Biological Functions and Chemical Properties of Lipids

Characteristics of Lipids and Fats

Lipids are a very heterogeneous group of chemicals, both from a structural point of view and regarding the functions they perform. They are primarily composed of carbon, hydrogen, and oxygen, though the latter is usually present in very low proportions. Some lipids also contain phosphorus, nitrogen, and sulfur. Lipids are a group of organic biomolecules that satisfy two distinguishing characteristics: they are insoluble in water and other polar solvents, and they are soluble in organic solvents (i.e., non-polar solvents such as gasoline or benzene).

Molecular Structure of Fatty Acids

Fatty acids are molecules composed of long aliphatic hydrocarbon chains. These are linear chains with an even number of carbon atoms, the last of which constitutes a carboxyl group (also called an acid group). The bonds form angles of 109.5°. They can be classified into two groups:

• Saturated: They only have single bonds between carbon atoms.
• Unsaturated: They have one or more double bonds between carbons in the hydrocarbon chain. These molecules have "elbows" or bends where the double bonds are located.

Physical Properties of Lipids

The main physical properties include:

• Amphipathic (Unfriendly) Character: They exhibit dual behavior. Part of the molecule is hydrophilic (water-soluble) and part is hydrophobic (water-insoluble). The hydrophilic part is the ionized carboxyl group (-COO-), while the hydrophobic part corresponds to the hydrocarbon chain (CH3-CH2-CH2-...).
• Solubility: This refers to the behavior when pouring the fatty acid into water. 1

Low Melting Point: The degree of unsaturation and chain length determine the melting point of a fatty acid. The melting point increases if you increase the chain length and decreases with the presence of double bonds. Packing of Molecules by Van der Waals Bonds: Fatty acid molecules tend to cluster because hydrogen bonds are established between the carboxyl groups, and especially because Van der Waals bonds are formed among the lipophilic portions of the hydrocarbon chains.

Chemical Properties: Esterification and Saponification

• Esterification: This is the process of formation of an ester and water by reacting a fatty acid with an alcohol. An ester and water are produced by reacting a fatty acid with an alcohol. An ester is the combination of a fatty acid and an alcohol by a covalent bond called an ester bond.
• Saponification: The reaction of a fatty acid with a strong base resulting in a salt of the fatty acid, commonly known as soap, and water.

Classification of Saponifiable and Unsaponifiable Lipids

Saponifiable Lipids

These contain long-chain fatty acids, and saponification reactions occur within them. They are esters; i.e., they are the product of the union of a fatty acid and an alcohol.

• Simple Lipids (Hololipids): These include fats and waxes.
• Complex Lipids (Heterolipids): These incorporate alcohol, fatty acids, and other molecules.

Unsaponifiable Lipids

These contain fatty acids in their composition. There are two primary types: terpenes and steroids-2.