Fatty Acids as Structural Building Blocks
Most lipids contain fatty acids (FAs), which share a general structural scheme: a hydrocarbon chain and a carboxyl group (R-COOH). In cells, they rarely exist in a free state, more often acting as components of more complex molecules or as vital 'fuel molecules' competing with glucose.
Based on the degree of carbon chain saturation with hydrogen, they are divided into three groups:
- Saturated. The most common in the body is palmitic acid (C16:0), accounting for about 30% of all FAs. Stearic acid (C18:0) is also significant.
- Monounsaturated (one double bond). The main representative is oleic acid (C18:1).
- Polyunsaturated (two or more double bonds). Includes arachidonic, eicosapentaenoic, and other acids.
Special nomenclatures are used to precisely record the structure of FAs:
- $\Delta$-nomenclature (delta): the position of double bonds is counted starting from the carboxyl carbon atom. For example, $18:2 \Delta 9, 12$ means the chain has 18 carbons and 2 double bonds at positions 9 and 12.
- $\omega$-nomenclature (omega): indicates the position of only the first double bond, but counting starts from the opposite end—the methyl group ($CH_3$). This is the origin of the well-known $\omega-3$ and $\omega-6$ families.
Major Classes of Complex Lipids
Fatty acids can bind to various alcohols to form functionally distinct classes of substances.
Triacylglycerols (TAGs) Consist of a glycerol molecule esterified with three fatty acid residues. This is the most compact and energy-dense form of calorie storage. TAGs are located primarily in adipose tissue (adipocytes). In addition to their energy function, they provide thermal insulation and mechanical protection for internal organs.
Glycerophospholipids Have an amphiphilic structure: two fatty acids are attached to glycerol, and the third position is occupied by a phosphate group linked to an alcohol (choline, ethanolamine, serine, or inositol bisphosphate). They form the lipid bilayers of cell membranes and the monolayer on the surface of lipoproteins.
Sphingolipids Their core is not glycerol, but the amino alcohol sphingosine. Sphingolipids are abundant in nervous tissue, forming the myelin sheaths of neurons, and are also components of cell receptors.
Steroids and Essential Nutritional Factors
Cholesterol is the body's primary steroid. It is an essential component of cell membranes, regulating the rigidity of their hydrophobic core, and serves as the basic substrate for the synthesis of bile acids and all steroid hormones.
Certain lipids cannot be synthesized by the human body and are therefore recognized as essential dietary factors:
- Essential Fatty Acids (Vitamin F). These include the polyunsaturated linoleic ($\omega-6$) and linolenic ($\omega-3$) acids. They must be obtained from the diet because they serve as the base for synthesizing other polyunsaturated acids (e.g., arachidonic acid) and local hormones—eicosanoids.
- Fat-Soluble Vitamins:
- Vitamin A: critical for photoreception; regulates cell differentiation and growth.
- Vitamin D: the main regulator of calcium metabolism.
- Vitamin E: a crucial antioxidant blocking the destructive action of free radicals on cell membranes.
- Vitamin K: necessary for adequate blood clotting.