fats are hydrophobic: fats separate from water because the water molecules hydrogen bond to one another and exclude the fat. 

In making a fat, three fatty acids each join to glycerol by an ester linkage, a bond between a hydroxyl group and a carboxyl group. The resulting fat, also called a triacylglycerol or triglyceride, thus consists of three fatty acids linked to one glycerol molecule. The fatty acids in a fat can be the same, or they can be of two or three different kinds. The length, number, and location of the double bonds in a fatty acid define its physical and chemical characteristics.

The terms saturated fats and unsaturated fats refer to the structure of the hydrocarbon tails of the fatty acids. If there are no double bonds between the carbon atoms composing the tail, then as many hydrogen atoms as possible are bonded to the carbon skeleton, creating a saturated fatty acid. The hydrocarbon tails can pack tightly together resulting in high melting points.

An unsaturated fatty acid has generally one to three double bonds at defined locations along the hydrocarbon chain. The fatty acid will have a kink in its shape wherever a double bond occurs that result in looser packing and lower melting points. This feature plays an important role in the relationship between temperature and the fluidity of the cell membrane.

The major function of fats is energy storage. A gram of fat stores more than twice as much energy as a gram of a polysaccharide, such as starch. In addition to energy storage, adipose tissue also serves to cushion vital organs, such as the kidneys, and provides insulation, by forming a layer of fat beneath the skin. This subcutaneous layer is especially thick in whales, seals, and most other marine mammals.

Phospholipids

Phospholipids are major components of the cell membrane.  They are similar to fats, but have only two fatty acids rather than three. The third hydroxyl group of glycerol is joined to a phosphate group, which is negative in electrical charge. Additional small molecules, usually charged or polar, can be linked to the phosphate group to form a variety of phospholipids.

Phospholipids are described as being amphipathic, having both a hydrophobic and a hydrophilic region. Their tails, which consist of hydrocarbons, are hydrophobic and are excluded from water. Their heads, however, which consist of the phosphate group and its attachments, are hydrophilic, and have an affinity for water.

Steroids

Steroids include cholesterol and certain hormones. They are characterized by a carbon skeleton consisting of four fused rings.  One steroid, cholesterol, is a common component of animal cell membranes and functions to help stabilize the membrane. It is also the precursor from which other steroids are synthesized. Thus cholesterol is a crucial molecule in animals although high levels of it in the blood may contribute to atherosclerosis.

Cholesterol molecules are interspersed among phospholipid tails in the bilayer.	Cholesterol is a steroid, lipid characterized by a carbon skeleton consisting of four fused rings.

The Structures of the Cell membrane

The Fluid Quality of Membranes

Membranes are not static sheets of molecules locked rigidly in place. A membrane is held together primarily by hydrophobic interactions, which are much weaker than covalent bonds. Most lipids are randomly mobile in the plane of the membrane with an average migration rate of 22 µm (micrometers) per second. It is rare, however, for a molecule to flip-flop transversely across the membrane, switching from one phospholipid layer to the other; to do so, the hydrophilic part of the molecule would have to cross the hydrophobic core of the membrane.

Temperature affects the fluidity of the membrane. A membrane remains fluid as temperature decreases, until finally, at some critical temperature, the membrane solidifies. The temperature at which a membrane solidifies depends on its fatty acid composition. A membrane rich in phospholipids with unsaturated hydrocarbon tails will remain fluid to a lower temperature because the kinks where the double bonds are located prevent the hydrocarbons from packing as closely together as saturated hydrocarbons. However, a cell can alter the lipid composition of its membranes to some extent as an adjustment to changing temperature. For instance, in many plants that tolerate extreme cold, such as winter wheat, the percentage of unsaturated phospholipids increases in autumn, an adaptation that keeps the membranes from solidifying during winter.

The steroid cholesterol, which is wedged between phospholipid molecules in the plasma membranes of animals, helps stabilize the membrane. At relatively warm temperatures, for example, 37C, the body temperature of humans, cholesterol makes the membrane less fluid by restraining the movement of phospholipids. However, because cholesterol hinders the close packing of phospholipids, it also lowers the temperature required for the membrane to solidify.

The Mosaic Quality of Membranes

Proteins

Proteins are the most structurally sophisticated molecules known, and account for more than 50% of the dry weight of most cells. Although they are diverse, humans have tens of thousands of different proteins, each with a specific structure and function. they are all polymers constructed from the same set of 20 amino acids.  Membrane proteins are classified into two major categories, Integral proteins and Peripheral proteins.  Integral proteins are generally transmembrane proteins, with hydrophobic regions that completely span the hydrophobic interior of the membrane. The hydrophilic ends of the molecule are exposed to the aqueous solutions on either side of the membrane. Proteins are much larger than lipids and move more slowly, but some do drift. Some membrane proteins seem to move in a highly directed manner, however, many others seem to be held virtually immobile by their attachment to the cytoskeleton.Peripheral proteins are not embedded in the lipid bilayer at all; they are loosely bound to the surface of the membrane, often to the exposed parts of integral proteins.

Carbohydrates

Membrane carbohydrates are usually branched oligosaccharides with fewer than 15 sugar units. Some of these oligosaccharides are covalently bonded to lipids, forming molecules called glycolipids. Most are covalently bonded to proteins, which are thereby glycoproteins. The oligosaccharides on the external side of the plasma membrane vary from species to species, among individuals of the same species, and even from one cell type to another in a single individual. The diversity of the molecules and their location on the cell's surface enable oligosaccharides to function as markers that distinguish one cell from another.

Membranes are asymmetric

Membranes have distinct inside and outside faces. The two lipid layers may differ in specific lipid composition, and each protein has directional orientation in the membrane. The plasma membrane also has carbohydrates, which are restricted to the exterior surface. This asymmetrical distribution of proteins, lipids, and carbohydrates is determined as the membrane is being built by the endoplasmic reticulum.  Molecules that start out on the inside face of the ER end up on the outside face of the plasma membrane.

The Mosaic of the Cell's Membrane

Summary: Membranes as Mosaics of Structure and Function

The biological membrane is a collage of many different proteins embedded in the fluid matrix of the lipid bilayer. The lipid bilayer is the main fabric of the membrane, and its structure creates a semi-permeable membrane. The hydrophobic core impedes the transport of hydrophilic structures, such as ions and polar molecules but enable hydrophobic molecules, which can dissolve in the membrane, cross it with ease. Proteins determine most of the membrane's specific functions. The plasma membrane and the membranes of the various organelles each have unique collections of proteins. For example, to date more than 50 kinds of proteins have been found in the plasma membrane of red blood cells.