Chemical Composition of the Body
29
fasting, leads to release of the glucose subunits into the blood,
thereby preventing blood glucose from decreasing to danger-
ously low levels.
Lipids
Lipids
are molecules composed predominantly (but not exclu-
sively) of hydrogen and carbon atoms. These atoms are linked
by neutral covalent bonds. Thus, lipids are nonpolar and have
a very low solubility in water. Lipids, which account for about
40 percent of the organic matter in the average body (15 per-
cent of the body weight), can be divided into four subclasses:
fatty acids, triglycerides, phospholipids, and steroids. Like car-
bohydrates, lipids are important in physiology partly because
they provide a valuable source of energy.
Fatty Acids
A
fatty acid
consists of a chain of carbon and hydrogen atoms
with a carboxyl group at one end (
Figure 2–11
). Thus, fatty
acids contain two oxygen atoms in addition to their comple-
ment of carbon and hydrogen. Fatty acids are synthesized in
the body by the bonding together of two-carbon fragments,
resulting most commonly in fatty acids of 16 or 18 carbon
atoms. When all the carbons in a fatty acid are linked by sin-
gle covalent bonds, the fatty acid is said to be a
saturated
fatty acid,
because all the carbons are saturated with cova-
lently bound H. Some fatty acids contain one or more double
bonds, and these are known as
unsaturated fatty acids.
If
one double bond is present, the fatty acid is
monounsatu-
rated,
and if there is more than one double bond,
polyun-
saturated
(
Figure 2–11a
).
Most naturally occurring unsaturated fatty acids exist
in the
cis
position, with both hydrogens on the same side of
the double-bonded carbons (see Figure 2–11). It is possible,
however, to modify fatty acids during the processing of cer-
tain fatty foods, such that the hydrogens are on opposite sides
of the double bond. These chemically altered fatty acids are
known as
trans fatty acids.
The
trans
confi guration imparts
stability to the food for longer storage, and alters its fl avor
and consistency. However,
trans
fatty acids have recently been
linked with a number of serious health conditions, including
elevated blood levels of cholesterol.
Some fatty acids can be altered to produce a special
class of molecules that regulate a number of cell functions. As
Chapter 5 will describe in more detail, these modifi ed fatty
OO
O
O
OO
C
O
H
C
OH
C
C
H
OH
H
H
C
CH
2
C
O
H
C
OH
C
C
H
H
OH
H
H
C
CH
2
OH
C
O
H
C
OH
C
C
H
H
OH
H
H
C
CH
2
OH
C
O
H
C
O
C
C
H
H
OH
H
H
C
Glucose subunit
Glycogen
OH
H
H
CH
2
Figure 2–10
Many molecules of glucose joined end-to-end and at branch points form the branched-chain polysaccharide glycogen, shown here in
diagrammatic form. The four red subunits in the glycogen molecule correspond to the four glucose subunits shown at the bottom.
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