Lipids Phospholipid bilayer Sphingolipid

As the final outer stratum comeum is formed the phospholipid bilayer deteriorates and intercellular lipid layers are formed.k l These contain principally ceramides, cholesterol, and free fatty acids. Some sphingolipids are covalently attached to proteins.3... 

They differ because phospholipids and sphingolipids have two nonpolar tails while detergents usually have only one. While detergents form micelles with one layer of molecules shaped into a sphere, the other two types form lipid bilayers such that a polar solvent can appear within and outside of the structure. 

The lipid bilayer of cell membranes is composed primarily of phospholipids and sphingolipids. After a discussion of the metabolism of these lipid classes, several aspects of membrane biogenesis are briefly described. 

Different cellular membranes vary in lipid composition (see Table 5-1). Phospholipids and sphingolipids are asymmetrically distributed in the two leaflets of the bilayer, whereas cholesterol is fairly evenly distributed in both leaflets. 

In aqueous systems, membrane lipids may exist in a gel-like solid state or as a two-dimensional liquid. In the case of pure phospholipids, these states interconvert at a well-defined transition temperature, Tc, that increases with alkyl chain length and decreases with introduction of alkyl chain unsaturation. In cell membranes, which have marked heterogeneity in both the polar and nonpolar domains of the bilayer, this state is described as liquid disordered . The presence of sufficient sphingolipids, with... 

Because membranes components participate in nearly every cell activity their structures are also dynamic and far from the equilibrium states that are most readily understood in biophysical terms. Newly synthesized bilayer lipids are initially associated with endoplasmic reticulum (Ch.3) whereas phospholipids initially insert into the cytoplasmic leaflet while cholesterol and sphingolipids insert into the luminal endoplasmic reticulum (ER) leaflet. Glycosylation of ceramides occurs as they transit the Golgi compartments, forming cerebrosides and gangliosides in the luminal leaflet. Thus, unlike model systems, the leaflets of ER membranes are asymmetric by virtue of their mode of biosynthesis. 

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