2
chapter 1
Water, Acids, Bases, and Buffers
FIGURE 1-1
Structure of the water molecule.
Physical Properties
Other properties of water uniquely suited to biological
systems include melting point, boiling point, heat of va-
porization (quantity of heat energy required to transform
1
g of liquid to vapor at the boiling point), heat of fusion
(quantity of heat energy required to convert
1
g of solid
to liquid at the melting point), specific heat (the amount
of heat required to raise the temperature of
1
g of sub-
stance by 1°C, and surface tension (Table 1-1). All these
values for water are much higher than those for other low-
molecular-weight substances because of the strong inter-
molecular hydrogen bonding of water. These properties
contribute to maintenance of temperature and to dissipa-
tion of heat in living systems. Thus, water plays a major
role in thermoregulation in living systems. The optimal
body temperature is a balance between heat production and
heat dissipation. Impaired thermoregulation causes either
hypothermia
or
hyperthermia
and has serious metabolic
consequences; if uncorrected, impaired thermoregulation
may lead to death (Chapter 39). Water freezes to form
ice at 0°C, but its maximum density is at 4°C. Aquatic
H
o
H
H
0
1
H
H
H
H
H
FIGURE 1-2
Tetrahedral hydrogen-bonded structure of water molecules in ice. The
tetrahedral arrangement is due to the fact that each water molecule has four
fractional charges: two negative charges due to the presence of a lone pair
of electrons on the oxygen atom and two positive charges, one on each of
the two hydrogen atoms. In the liquid phase this tetrahedral array occurs
transiently.
TABLE 1-1
Physical Properties of Water
*
Density (at 4°C)
1.0 g/mL
Molecular weight
18
Liquid range
0°-100°C
Melting point
0°C
Boiling point
100°C
Heat of fusion
80 cal/g
Heat of vaporization
540 cal/g
Dipole moment
1.86 Debye unit
Dielectric constant (E)
78.4
Solid/liquid density ratio
0.92
*Some of these properties are measured at 1 atm pressure.
organisms survive cold winters because ice floats over and
insulates liquid water from sub-zero-degree temperature.
If ice were denser than liquid water—which is the case
for most liquid-to-solid transformations—the solid form
would sink and the entire amount of fluid would solid-
ify rapidly in freezing weather. During hot weather, deep
lakes and oceans remain cool because heat generated by
sunlight can be dissipated by evaporation of surface water.
Water is transported across cell membranes in one of
two ways:
1
. by simple diffusion through the phospholipid bilayer
and
2
. by the action of membrane-spanning transport
proteins known as
aquaporins.
Thus, the concentration of water is in thermodynamic
equilibrium across the cell membrane. In the renal collect-
ing duct, water is reabsorbed through a specific aquaporin
channel protein (aquaporin 2). This reabsorption of water
is regulated by the antidiuretic hormone (also known as
vasopressin
). A defect or lack of functional aquaporin 2,
vasopressin, or its receptor leads to enormous loss of
water in the urine, causing the disease known as
diabetes
insipidus
(Chapter 39). Water plays a significant role in en-
zyme functions, molecular assembly of macromolecules,
and allosteric regulation of proteins. For example, the ef-
fect of protein solvation in allosteric regulation is im-
plicated in the transition of deoxyhemoglobin to oxy-
hemoglobin. During this process about 60 extra water
molecules bind to oxyhemoglobin (Chapter 28).
Solutes, Micelles, and Hydrophobic Interactions
Water is an excellent solvent for both ionic compounds
(e.g., NaCl) and low-molecular-weight nonionic polar
