Textbook - BIOLOGICAL CHEMISTRY - Y.I. Hubsky - 2000

Chapter I. BIOMOLECULES AND CELLULAR STRUCTURES

CHAPTER 5. LIPIDS. BIOMEMBRANES

Lipids are a Class of bioorganic compounds characterized by their insolubility in Water and other polar Solvents, alongside their ability to dissolve in non-polar (hydrophobic) liquids. Non-polar solvents (such as diethyl ether, carbon tetrachloride, chloroform, etc.) are used to extract lipids from biological objects (Blood, Tissues, etc.).

In the Human and Animal body, lipids perform essential Functions as energy stores and suppliers, as well as Structural components of Cells, particularly Introduction/36.html">Biological Membranes. Certain classes of lipids also act as physiologically active substances (Vitamins, Hormones).

5.1. GENERAL CHARACTERISTICS OF LIPIDS. FATTY ACIDS

In terms of their chemical Structure, most lipids are esters of higher carboxylic (fatty) acids and alcohols (glycerol, sphingosine, Cholesterol, etc.). Many classes of lipids (Complex Lipids) also incorporate residues of phosphoric acid, nitrogenous bases (colamine, Choline), CARBOHYDRATES, etc.

Fatty acids. The most critical factor determining the physicochemical and biological properties of lipids is their fatty acid composition. The number of carbon atoms and, consequently, the length of the hydrocarbon chain, together with the degree of saturation of the fatty acids that make up natural lipids (neutral fats, Phospholipids, Sphingolipids, etc.), determine their consistency (liquid or solid) and surface activity. Specifically, these factors govern their ability to complex with Proteins, leading to The formation of micelles, bilayers, transport Lipoproteins, and the lipid matrix of biological membranes.

The lipids of The Human Body and higher animals contain fatty acids with an even number of carbon atoms, ranging from 12 to 24 C atoms, predominantly from C₁₆ to C₂₀ (Higher Fatty Acids) (Table 5.1).

In the human body, fatty acids exist partly in a free, i.e., non-esterified state (so-called non-esterified fatty acids — NEFAs), primarily in Blood Plasma, while the bulk are incorporated into complex structural lipids and adipose tissue triacylglycerols serving as an energy reserve (Table 5.2).

Among saturated fatty acids in human fats, palmitic acid (С₁₅Н₃₁СООН) predominates, while among unsaturated ones, oleic acid (С₁₇Н₃₃СООН) is the most abundant, accounting for about 60 % of the total major fatty acids found in human adipose tissue triacylglycerols. The presence of a significant amount of low-melting oleic acid in lipids determines the liquid state of human body fats, which have an average melting point of 10-15 °C. For instance, lipids stored in adipose tissue adipocytes consist of spherical lipid droplets composed mainly of triglycerides, whereas blood plasma lipids exist as a solution of lipid micelles stabilized by proteins in the form of lipoproteins.

Table 5.1. Most Common Natural Fatty Acids (Y.I. Hubsky et al., 1997)

Table 5.2. Fatty Acid Content in Human Adipose Tissue Triglycerides