phospholipids
plants. Saturated triglycerides are those containing a higher proportion of saturated fatty acid chains fewer carbon-to-carbon double bonds ; unsaturated triglyceridescontain a higher proportion of unsaturated fatty acid chains. Saponification is the hydrolysis of a triglyceride in a basic solution to form glycerol and three carboxylate anions or soap molecules. Other important reactions are the hydrogenation and oxidation of double bonds in unsaturated fats and oils. Phospholipids are lipids containing phosphorus. In phosphoglycerides, the phosphorus is joined to an amino alcohol unit. Some phosphoglycerides, like lecithins, are used to stabilize an emulsiona dispersion of two liquids that do not normally mix, such as oil and water. Sphingolipids are lipids for which the precursor is the amino alcohol sphingosine, rather than glycerol. A glycolipidhas a sugar substituted at one of the OH groups of either glycerol or sphingosine. All are highly polar lipids found in cell membranes. Polar lipids have dual characteristics: one part of the molecule is ionic and dissolves in water; the rest has a hydrocarbon structure and dissolves in nonpolar substances. Often, the ionic part is referred to as hydrophilic literally, water loving and the nonpolar part as hydrophobic water fearing . When placed in water, polar lipids disperse into any one of three arrangements: micelles,monolayers, and bilayers. Micelles are aggregations of molecules in which the hydrocarbon tails of the lipids, being hydrophobic, are directed inward away from the surrounding water , and the hydrophilic heads that are directed outward into the water. Bilayers are double layers arranged so that the hydrophobic tails are sandwiched between the two layers of hydrophilic heads, which remain in contact with the water. Every living cell is enclosed by a cell membrane composed of a lipid bilayer. In animal cells, the bilayer consists mainly of phospholipids, glycolipids, and the steroid cholesterol. Embedded in the bilayer are integral proteins, andperipheral proteins are loosely associated with the surface of the bilayer. Everything between the cell membrane and the membrane of the cell nucleus is called the cytoplasm.