Essential Cell Biology 5th edition

Chemical Bonds
49
Some Polar Molecules Form Acids and Bases in Water
One of the simplest kinds of chemical reaction, and one that has profound
significance for cells, takes place when a molecule with a highly
polar covalent bond between a hydrogen and another atom dissolves in
water. The hydrogen atom in such a bond has given up its electron almost
entirely to the companion atom, so it exists as an almost naked positively
charged hydrogen nucleus—in other words, a proton (H + ). When the polar
molecule becomes surrounded by water molecules, the proton will be
attracted to the partial negative charge on the oxygen atom of an adjacent
water molecule (see Figure 2–11); this proton can thus dissociate
from its original partner and associate instead with the oxygen atom of
the water molecule, generating a hydronium ion (H 3 O + ) (Figure 2–15A).
The reverse reaction—in which a hydronium ion releases a proton—also
takes place very readily, so in an aqueous solution, billions of protons are
constantly flitting to and fro between one molecule and another.
Substances that release protons when they dissolve in water, thus forming
H 3 O + , are termed acids. The higher the concentration of H 3 O + , the
more acidic the solution. Even in pure water, H 3 O + is present at a concentration
of 10 –7 M, as a result of the movement of protons from one water
molecule to another (Figure 2–15B). By tradition, the H 3 O + concentration
is usually referred to as the H + concentration, even though most protons
in an aqueous solution are present as H 3 O + . To avoid the use of unwieldy
numbers, the concentration of H + is expressed using a logarithmic scale
called the pH scale. Pure water has a pH of 7.0 and is thus neutral—that
is, neither acidic (pH <7) nor basic (pH >7).
Acids are characterized as being strong or weak, depending on how
readily they give up their protons to water. Strong acids, such as hydrochloric
acid (HCl), lose their protons easily. Acetic acid, on the other
hand, is a weak acid because it holds on to its proton fairly tightly when
dissolved in water. Many of the acids important in the cell—such as molecules
containing a carboxyl (COOH) group—are weak acids (see Panel
2–2, pp. 68–69). Their tendency to give up a proton with some reluctance
is exploited in a variety of cellular reactions.
Because protons can be passed readily to many types of molecules in
cells, thus altering the molecules’ characters, the H + concentration inside
a cell—its pH—must be closely controlled. Acids will give up their protons
more readily if the H + concentration is low (and the pH is high) and will
hold onto their protons (or accept them back) when the H + concentration
is high (and the pH is low).
C
polar
O covalent H
O
bond
+ O
CH 3 C + H O
+
O H H
O
δ – δ +
H
H
CH 3
H 3 O + OH –
acetic acid
(A)
hydrogen bond
H
O
H
H O
H
H 2 O H 2 O
(B)
water
proton moves
from one H 2 O
molecule to
the other
acetate
ion
H
O H
H +
hydronium
ion
+
O
H
hydroxyl
ion
hydronium
ion
Figure 2–15 Protons move continuously
from one molecule to another in aqueous
solutions. (A) The reaction that takes place
when a molecule of acetic acid dissolves in
water. At pH 7, nearly all of the acetic acid
molecules are present as acetate ions.
(B) Water molecules are continually
exchanging protons with each other to form
hydronium and hydroxyl ions. These ions
in turn rapidly recombine to form water
molecules.