Phospholipids are abundant in membranes and form the framework in which the other components of the membrane are embedded. Refer to the figure to the right. At the center of a phospholipid lies glycerol, which is a small three carbon molecule. A fatty acid is bonded to each of two carbons of the glycerol molecule. At the remaining carbon of the glycerol molecule is a phosphate, which is bonded in turn to a small polar molecule such as choline.
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QUESTION: Which one of the following is LEAST polar?
A phospholipid molecule is termed amphipathic because it has two conflicting tendencies. One end of the molecule, which is comprised of the fatty acid chains, is highly nonpolar because only carbons and hydrogens are present. The other end of the molecule, by contrast, is quite polar. It contains the highly polar phosphate group and a polar molecule such as choline.
(Another class of phospholipids, which we will not examine, is dervived from sphingosine, a somewhat larger molecule than glycerol. These phospholipids likewise have two fatty acids at one end and a phosphate and polar molecule at the other, producing amphipathetic molecules with similar overall properties to those discussed above.)
Cholesterol is a lipid molecule comprised almost entirely of carbons and hydrogens. At one end is a single oxygen molecule, which is part of a hydroxyl group. Cholesterol is most abundant in plasma membranes, and influences the physical properties of the membrane. (Each line in the figure to the right represents a covalent bond between carbon atoms. Hydrogen atoms are assumed.)
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