Organic Chemistry

Section 26.4 Membranes 009

polar

head

CH 2 OR^1

R^2

C

O

= CHO C

nonpolar

fatty acid

chains

O

O−

CH 2 O P

O

O(CH 2 ) 2 NH 3

+

cholesterol

molecule

enlargement of a

phosphoacylglycerol

Figure 26.1

A lipid bilayer.

IS CHOCOLATE A HEALTH FOOD?

We have long been told that our diets should in-

clude lots of fruits and vegetables because they are

good sources of antioxidants. Antioxidants protect against car-

diovascular disease, cancer, and cataracts, and they slow the ef-

fects of aging. Recent studies show that chocolate has high levels

of antioxidants—complex mixtures of phenolic compounds

(Section 9.8). On a weight basis, the concentration of antioxi-

dants in chocolate is higher than the concentration in red wine or

green tea and 20 times higher than the concentration in tomatoes.

Dark chocolate contains more than twice the level of antioxi-

dants as milk chocolate. Unfortunately, white chocolate contains

no antioxidants. Another piece of good news is that stearic acid,

the main fatty acid in chocolate, does not appear to raise blood

cholesterol levels the way other saturated fatty acids do.

The fluidity of a membrane is controlled by the fatty acid components of the phos-
phoacylglycerols. Saturated fatty acids decrease membrane fluidity because their
hydrocarbon chains can pack closely together. Unsaturated fatty acids increase fluidity
because they pack less closely together. Cholesterol also decreases fluidity (Sec-
tion 26.9). Only animal membranes contain cholesterol, so they are more rigid than
plant membranes.
The unsaturated fatty acid chains of phosphoacylglycerols are susceptible to reac-
tion with similar to the reaction described on p. 7 for fats and oils. Oxidation of
phosphoacylglycerols can lead to the degradation of membranes. Vitamin E is an im-
portant antioxidant that protects fatty acid chains from degradation via oxidation. Vit-
amin E, also called is classified as a lipid because it is soluble in
nonpolar organic solvents. Because vitamin E reacts more rapidly with oxygen than
triacylglycerols do, the vitamin prevents biological membranes from reacting with
oxygen (Section 9.8). There are some who believe that vitamin E slows the aging
process. Because vitamin E also reacts with oxygen more rapidly than fats do, it is
added to many foods to prevent spoilage.

-tocopherol

vitamin E

CH 3

CH 3

H 3 C

HO

O

a-tocopherol,

O 2 ,

3-D Molecule:

Vitamin E

AU: Sentence above is not definite,

whereas sentence in box

is quite definite. Want to be consistent?

Pg number to be fixed