Lipids and Fatty Acids: Structure, Properties, and Reactions
Lipids: An Overview
Lipids are a group of organic biomolecules characterized by:
- Solubility in non-polar solvents and insolubility in polar solvents.
- Composition mainly of carbon, hydrogen, and oxygen.
Fatty Acids
Fatty acids are molecules consisting of a hydrocarbon chain (-CH2-CH2-CH2-), which is aliphatic and linear, with an even number of carbon atoms, and a carboxyl group (-COOH), also called an acid group.
Saturated Fatty Acids
Saturated fatty acids have only single bonds between the carbon atoms in the hydrocarbon chain. The chain is linear. Form: CH3-(CH2)16-COOH
Unsaturated Fatty Acids
Unsaturated fatty acids have one or more double bonds between carbons in the chain. Those with one double bond are called monounsaturated (oleic acid), and those with more are polyunsaturated (linoleic acid). Form: CH3-(CH2)7-CH=CH-(CH2)7-COOH.
Esterification
Esterification is the reaction of a fatty acid with an alcohol, resulting in an ester and a water molecule. Form:
CH3-(CH2)14-C + HO-CH2-CH2-CH3 → CH3-(CH2)14-CO-CH2-CH3 + H2O
Saponification
Saponification is the reaction of a fatty acid with a strong base, leading to a fatty acid salt called soap. This reaction is very important because fatty acids, which are insoluble in water, form micelles dispersed in water. Form: CH3-(CH2)14-C + NaOH → CH3-(CH2)14-C-ONa + H2O
Solubility
Solubility: Aliphatic acids with 4 to 6 carbons are soluble in water (not fatty acids), while those with 8 or more are practically insoluble (fatty acids). This is because the carboxyl group (-COOH), the hydrophilic part, ionizes very little and is hardly significant. When soap is put in water, its molecules disperse with the carboxyl groups located in the water and the hydrocarbon chain extending beyond, forming a layer which is a thin surface film.
Melting Point
Melting Point: Fatty acids tend to cluster because of hydrogen bonds between the carboxyl groups and Van der Waals forces between the lipophilic chains.
Simple Lipids (Hololipids)
Simple lipids or hololipids are esters formed by fatty acids and an alcohol.
Acylglycerides
Acylglycerides are simple lipids formed by the esterification of one, two, or three molecules of fatty acids with a glycerol molecule. They are insoluble in water and float on it due to their low density.
Cerides
Cerides are lipids obtained by the esterification of a long-chain monovalent alcohol and a fatty acid molecule. They have a specific lipophilic character on both ends of the molecule and originate waterproof layers that protect the epidermis and dermal formations of animals.
Complex Lipids
Complex lipids are formed by fatty acid esters, an alcohol, and a third type of molecule. These molecules are major constituents of lipid bilayer membranes and are therefore also called membrane lipids. They are divided into three groups:
Phosphoglycerides
Phosphoglycerides are esters formed by two fatty acids, a glycerol, phosphoric acid, and an alcohol. Usually, the alcohol is an aminoalcohol or bears an amino group (-NH2).
Fosfoesfingolipids
Fosfoesfingolipids are esters formed by the combination of a fatty acid, a sphingosine, a phosphate group, and an aminoalcohol. The most abundant is sphingomyelin.
Glicoesfingolipids
Glicoesfingolipids are esters formed by the union of an acid, a sphingosine, and a glucide. They are not present in lipid bilayers in the membrane, where they function as receptors for external molecules. They are divided into two classes: brain and Gangl.