Chemical Composition of the Body
23
that destroy pathogens. The free radicals are highly reactive,
removing electrons from the outer orbits of molecules present
in the pathogen cell membrane, for example. This mechanism
begins the process whereby the pathogen is destroyed.
In addition, however, free radicals can be produced in
the body following exposure to radiation or toxin ingestion.
These free radicals can do considerable harm to the cells of
the body. For example, oxidation due to long-term buildup
of free radicals has been proposed as one cause of several dif-
ferent human diseases, notably eye, cardiovascular, and neural
diseases associated with aging. Thus, it is important that free
radicals be inactivated by molecules that can donate electrons
to free radicals without becoming free radicals themselves.
Examples of such protective molecules are the antioxidant
vitamins C and E.
Free radicals are diagrammed with a dot next to the
atomic symbol. Examples of biologically important free radi-
cals are superoxide anion, O
2
·
; hydroxyl radical, OH
·
;
and nitric oxide, NO
·
. Note that a free radical confi
gura-
tion can occur in either an ionized (charged) or an un-ionized
molecule.
Polar Molecules
As we have seen, when the electrons of two atoms interact, the
two atoms may share the electrons equally, forming an electri-
cally neutral covalent bond. Alternatively, one of the atoms
may completely capture an electron from the other, forming
two ions. Between these two extremes are bonds in which the
electrons are not shared equally between the two atoms, but
instead reside closer to one atom of the pair. This atom thus
acquires a slight negative charge, while the other atom, having
partly lost an electron, becomes slightly positive. Such bonds
are known as
polar covalent bonds
(or, simply, polar bonds)
because the atoms at each end of the bond have an opposite
electric charge. For example, the bond between hydrogen and
oxygen in a
hydroxyl group
(—OH) is a polar covalent bond
in which the oxygen is slightly negative and the hydrogen
slightly positive:
(
δ
) (
δ
+
)
R—O—H
(The
δ
and
δ
+
symbols refer to atoms with a partial negative
or positive charge, respectively. The R symbolizes the remain-
der of the molecule.) The electric charge associated with the
ends of a polar bond is considerably less than the charge on
a fully ionized atom. For example, the oxygen in the polar-
ized hydroxyl group has only about 13 percent of the nega-
tive charge associated with the oxygen in an ionized carboxyl
group, R—COO
. Polar bonds do not have a
net
electric
charge, as do ions, because they contain equal amounts of
negative and positive charge.
Atoms of oxygen and nitrogen, which have a relatively
strong attraction for electrons, form polar bonds with hydro-
gen atoms. In contrast, bonds between carbon and hydrogen
atoms and between two carbon atoms are electrically neutral
(
Table 2–4
).
Different regions of a single molecule may contain non-
polar bonds, polar bonds, and ionized groups. Molecules con-
taining signifi cant numbers of polar bonds or ionized groups
are known as
polar molecules,
whereas molecules composed
predominantly of electrically neutral bonds are known as
non-
polar molecules.
As we will see, the physical characteristics
of these two classes of molecules, especially their solubility in
water, are quite different.
Hydrogen Bonds
The electrical attraction between the hydrogen atom in a polar
bond in one molecule and an oxygen or nitrogen atom in a
polar bond of another molecule forms a
hydrogen bond.
Such
bonds may also form between atoms within the same mol-
ecule. Hydrogen bonds are represented in diagrams by dashed
or
dotted
l
ines
to
d
ist
ingu
ish
them
from
cova
lent
bonds
(
Figure 2–4
). Hydrogen bonds are very weak, having only
about 4 percent of the strength of the polar bonds between
the hydrogen and oxygen atoms in a single molecule of water.
Although hydrogen bonds are weak individually, when pres-
ent in large numbers, they play an extremely important role in
molecular interactions and in determining the shape of large
Table 2–4
Examples of Nonpolar and Polar
Bonds, and Ionized Chemical Groups
Nonpolar
Bonds
A
—C—H
A
Carbon-hydrogen bond
A
A
—C—C—
A
A
Carbon-carbon bond
Polar
Bonds
(
δ
) (
δ
+
)
R—O—H
Hydroxyl group (R—OH)
(
δ
) (
δ
+
)
R—S—H
Sulfhydryl group (R—SH)
H
(
δ
+
)
A
(
δ
)
R—N—R
Nitrogen-hydrogen bond
Ionized
Groups
O
B
R—C—O
Carboxyl group (R—COO
)
H
A
+
R—N—H
A
H
Amino group (R—NH
3
+
)
O
B
R—O—P—O
A
O
Phosphate group (R—PO
4
2–
)
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