Amphipathic Lipids


An amphipathic substance is one that is polar at one end of the molecule (hydrophilic) and nonpolar (hydrophobic) at the other. Two examples which illustrate this type of lipid are cholesterol, a sterol, and phosphatidyl choline, a glycerophospholipid. Looking at cholesterol you can see the four rings that are characteristic of a sterol and the aliphatic chain extending from the five membered ring (D ring). Another important feature is the hydroxyl group on ring A. It is the oxygen of this group that makes the A ring end of the molecule slightly polar. The remainder of the molecule is hydrocarbon and therefore nonpolar. In order to orient the molecule in the way in which it is usually shown in a textbook, position the hydroxyl group in the lower left portion of the structure with the aliphatic chain in the upper right. The two methyl groups which are attached to the bridge carbons should be projecting toward you. Notice that since the rings are not aromatic they are not planar.


Phosphatidylcholine, as with all glycerophospholipids, contains a diacylglycerol, a phosphate, and a nitrogen containing component which in this case is choline. The two acyl groups can easily be identified by looking for the two hydrocarbon chains that are somewhat twisted around each other. These two chains are attached to a glycerol,the three carbon segment with three oxygens, by carboxylic esters. The third oxygen of glycerol is connected to the phosphate (orange) which being a diester is also bonded to the choline which is a two carbon unit containing a nitrogen (blue) bonded to three methyl groups. The nitrogen with its four bonds has a permanent positive charge. The phosphate is negative since at physiological pH its third acidic hydrogen is ionized. This phosphate/choline end of the molecule being charged is polar (hydrophilic), but the opposite end consisting of only hydrocarbon chains is nonpolar (hydrophobic).


Lipid Bilayer