Essential Cell Biology 5th edition

50 CHAPTER 2 Chemical Components of Cells
Figure 2–16 In aqueous solutions, the
concentration of hydroxyl (OH – ) ions
increases as the concentration of H 3 O +
(or H + ) ions decreases. The product of
the two values, [OH – ] x [H + ], is always 10 –14
(moles/liter) 2 . At neutral pH, [OH – ] = [H + ],
and both ions are present at 10 –7 M. Also
shown are examples of common solutions
along with their approximate pH values.
ACIDIC
[H + ]
moles/liter
pH
1 0
10 –1 1
10 –2 2
10 –3 3
10 –4 4
[OH – ]
moles/liter
10 –14
10 –13
10 –12
10 –11
10 –10
some solutions and their
pH values
battery acid (0.5)
stomach acid (1.5)
lemon juice (2.3), cola (2.5)
orange juice (3.5)
beer (4.5)
10 –5 5
10 –9
black coffee (5.0), acid rain (5.6)
10 –6 6
10 –8
urine (6.0), milk (6.5)
NEUTRAL
10 –7 7
10 –7
pure water (7.0)
10 –8 8
10 –6
sea water (8.0)
10 –9 9
10 –5
hand soap (9.5)
10 –10 10
10 –4
milk of magnesia (10.5)
BASIC
10 –11 11
10 –3
household ammonia (11.9)
10 –12 12
10 –2
non-phosphate detergent (12.0)
10 –13 13
10 –1
bleach (12.5)
10 –14 14
1
caustic soda (13.5)
QUESTION 2–5
A. Are there H 3 O + ions present in
pure water at neutral pH (i.e., at pH
= 7.0)? If so, how are they formed?
B. If they exist, what is the ratio
of H 3 O + ions to H 2 O molecules at
neutral pH? (Hint: the molecular
weight of water is 18, and 1 liter of
water weighs 1 kg.)
Molecules that accept protons when dissolved in water are called bases.
Just as the defining property of an acid is that it raises the concentration
of H 3 O + ions by donating a proton to a water molecule, so the defining
ECB5 n2.100-2.16
property of a base is that it raises the concentration of hydroxyl (OH – )
ions by removing a proton from a water molecule. Sodium hydroxide
(NaOH) is basic (the term alkaline is also used). NaOH is considered a
strong base because it readily dissociates in aqueous solution to form
Na + ions and OH – ions. Weak bases—which have a weak tendency to
accept a proton from water—however, are more important in cells. Many
biologically important weak bases contain an amino (NH 2 ) group, which
can generate OH – by taking a proton from water: –NH 2 + H 2 O → –NH
+ 3 +
OH – (see Panel 2–2, pp. 68–69).
Because an OH – ion combines with a proton to form a water molecule,
an increase in the OH – concentration forces a decrease in the H + concentration,
and vice versa (Figure 2–16). A pure solution of water contains
an equal concentration (10 –7 M) of both ions, rendering it neutral (pH 7).
The interior of a cell is kept close to neutral by the presence of buffers:
mixtures of weak acids and bases that will adjust proton concentrations
around pH 7 by releasing protons (acids) or taking them up (bases) whenever
the pH changes. This give-and-take keeps the pH of the cell relatively
constant under a variety of conditions.
SMALL MOLECULES IN CELLS
Having looked at the ways atoms combine to form small molecules and
how these molecules behave in an aqueous environment, we now examine
the main classes of small molecules found in cells and their biological
roles. Amazingly, we will see that a few basic categories of molecules,
formed from just a handful of different elements, give rise to all the
extraordinary richness of form and behavior displayed by living things.
A Cell Is Formed from Carbon Compounds
If we disregard water, nearly all the molecules in a cell are based on carbon.
Carbon is outstanding among all the elements in its ability to form
large molecules. Because a carbon atom is small and has four electrons
and four vacancies in its outer shell, it readily forms four covalent bonds