All cells are enclosed by a membrane composed of lipids, proteins and carbohydrates. Eucaryotic cells also have internal membranes that divide the cell into different compartments. Biological membranes have several important functions, including:
- They separate the contents of a cell or organelle from the surrounding environment.
- They control import and export of molecules (e.g., nutrients, waste, ions) into and out of the cell or organelle, using proteins that span the membrane.
- They contain sensors or receptors that allow the cell to respond to external stimuli including communications from other cells.
- They are involved in cell movement.
Biological membranes are based on certain types of lipid molecules. Lipids are biological molecules that have little or no solubility in water, but are soluble in organic solvents. In addition to being structural components of biological membranes, lipids also function as energy storage molecules, enzyme cofactors, signaling molecules, and pigments. Structurally, lipids may be divided into several different classes, some of which are described below.
Fatty acids: Hydrocarbon chains ending in a carboxylic acid group. Fatty acids usually contain an even number of carbon atoms, and range in length from 4 to 36 carbons. If the hydrocarbon chain has no double bonds, it is called saturated. If it has one double bond, it is called monounsaturated. If it has two or more double bonds, it is called polyunsaturated. The first C-C double bond in unsaturated fatty acids is usually between C9 and C10, counting from the COOH end; it is almost always cis, forming a kink in the chain. Any additional double bonds are usually at every third carbon, as in the 18-carbon fatty acid shown below.
The structure of a fatty acid can be indicated by a short form in which the letter C is followed by the number of carbon atoms and double bonds in the fatty acid, separated by a colon. For example, the saturated fatty acid with 16 carbons would be represented C16:0. The 18-carbon unsaturated fatty acid shown below is most properly abbreviated C18:2 cis,cis-Δ9, 12, although sometimes the cis isomer or the positions of the double bonds are assumed, such that the abbreviation could be simply C18:2. In another nomenclature system, the double-bond position is counted from the methyl end of the molecule, and the number of the double-bonded carbon closest to the methyl end is given in brackets. In this system, the 18-carbon fatty acid shown below would be represented as C18:2(n-6). In older nomenclature, the terminal carbon is called the ω (omega) carbon, and sometimes you will see the abbreviation in the form C18:2(ω-6).
Triacylglycerols: Glycerol is a three-carbon molecule with hydroxyl groups at each carbon (see figure). Triacylglycerols (also called triglycerides) are obtained by attaching a fatty acid to each hydroxyl via an ester linkage. Most triacylglycerol molecules contain two or three different types of fatty acids. Triacylglycerol is used to store fatty acids as energy reservoirs in adipocytes.
Glycerophospholipids: These are like triacylglycerols, except that the fatty acid on one end of glycerol is replaced with phosphate. The phosphate group is often conjugated to a polar alcohol like ethanolamine (to make phosphatidylethanolamine, serine (to make phosphatidylserine, or choline (to make phosphatidylcholine,.
Sphingolipids: These are based on sphingosine (see figure). If a fatty acid is attached to the nitrogen of sphingosine via an amide linkage, the molecule is called a ceramide. The terminal hydroxyl group can be modified with phosphoethanolamine or phosphocholine to make sphingomyelins (found in the myelin sheath of nerve cells), or with carbohydrates to make glycosphingolipids.
Steroids: Steroids are based on a system of four fused rings, three with six carbons and one with five; the ring system is almost planar. Testosterone is an example of a steroid. Steroids that have a hydroxyl group at C3 are called sterols (e.g., cholesterol).
Other lipids: Some lipids have structures that do not fit into the other classes. For example, retinol, or vitamin A (see figure), is a polyisoprenoid that our bodies convert to retinal, which is a photoreceptor in the rod cells of the retina.
Glycerophospholipids, sphingolipids and the steroid cholesterol are amphipathic: one part of the molecule is hydrophobic and the other is hydrophilic. Because of their amphipathic nature, these lipid molecules are well suited to be the basic structural components of biological membranes. In water, glycerophospholipids and ceramides spontaneously assemble into bilayers such that the hydrophobic groups do not contact the polar solvent. The hydrophilic “head” groups are on the outside of the bilayer, next to the solvent, while the hydrophobic “tail” groups form the interior of the bilayer. Lipid bilayers are about 50 Å (5 nm) thick. Because they are composed of many individual molecules, lipid bilayer sheets are flexible. This flexibility allows them to protect their hydrophobic edges by spontaneously closing to form spherical vesicles or liposomes. Individual lipid molecules can move and diffuse within the plane of the bilayer. The rate of diffusion is determined by the bilayer’s fluidity. At constant temperature, fluidity is determined by three main factors:
- The proportion of unsaturated fatty acyl chains within the glycerophospholipids and sphingolipids. Cis double bonds impede tight packing of the fatty acyl chains, increasing fluidity.
- The length of fatty acyl chains present in the glycerophospholipids and sphingolipids. Shorter chains mean more mobility and fluidity.
- The amount of cholesterol in the bilayer. At 37°C and higher temperatures, increasing the amount of cholesterol in the membrane reduces the fluidity of the bilayer. Cholesterol fills gaps created by unsaturated fatty acyl chains and restricts the motion of other lipid molecules. At low temperatures, cholesterol increases the fluidity of the bilayer by interfering with orderly packing of other lipids.
Biological lipid bilayers are asymmetrical. That is, the lipid composition of one half or surface of the bilayer is different than the composition of the other half. For example, in human erythrocyte membranes, sphingomyelin and phosphatidylcholine are mostly found on the outer half, and phosphatidylethanolamine and phosphatidylserine are mostly found on the inner half. Lipids with attached carbohydrates are called glycolipids; these are not usually found on the cytosolic face of the membrane. However, cholesterol is evenly distributed on both halves.
Spontaneous “flip-flop” of a lipid from one side of the bilayer to the other occurs only rarely. The cell moves lipids from one side to the other using two classes of proteins: flippases and phospholipid translocases.