Amphipathic lipids Sphingolipids

A. The three major types of amphipathic lipids found in membranes are the gly-cerophospholipids (also called phosphoglycerides), the sphingolipids, and cholesterol. 

While the overall structural scheme of a polar head and two nonpolar tails is also found in sphingolipids, this class of amphipathic lipids has its own unique set of distinguihing structures. 

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. 

Phospholipids and sphingolipids are major components of cell membranes. They are amphipathic molecules that is, one portion of the molecule is hydrophilic and associates with H20, and another portion contains the hydrocarbon chains derived from fatty acids, which are hydrophobic and associate with lipids (see Figure 6-4). 

The structures of lipids are quite varied triacylglycerols (fats and oils), waxes, phospholipids, sphingolipids, steroids, eicosanoids, fat soluble vitamins, and pigments. Some lipids are simple in structure while others are more complex. Among these molecules are those which are esters in nature and therefore saponifiable in aqueous base. Others are nonsaponifiable. Many are completely nonpolar while others are amphipathic, that is, they have a polar/nonpolar nature. 

Cell membranes are constructed principally of phospholipids, sphingolipids, and proteins, together with small amounts of cholesterol. The common feature of the membrane lipids is their similarity to soap and detergent molecules—the presence of both hydrophilic and hydrophobic portions in the same molecule. Such molecules are referred to as amphipathic molecules. 

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