60
UNIT ONE
2.3
CHEMICAL CONSTITUENTS
OF CELLS
Chemicals, including those that take part in metabolism (the
cell’s energy reactions), are of two general types.
Organic
(or-gan
ik) compounds have carbon and hydrogen. All other
chemicals are
inorganic
(in
or-gan
ik). Many organic mole-
cules have long chains or ring structures that can form because
of a carbon atom’s ability to form four covalent bonds.
Inorganic substances usually dissociate in water, forming
ions; thus, they are
electrolytes.
Many organic compounds
dissolve in water, but most dissolve in organic liquids such
Many bases are present in body fl
uids, but because of the
way bases react in water, the concentration of hydroxide
ions is a good estimate of the total base concentration.
The concentrations of hydrogen ions and hydroxide ions
are always in balance such that if one increases, the other
decreases, and vice versa. Solutions with more hydrogen
ions than hydroxide ions are
acidic.
That is, acidic solutions
have pH values less than 7.0
(f
g. 2.10)
. Solutions with fewer
hydrogen ions than hydroxide ions are
basic
(alkaline); they
have pH values greater than 7.0.
Table 2.5
summarizes the relationship between hydro-
gen ion concentration and pH. Chapter 21 (p. 819) discusses
the regulation of hydrogen ion concentrations in the internal
environment.
Many fl uids in the human body function within a nar-
row pH range. Illness results when pH changes. The normal
pH of blood, for example, is 7.35 to 7.45. Blood pH of 7.5
to 7.8, called
alkalosis
(al
kah-lo
sis), makes one feel agi-
tated and dizzy. This can be caused by breathing rapidly at
high altitudes, taking too many antacids, high fever, anxiety,
or mild to moderate vomiting that rids the body of stomach
acid.
Acidosis
(as
ı˘-do
sis), in which blood pH falls to 7.0 to
7.3, makes one feel disoriented and fatigued, and breathing
may become difF
cult. This condition can result from severe
vomiting that empties the alkaline small intestinal contents,
diabetes, brain damage, impaired breathing, and lung and
kidney disease.
Buffers
are chemicals that resist pH change. They com-
bine with hydrogen ions when these ions are in excess, or
they donate hydrogen ions when these ions are depleted.
Buffers are discussed in chapter 21 (pp. 820–823).
PRACTICE
13
Describe three types of chemical reactions.
14
Compare the characteristics of an acid, a base, and a salt.
15
What does the pH scale measure?
16
What is a buF
er?
OH
concentration increases
H
+
concentration increases
Acidic
H
+
Relative
amounts
of H
+
(red)
and OH
(blue)
Basic
OH
2.0
gastric
juice
3.0
apple
juice
4.2
tomato
juice
5.3
cabbage
6.0
corn
6.6
cow’s
milk
7.0
distilled
water
7.4
human
blood
8.0
egg
white
8.4
sodium
bicarbonate
10.5
milk of
magnesia
11.5
household
ammonia
pH
0123456789
1
0
1
1
1
2
1
3
1
4
Basic (alkaline)
Neutral
Acidic
TABLE
2.5
|
Hydrogen Ion Concentrations
and pH
Grams of H
+
per Liter
pH
0.00000000000001
14
0.0000000000001
13
0.000000000001
12
0.00000000001
11
Increasingly basic
0.0000000001
10
0.000000001
9
0.00000001
8
0.0000001
7
Neutral—neither acidic nor basic
0.000001
6
0.00001
5
0.0001
4
0.001
3
Increasingly acidic
0.01
2
0.1
1
1.0 0
0
FIGURE 2.10
The pH scale re±
ects the hydrogen ion (H
+
) concentration. As the concentration of H
+
increases, a solution becomes more acidic and
the pH value decreases. As the concentration of ions that bond with H
+
(such as hydroxide ions) increases, a solution becomes more basic (alkaline)
and the pH value increases. The pH values of some common substances are shown.
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