30
Chapter 2
acids—collectively termed eicosanoids—are derived from the
20-carbon, polyunsaturated fatty acid arachidonic acid.
Triglycerides
Triglycerides
(also known as triacylglycerols) constitute the
majority of the lipids in the body, and it is these molecules that
are generally referred to simply as “fat.” Triglycerides form
when
glycerol,
a three-carbon alcohol, bonds to three fatty
acids (see Figure 2–11a). Each of the three hydroxyl groups
in glycerol is bonded to the carboxyl group of a fatty acid by a
dehydration reaction.
The three fatty acids in a molecule of triglyceride need
not be identical. Therefore, a variety of fats can be formed with
fatty acids of different chain lengths and degrees of saturation.
Animal fats generally contain a high proportion of saturated
fatty acids, whereas vegetable fats contain more unsaturated
fatty acids. Saturated fats tend to be solid at low temperatures.
Unsaturated fats, on the other hand, have a very low melting
point, and thus they are liquids (oil) even at very low tem-
peratures. Thus, heating a hamburger on the stove melts the
saturated animal fats, leaving grease in the frying pan. When
allowed to cool, however, the oily grease returns to its solid
form.
Hydrolysis of triglycerides releases the fatty acids from
glycerol and allows these products to then be metabolized to
provide energy for cell functions. Thus, to store energy in the
form of triglycerides and polysaccharides requires dehydration
reactions, and both polymers break down to usable forms of
fuel through hydrolysis.
Phospholipids
Phospholipids
are similar in overall structure to triglycerides,
with one important difference. The third hydroxyl group of
glycerol, rather than being attached to a fatty acid, is linked to
C
+
C
H
H
O
(CH
2
)
3
H
CC
H
3
CH
2
CH
2
CH
CH
CH
2
CH
CH
(CH
2
)
5
CH
2
O
(CH
2
)
3
C
CH
3
CH
3
CH
2
CH
2
CH
2
CH
2
CH
2
CH
2
(CH
2
)
5
CH
2
Polyunsaturated fatty acid
Saturated fatty acid
Triglyceride (fat)
Water
+
Phospholipid (phosphatidylcholine)
H
H
C
H
H
(a)
(b)
CN
H
C
C
H
O
CC
H
2
CH
2
CH
2
O
CC
H
3
OC
H
2
CH
2
O
P
CH
3
O
CH
2
CH
2
H
H
H
O
CO
H
C
C
H
O
CC
H
3
OC
H
2
CH
2
O
CC
H
3
OC
H
2
CH
3
2
C
H
O
CH
2
CH
2
O
CH
3
CH
3
CH
3
O
(CH
2
)
3
CH
3
CH
2
CH
2
CH
2
CH
2
CH
2
CH
2
(CH
2
)
5
CH
2
O
CC
H
2
OH
HO
HO
HO
OH
OH
Dehydration
Glycerol
Fatty acids
+
+
+
H
2
O
Figure 2–11
Lipids. (a) Glycerol and fatty acids are the major subunits that
combine to form triglycerides.
Note:
The reaction presented here is
shown in simplifi ed form. Details are provided in Chapter 16. (b)
Phospholipids are formed from glycerol, two fatty acids, and one
or more charged groups.
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