64
UNIT ONE
Proteins
Proteins
(pro
te-inz) have a great variety of functions. The
human body has more than 200,000 types of proteins. They are
structural materials, energy sources, and chemical messengers
(hormones). Other proteins combine with carbohydrates (gly-
coproteins) and function as receptors on cell surfaces, allowing
cells to respond to particular types of molecules that bind to
them. Antibody proteins recognize and destroy substances for-
eign to the body, such as certain molecules on the surfaces of
infecting bacteria. Proteins such as hemoglobin and myoglobin
transport oxygen in the blood and muscles, respectively, and
actin and myosin are contractile proteins that provide mus-
cle action. Many proteins play vital roles in metabolism as
enzymes
(en
zı¯mz), catalysts in living systems. That is, they
speed speciF c chemical reactions without being consumed.
(Enzymes are discussed in chapter 4, p. 117.)
A
phospholipid
molecule is similar to a fat molecule in
that it includes a glycerol and fatty acid chains. The phospho-
lipid, however, has only two fatty acid chains and, in place
of the third, has a portion containing a phosphate group.
This phosphate-containing part is soluble in water (hydro-
philic) and forms the “head” of the molecule, whereas the
fatty acid portion is insoluble in water (hydrophobic) and
forms a “tail”
(f g. 2.15)
.
Steroid
molecules are complex structures that include
connected rings of carbon atoms
(f
g. 2.16)
. Among the more
important steroids are cholesterol, in all body cells and used
to synthesize other steroids; sex hormones, such as estrogen,
progesterone, and testosterone; and several hormones from
the adrenal glands. Chapters 13, 18, 20, 21, and 22 discuss
these steroids.
Table 2.7
summarizes the molecular struc-
tures and characteristics of lipids.
FIGURE 2.13
Fatty acids. (
a
) A molecule of saturated fatty acid and (
b
) a molecule of unsaturated fatty acid. Double bonds between carbon atoms
are shown in red. They cause a “kink” in the shape of the molecule.
(a)
Saturated fatty acid
(b)
Unsaturated fatty acid
C
H
H
H
H
C
H
H
C
H
H
C
H
H
C
H
H
C
H
H
C
H
H
C
H
H
C
H
H
C
H
H
C
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H
C
H
H
C
H
H
C
H
H
C
H
C
O
O
H
C
H
H
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H
C
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C
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C
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H
C
H
H
C
H
H
C
H
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H
C
H
H
C
H
C
H
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H
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C
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C
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O
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C
O
O
HC
C
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C
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C
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C
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C
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C
H
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C
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C
O
O
HC
C
H
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C
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C
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C
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C
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H
C
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H
C
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H
C
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H
C
H
H
C
H
H
H
C
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C
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C
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C
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C
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O
O
HC
C
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C
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C
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C
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C
Glycerol
portion
Fatty acid
portions
FIGURE 2.14
A triglyceride molecule (fat) consists of a glycerol portion and three fatty acid portions. This is an example of an unsaturated fat. The
double bond between carbon atoms in the unsaturated fatty acid is shown in red.
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