Phospholipids, Cholesterol

Phospholipids

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.

To inspect the entire molecule first, observe:

A phospholipid molecule, in which each atom is represented by a sphere.
A phospholipid molecule, in which each covalent bond is represented by a stick.

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QUESTION: Which one of the following is LEAST polar?

a. oxygen and hydrogen covalent bone
b. nitrogen and hydrogen covalent bond
c. carbon and oxygen covalent bond
d. hydrogen and carbon covalent bond
e. phosphate group

To see where each component is located in the molecule, observe:

The three carbons of glycerol, which are shown as spheres, while the rest of the phospholipid is represented by sticks.
The carbons of one of the two fatty acids, which are shown as spheres, while the rest of the phospholipid is represented by sticks.
The phosphorus atom of the phosphate, which is shown as a sphere, while the rest of the molecule is respresented by sticks.

Amphipathic Nature of Phospholipids

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.

To locate the polar regions of the molecule, observe:

The positions of the oxygen (red), nitrogen (blue) and phosphorus (orange) atoms, which are shown as spheres. These are the atoms forming polar covalent bonds and clearly are found at one end of the molecule. The other end of the molecule is all carbons and hydrogens.

(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.)

Saturated and Unsaturated Fatty Acids

A fatty acid is said to be saturated if each carbon is joined to its neighboring carbons by a single bond. If one or more double bonds is present, the fatty acid is said to be unsaturated. (This term refers to the fact that further hydrogens could be added to the fatty acid.)

While a saturated fatty acid is a straight molecule on the average, the double bond in an unsaturated fatty acid produces a kink in the molecule. Due to these kinks, unsaturated fatty acids don't pack to together as compactly, making their phospholipids more fluid at room or body temperature.

The term trans fatty acid refers to polyunsaturated fatty acids that have been "partially hydrogenated" chemically. This is done commercially to make plant fatty acids more solid and to improve self-life. Margarine, for example, is manufactured from plant oils in this way. Trans fatty acids are also widely present in snack foods and fast foods. The chemical addition of the hydrogens puts some of the remaining double bonds into the "trans" configuration rather than the "cis" configuration. The trans double bond puts less of a kink in the molecule than does a cis. Epidemiological studies correlated consumption of trans fatty acids with increased risk of heart disease.

The term omega-3 fatty acid refers to the location of the first double bond in an unsaturated fatty acid. The counting starts at the end of the fatty acid molecule without the oxygens. Thus, in an omega-3 fatty acid, the first double bound is at the third position between carbons, counting from the end.

Cholesterol

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, as discussed under phospholipid bilayers, influences the physical properties of the lipid portion 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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