Small Biomolecules : Fatty Acids and Molecular Structural Polarity

Key Points

    1.    Why are fats and lipids insoluble in aqueous environments
    2. How does the structure of a fatty acid allow for different physical properties?
    3.    Where does one find fatty acids, fats and lipids in cells?

            The second group of small biomolecules is the fatty acids, which make up the fats and lipids, with the common property of insolubility in water. A fatty acid is a dipolar molecule with a straight chain of an even number of carbon atoms, with hydrogen atoms along the length of the chain at one end of the chain.  At the other end of the hydrocarbon chain is a carboxyl group (-COOH) at the other end. The carboxyl group makes the molecule an acid (see panel 2.4) If the carbon-to-carbon bonds are all single, the fatty acid is saturated; if any of the bonds is double or triple, the fatty acid is unsaturated and is more reactive. Some fatty acids have branched chains, while other fatty acids contain ring structures (e.g., prostaglandins).

        The most widely distributed fatty acid in cells is oleic acid, which is abundant in some vegetable oils (e.g., olive, palm, peanut, and sunflower seed) and which makes up about 46 percent of human fat.

            Many animals cannot synthesize one or more of the fatty acids and must ingest them in foods. Such fatty acids, as linoleic and arachodonic acid, are called essential fatty acids.

        Fatty acids are not found in a free state in cells. Commonly they exist in fats or lipids, which are composed of three fatty acids connected to a alcohol glycerol molecule via ester bonds forming a triglyceride.  A condensation reaction links the carboxyl end of one fatty acid to the hydroxyl of on one of the glycerol carbons, forming an ester bond, while eliminating water (see panel 2.5 triacylglycerols). Fats and lipids play the same general biological role as carbohydrates, one of structural molecules, as well as a storage form of energy reserve.

Hundreds of different fatty acids can contribute to lipid formation.  Some fatty acids have one or more double bonds in their hydrocarbon chain and are said to be unsaturated. panel 2.4 sat. vs. unsat. shows that the presence of a double bond in a fatty acid chain creates a rigid kink in the chain, while the rest of the fatty acid can freely rotate about the C-C bonds. Fatty acids with no double bonds are said to be saturated. Saturated fats remain solid at temperature at which unsaturated fats are melted liquids (shortening vs. vegetable oils).  Thus cold water sea animals with lots of saturated fats would have a reduced mobility.

           Another example of a lipid is a phospholipid, a major structural component of all cell membranes.  It is composed of a glycerol molecule, two fatty acids, a phosphate, and a large organic alcohol, such as choline or ethanolamine (see panel 2.4 P-lipid).

        One of the unique physical properties of phospholipids is a polarity in its molecular structure.  One end of the molecule contains only long chain hydrocarbon atoms (the fatty acid end) and as such exhibits a property of hydrophobicity. Hydrophobicity is best described being fat-soluble, but water insoluble or lacking an affinity for water. The other end of the molecule holds the phosphate group and an alcohol.  It is this end which is polar, i.e., has an electric charge, and is attracted to water (hydrophilic). The hydrophilic end of a phospholipid contains electronegative oxygen atoms and ionizable groups and thus exhibits an affinity for an aqueous environment, which is termed hydrophilicity.

        Phospholipids occur in plants, animals, and bacteria. Because phospholipids do not dissolve in water, but can form an interface with water, they make excellent structural components in which water-soluble species can be sequestered within an aqueous environment. It is just this amphipathic nature of phospholipids, i.e., containing both hydrophobic and hydrophilic groups, which makes then important in membrane structure.  Phospholipids can form a two-layer structure, called the lipid bilayer, with the polar head facing out on each surface to interact with water, and with the neutral "tails" driven inward and pointing toward one another. The lipid bilayer is the structural basis of all cell membranes and is nearly impermeable to ions and most polar molecules. Proteins embedded in the phospholipid matrix transport many substances through the membrane.

        All cell membranes contain phospholipids. Liver membranes are about 40 percent phospholipid and 60 percent protein. Some typical phospholipids of widespread occurrence in plants and animals include lecithin (phosphatidyl choline) and the cephalins (phosphatidyl ethanolamine and phosphatidyl serine). Other phospholipids include plasmalogens and  phosphoinositides, present in the brain, and cardiolipin, from heart tissue. In each of these phospholipids, a glycerol is acylated at two adjacent hydroxyl groups, with the third hydroxyl forms a phosphate ester to the substance indicated in the name. For example, in phosphatidylserine the phosphate is joined through oxygen to serine and a diacyl glycerol.

        So a phospholipid is a dimorphic molecule, where one end is soluble in an aqueous environment and the other end is insoluble in water.  This unique diversity of biological properties in a single molecule is what probably helped make its role in membranes so important at the interface between the aqueous environment and the interior of cells.