Fatty acids as structural basis
Most lipids contain fatty acids (FAs), which share a common structural pattern: a radical and a carboxyl group (R-COOH). In cells, they rarely exist in a free state, more commonly acting as components of more complex molecules or as vital "fuel molecules" competing with glucose.
According to 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 accurately record the structure of FAs:
- **$
\Delta$-nomenclature (delta):** the position of double bonds is counted from the carboxyl carbon atom. For example, the notation $18:2 \Delta 9, 12$ means that the chain has 18 carbons and 2 double bonds at positions 9 and 12.
- **$
\omega$-nomenclature (omega):** indicates the position of only the first double bond, but counting starts from the opposite end — from the methyl group ($CH_3$). This is the origin of the well-known $\omega-3$ and $\omega-6$ families.
Main classes of complex lipids
Fatty acids can bind to various alcohols, forming 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 predominantly 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 backbone is not glycerol, but the amino alcohol sphingosine. Sphingolipids are present in vast amounts in nervous tissue, forming the myelin sheaths of neurons, and are also components of cell receptors.
Steroids and essential nutritional factors
Cholesterol is the main steroid in the body. It is an essential component of cell membranes, regulating the rigidity of their hydrophobic layer, and serves as the basic substrate for the synthesis of bile acids and all steroid hormones.
The human body is unable to synthesize certain lipids, which is why they are recognized as essential nutritional factors:
- Essential fatty acids (Vitamin F). These include the polyenoic linoleic ($\omega-6$) and linolenic ($\omega-3$) acids. They must be obtained from diet as they serve as the basis for synthesizing other polyenoic acids (e.g., arachidonic) 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 vital antioxidant that blocks the destructive action of free radicals on cell membranes.
- Vitamin K: necessary for adequate blood clotting.
Briefly: answers to topical questions
What is the chemical structure of sphingolipids?
Sphingolipids are based on the amino alcohol sphingosine. A fatty acid radical is attached to the amino group of sphingosine, forming ceramide (acylated sphingosine), which is the precursor for more complex sphingolipids. Various groups are attached to the hydroxyl group of ceramide (polar "head"):
- Sphingomyelins (sphingophospholipids) — contain a phosphoric acid residue and choline (or ethanolamine or serine).
- Glycolipids (cerebrosides and gangliosides) — contain a carbohydrate residue (a mono- or oligosaccharide in cerebrosides, a branched oligosaccharide with N-acetylneuraminic acid in gangliosides).
What classes of substances are synthesized from essential fatty acids?
Other polyenoic acids are synthesized from essential fatty acids (linoleic and linolenic); for instance, arachidonic acid is synthesized from linoleic acid. Furthermore, essential fatty acids (polyenoic acids with two or more double bonds) serve as substrates for the synthesis of local hormones — eicosanoids. Eicosanoids include prostaglandins, leukotrienes, and thromboxanes.
Which specific lipids form the myelin sheaths of neurons?
The myelin sheaths of neurons are formed by sphingolipids, primarily sphingophospholipids (sphingomyelins).
How to memorize essential acids: the word "Linoleic" is shorter and has 2 double bonds ($C_{18:2}$). The word "Linolenic" is longer — it has 3 double bonds ($C_{18:3}$).
Class and exam questions
What is the difference between omega and delta fatty acid nomenclature?
In the delta nomenclature ($ \Delta$), double bonds are counted starting from the carboxyl carbon, and all positions are listed. In the omega nomenclature ($ \omega$), the position of only the first double bond is indicated, counting from the opposite (methyl) end.
Why is arachidonic acid not always considered truly essential?
Although it is essential for life, the body is capable of synthesizing arachidonic acid on its own if an adequate amount of its precursor — essential linoleic acid — is provided in the diet.
What are glycerophospholipids used for?
These are the main structural components of cell membranes. In addition, phosphatidylcholine is the basis of pulmonary surfactant, which prevents alveoli from sticking together during exhalation.
What else to study on this topic
- Mechanisms of lipid peroxidation and the protective role of vitamin E
- Synthesis of eicosanoids (prostaglandins, leukotrienes) from polyenoic acids
- The role of cholesterol in cell membrane formation
- Structure and functions of sphingolipids in nervous tissue
- Metabolism of fat-soluble vitamins
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