Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

46 Chapter 2: Cell Chemistry and Bioenergetics
Figure 2–5 Protons readily move in
CH 3
3
O
H
O
H
C + O
CH 3 C + H
aqueous solutions. (A) The reaction that
takes place when a molecule of acetic
O
+
O H H
O
H
acid dissolves in water. At pH 7, nearly all
δ – δ + of the acetic acid is present as acetate
ion. (B) Water molecules are continuously
acetic acid
water
acetate
hydronium
ion
ion
exchanging protons with each other to
(A)
form hydronium and hydroxyl ions. These
ions in turn rapidly recombine to form water
H
H
molecules.
O H O
O H + O
H
H 2 O
H
H 2 O
proton moves
from one
molecule to
the other
H
H
+
O + H
OH –
(B)
hydronium
ion
hydroxyl
ion
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
O atom of an adjacent water molecule. This proton can easily dissociate from its
original partner and associate instead with the oxygen atom of the water molecule,
generating a hydronium ion (H 3 O + ) (Figure 2–5A). The reverse reaction also
takes place very readily, so in the aqueous solution 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. H 3 O + is present even in pure water, at a concentration of 10 –7 M, as
a result of the movement of protons from one water molecule to another (Figure
2–5B). By convention, 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 MBoC6 numbers, e2.14/2.05 the concentration of H 3 O + is expressed
using a logarithmic scale called the pH scale. Pure water has a pH of 7.0 and is said
to be 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 quickly. Acetic acid, on the other hand, is a weak acid because
it holds on to its proton more 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. 92–93).
Because the proton of a hydronium ion can be passed readily to many types of
molecules in cells, altering their character, the concentration of H 3 O + inside a cell
(the acidity) must be closely regulated. Acids—especially weak acids—will give up
their protons more readily if the concentration of H 3 O + in solution is low and will
tend to receive them back if the concentration in solution is high.
The opposite of an acid is a base. Any molecule capable of accepting a proton
from a water molecule is called a base. Sodium hydroxide (NaOH) is basic (the
term alkaline is also used) because it dissociates readily in aqueous solution to
form Na + ions and OH – ions. Because of this property, NaOH is called a strong
base. More important in living cells, however, are the weak bases—those that
have a weak tendency to reversibly accept a proton from water. Many biologically
important molecules contain an amino (NH 2 ) group. This group is a weak base
that can generate OH – by taking a proton from water: –NH 2 + H 2 O → –NH
+ 3 + OH –
(see Panel 2–2, pp. 92–93).
Because an OH – ion combines with a H 3 O + ion to form two water molecules,
an increase in the OH – concentration forces a decrease in the concentration of
H 3 O + , and vice versa. A pure solution of water contains an equal concentration
(10 –7 M) of both ions, rendering it neutral. The interior of a cell is also kept close
to neutrality by the presence of buffers: weak acids and bases that can release or
take up protons near pH 7, keeping the environment of the cell relatively constant
under a variety of conditions.