Cholesterol amphipathic nature

Hydrophobic and Hydrophilic Components of Membrane Lipids A common structural feature of membrane lipids is their amphipathic nature. For example, in phosphatidylcholine, the two fatty acid chains are hydrophobic and the phosphocholine head group is hydrophilic. For each of the following membrane lipids, name the components that serve as the hydrophobic and hydrophilic units (a) phos-phatidylethanolamine (b) sphingomyelin (c) galactosyl-cerebroside (d) ganglioside (e) cholesterol. 

Sterols are a class of lipids that consist of a steroid and an alcohol. The most physiologically abundant sterol in humans is cholesterol, which is an integral part of the plasma membranes, confaring the fluidity of the lipid bilayer. Due to its amphipathic nature, cholesterol is an important structural component of cell membranes and the outer layer of plasma lipoproteins. Cholesterol plays an essential role both in the structure of cells and as a precursor to corticosteroids, mineral corticoids, bUe acids, vitamin D, and sex hormones such as testosterone, estrogen, and progesterone. As excess free cholesterol has cytotoxicity in cells, it is estrailied to a fatty acid to become a cholesteryl ester, a neutral form of cholesterol. Cholesteryl esters can be stored in the hpid droplets of cells without cytotoxicity, and therefore cholesterol esterification is increased whrai free cholesterol content in cells becomes excessive. 

The naturally occurring fatty acids are carboxylic acids with unbranched hydrocarbon chains of 4-24 carbon atoms. They are present in all organisms as components of fats and membrane lipids, in these compounds, they are esterified with alcohols (glycerol, sphingosine, or cholesterol). However, fatty acids are also found in small amounts in unesterified form. In this case, they are known as free fatty adds (FFAs). As free fatty acids have strongly amphipathic properties (see p. 28), they are usually present in protein-bound forms. 

The biochemical structure of a membrane is that of a lipid bilayer composed of phospho- and sphingolipids, as well as cholesterol. These lipids are amphipathic in nature, that is, they each have a polar and a nonpolar end. In water the nonpolar (hydrophobic, lipophilic) ends will seek to avoid the polar solvent and aggregate into a bilayer with the polar (hydrophilic, lipophobic) ends oriented towards the outside of the bilayer. As this structure extends in all directions the exposed nonpolar regions will close up and form a sphere (or ellipsoid) with water trapped inside and excluded outside. See Figures 2a and 2b. 

Several naturally occurring amphipathic molecules (in the body) exist, such as bile salts, phospholipids, cholesterol, which play an important role in various biological processes. Their interactions with other solutes, such as drug molecules, and with membranes are also very important. Below a brief summary of some of these biological surfactants is given, illustrating their interactions. 

Phosphoacylglycerols, sphingomyelins and cholesterol are incorporated into the lipid bilayer because of their amphipathic character (Chapter 8). The plasma membrane of human erythrocytes has a high cholesterol content, about 45% of the total lipid content. The bilayer formed from natural phospholipids is essentially a liquid in that it exhibits random motions consistent with the liquid state. Their hydrocarbon tails may move more freely about in the plane of the monolayer without losing their hydrophobicity which is responsible for their mutual attraction. Membranous proteins embedded within the membrane can also move laterally but are limited in the magnitude of such migrations by their interactions with other components located internal to the membrane. 

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