Lipids: Chemical Structure and Biological Functions

Study Notes

Lipids are a diverse group of compounds, including fats, oils, steroids, waxes, and related compounds.
They are relatively insoluble in water and soluble in nonpolar solvents such as ether and chloroform.

Lipids are important dietary constituents due to their high energy value.
Fat-soluble vitamins and essential fatty acids in natural foods are vital.
Fat is stored in adipose tissue, serving as a thermal insulator in subcutaneous tissues and around organs.
Nonpolar lipids act as electrical insulators, allowing rapid propagation of depolarization waves along myelinated nerves.
Lipoproteins are important cellular constituents, serving as the means of transporting lipids in the blood.

Fatty acids are hydrocarbon chains of various lengths (C4 to C36) and degrees of unsaturation, terminating with carboxylic acid groups.
Systematic names for fatty acids are derived from the parent hydrocarbon by substituting "oic" for the final.
Fatty acid notation (e.g., 18:0 or 18:2) indicates the carbon chain length and number of double bonds.

Most commonly occurring fatty acids have even numbers of carbon atoms in an unbranched chain of 12 to 24 carbons.
Double bonds in monounsaturated fatty acids are typically between C-9 and C-10 (Δ9), and in polyunsaturated fatty acids, generally Δ12 and Δ15.
Double bonds in polyunsaturated fatty acids are separated by a methylene group (-CH=CH-CH2-CH=CH-).

Palmitic acid (16:0)
Oleic acid (18:1)
Linoleic acid (18:2)
α-Linolenic acid (18:3)
Arachidonic acid (20:4)
Lignoceric acid (24:0)
Nervonic acid (24:1)

Essential fatty acids are precursors of prostaglandins.
Fatty acids with chain lengths of 4-10 carbons are found in significant quantities in milk.
Trans-unsaturated fatty acids are formed by microorganisms in the gastrointestinal tract of ruminant mammals or through chemical partial hydrogenation of fats and oils.