Lipids are a large group of organic compounds of various composition and structure. A lipid molecule always contains a long chain composed of fatty acids providing a poor solubility in water, which is related to the presence of a large number of hydrophobic nonpolar hydrocarbon groups (-CH2-). Besides to the hydrocarbon chains, lipid molecules comprise hydrophilic polar groups (i.e., ester, phosphate, hydroxy, or amino ones), which are capable to interact with water by the formation of hydrogen bonds. Due to their amphipathic (i.e., partly hydrophobic and partly hydrophilic) properties, lipids are capable to form a lipid bilayer where the hydrophobic effect and van der Waals
interactions cause that the polar head groups are oriented towards water (outside of the bilayer) while hydrophobic tails are hidden [1, 2]. The lipid bilayer of most animal membranes is composed mainly of three classes of molecules: phospholipids, glycolipids, and steroids (Fig. 2.7).
Figure 2.7 (a) Schematic structures of the three most common groups
of lipids, i.e., phospholipids, glycolipids, and steroids. (b) Chemical structure of one molecule representing three classes of membrane lipids (phosphatidylocholine, cholesterol, and galactocerebroside).
Phospholipids represent a large group of lipids, whose characteristic feature is the presence of phosphate group in the hydrophilic part of the molecules [1, 2]. The hydrophobic part consists of two fatty acid chains (Fig. 2.7a). The differences in structure of various phospholipids exist in both polar and hydrophobic parts. The former can have either ionic or amphipathic character, whereas hydrophobic tails can contain hydrocarbon chains of different lengths and degrees of unsaturation
. Within phospholipids, two sub-families of molecules can be distinguished, namely phosphoglycerides and sphingolipids.
Phosphoglycerides are derivatives of glycerol-3-phosphate. They have two hydrocarbon chains in the glycerol backbone and one phosphoric acid (or one phosphoric acid esterified group). The main members of phosphoglycerides are phosphatic acid (PA), phosphatidylocholine (PC), phosphatidyl-ethanolamine (PE), phosphatidylserine (PS, Fig. 2.7b), and phosphatidylinositol (PI). Hydrophilic regions of phospholipid molecules contain acidic phosphate groups possessing a negative charge (at pH 7.0). The other component of the hydrophilic region may be serine, choline, ethanolamine, or inositol. Both choline and ethanolamine contain the positively charged amino groups. The presence of two oppositely charged groups in the polar head of a lipid gives it the character of a dipolar ion with no net charge. Phospholipids containing serine or inositol have a negative net charge. Sphingolipids are derivatives of sphingosine, which is a long unsaturated alcohol chain. The most abundant, representative molecule is a sphingomyelin residing mostly in nerve cells but also present in kidney tissue and blood. This molecule contains very little amount of unsaturated fatty acids attached with sphingosine through nitrogen atom.
Glycolipids form the second group of lipids. They have covalently bound mono- or polysaccharides in the hydrophilic region (through glycosidic bonds). The most common saccharides molecules are galactose, glucose or lactose [1, 2]. Glycolipids can be found in lipid membranes of various organs like brain, nerve system, spleen, kidney, lung, liver, and erythrocytes. Analogously as phospholipids, glycolipids have amphipathic character (Fig. 2.7a). There are several sub-families of glycolipids such as glycosphingolipids, commonly found in nerve tissue, made up of ceramides connected to a carbohydrate moiety. Galactocerebroside (Fig. 2.7b) is a type of glycolipid that is commonly found in the myelin sheath around the nerves of vertebrates.
The third main groups of cell membrane lipids are steroids (Fig. 2.7a). The main feature of these molecules is the ring system of three cyclohexanes and one cyclopentane . There exist a variety of functional groups that may be attached. Like other membrane lipids, they are amphipathic. They contain the hydrophilic head (-OH) that is directly linked with hydrophobic part of the molecule. The most common molecule is cholesterol, in which a hydroxyl group functions as polar head group (Fig. 2.7b). Cholesterol is a ring compound containing a branched side chain. In the body, it is formed from two-carbon elements (acetic acid radical) and it is a raw material for the production of bile acids and various hormones.
Lipid composition of various cell membranes varies among distinct structures within a cell, depending on functions performed by specialized organelles . The Fig. 2.8 shows the synthesis site of lipids that are involved in signaling and recognition pathways.
The major glycerophospholipids assembled in the endoplasmic reticulum (ER) are phosphatidylcholine, phosphatidylethanolamine, phosphatidylinositol, phosphatidylserine, and phosphatidic acid. In addition, the ER synthesizes ceramides, galactosylceramide, cholesterol, and ergosterol. Both the ER and lipid droplets participate in steryl ester and triacylglycerol synthesis. The Golgi lumen is the site of synthesis of sphingomyelin, complex glycosphingolipids, and yeast inositol sphingolipid. Phosphatidylcholine is also synthesized in the Golgi, and may be coupled to protein secretion at the level of its diacylglycerol precursor. Approximately 45% of the phospholipids (mostly phosphatidylethanolamine, phosphatidic acid, and cardiolipin) in mitochondria are autonomously synthesized by this organelle. The inner membrane of late endosomes produces mostly bis(mono-acylglycero) phosphate (BMP).
Figure 2.8 Lipid composition of various cell membranes expressed as a percentage of the total amount of phospholipids (PL) in mammals (blue) and in yeast (light blue). As a measure of sterol content, the molar ratio of cholesterol (CHOL in mammals) and estrosterol (ERG in yeast) to phospholipid is also included. The synthesis sites of a particular lipids shows various organelles involved in this process (PA—phospha- tidic acid, PC—phosphatidylcholine, PE—phosphatidyleth- anolamine, PI—phosphatidylinositol, PS—phosphatidylserine, PG—phosphatidylglycerol, CL—cardiolipin, GalCer—galactos- ylceramide, TG—triacylglycerol, SM—sphingomyelin, DAG— diacylglycerol, GSLs—glycosphingolipids, and R—remaining lipids). Reprinted with permission from .
-  In the case of lysophospholipids, there is only one fatty acid chain.