The unifying feature of lipids is having little or no affinity to water. This property is designated as hydrophobicity. Hydrophobic nature comes from that lipids mostly consist of hydrocarbons, which form nonpolar covalent bonds. In aqueous environment, lipids try to clump together in order to stay away from water.
When lipid molecules sufficiently approach each other, weak but additive van der Waals forces forms and holds them together. Unlike other biological polymers, lipids are not held together by strong covalent bonds. Therefore, lipids are not true polymers but they can be considered polymers of individual lipid units.
Functions of Lipids:
- Phospholipids take structural role in cellular membranes.
- Fats and oils store energy.
- Fats in animal body provide thermal insulation.
- Lipid coating around nerves provide electrical insulation.
- Steroids like vitamins and hormones take regulatory role.
Triglycerides are known as fats and oils. Fats are solid, oils are liquid at room temperature. Triglycerides are composed of 1 glycerol molecule and 3 fatty acid molecules. Glycerol is a three-carbon alcohol with a hydroxyl (-OH) group attached to each carbon. A fatty acid consists of a carboxyl group (-COOH) attached to a long nonpolar hydrocarbon skeleton. Carboxyl group of fatty acids and hydroxyl group of glycerol can form covalent bond which is called ester linkage.
Fatty acids can either be saturated or unsaturated. In the case of a saturated fatty acid, all the carbons in the hydrocarbon chain are saturated with hydrogens, there are no double bonds. Animal fats are saturated fatty acids, their melting point is high and solid at room temperature. Whereas, in unsaturated fatty acids, there are one or more double bonds in the hydrocarbon chain. Double bond makes kink in the hydrocarbon chain. Unsaturated fatty acids tend to be liquid at room temperature and have low melting point due to the kinks of the structure. Triglycerides of plants belong to this category.
In food industry, there is a big desire for fats that are solid at room temperature. But these molecules come from expensive sources like animal fat. Plants are easy source but these always tend to be cis double bonding which will make them liquid at room temperature. Decades ago, some chemists decided to partially hydrogenated these plant oils and this process end up with trans double bonding and make fatty acids as solid at room temperature. The carbon chain extends from the same side of the double bond in cis-fats, while, the carbon chain extends from opposite sides of the double bond in trans-fat.
In a phospholipid, two fatty acids are attached to a glycerol but the third hydroxyl position of glycerol is attached to a phosphate which is subsequently attached to a choline group. Phosphate has negative charge and choline has positive charge so this portion of the molecule is polar as a result of this hydrophilic. While the two fatty acid tails are hydrophobic. In an aqueous environment, hydrophobic tails aggregate together, while hydrophilic heads face water and thus form a bilayer structure. Biological membranes are composed of phospholipid bilayer.
Another class of lipids is steroids. Steroids are lipids characterized by a carbon skeleton consisting of four fused rings. Cholesterol, an important steroid, is a component in animal cell membranes and gives rigidity to the membrane. Estrogen and testosterone are also steroids which are important for signaling of differentiation of traits.
Some vitamins are lipid-based; vitamin A, vitamin D, vitamin E, vitamin K.