- Fatty acids are a special class of carboxylic acids that possess a long hydrocarbon chain. If the hydrocarbon chain is saturated - that is, if it contains no double bond - the fatty acid is called saturated fatty acid. Fatty acids whose hydrocarbon chain contains double bonds are known as unsaturated fatty acids. If the hydrocarbon chain contains more than one double bond, the acid is also called a polyunsaturated fatty acid.
Fatty acids may be obtained through the hydrolyzation of naturally occuring fats. Fats are triesters that consist of the trihydric alcohol glycerol (1,2,3-propanetriol) that is esterified with three fatty acids. The hydrocarbon chain of fatty acids usually contains eleven to nineteen carbons. In fatty acid biosynthesis, the hydrocarbon chain is constructed of acetic acid units, which are supplied by acetyl CoA (polyketide synthesis). As a result, naturally occuring fatty acids always contain an even number of carbons.
Enzymic cleavage of fatty acids is an important reaction of human metabolism. Through enzymatic cleavage, fatty acids are broken down into acetyl units which are subsequently incorporated into acetyl CoA. Acetyl CoA is an important metabolite that is the main starting product of the citric acid cycle (also called tricarboxylic acid cycle (TCA cycle) or Krebs cycle). Thus, in metabolism, fatty acids are synthesized as well as decomposed. Why is this? Fatty acids are energy-rich molecules. When the energy supply in the organism is large, fatty acids are synthesized. Fatty acids are later decomposed when the organism is in demand for energy.
The salts of fatty acids are soaps. Soap molecules contain hydrophobic as well as hydrophilic parts. The long, non-polar hydrocarbon chain is hydrophobic, while the polar, electrically charged carboxylate terminus is hydrophilic. Thus, fatty acid salt anions display qualities that are actually typical of surface-active agents (surfactants).
Surfactant molecules (detergents) in water assemble to micelles. Micelles are spherical conglomerates in which the hydrophobic chains (tails) of the fatty acids point to the micelle's center, while the hydrophilic heads are located at the micelle's surface where they come into contact with water molecules.
The detergent power of surfactants is the result of the inclusion of non-polar, hydrophobic dirt molecules into the hydrophobic center of micelles. Thus, the dirt is indirectly altered into a water-soluble, as micelles, due to their polar surface, are also water-soluble. Thus, dirt can be removed in this way.