Water, Acids, Bases, and Buffers
Properties of Water
Acid and base concentrations in living systems are care-
fully regulated to maintain conditions compatible with
normal life. Biochemical reactions involving acids and
bases occur in the body water, whereas buffer systems pro-
tect the body from significant variations in the concentra-
tions of acids and bases. This chapter introduces basic con-
cepts of the properties of water, acids, bases, and buffers,
and Chapter 39 presents a detailed discussion of both nor-
mal and pathological aspects of acid-base metabolism.
Life cannot be sustained without water. Water consti-
tutes 45-73% of total human body weight. It is distributed
in intracellular (55%) and extracellular (45%) compart-
ments and provides a continuous solvent phase between
body compartments. As the biological solvent, water plays
a major role in all aspects of metabolism: absorption, trans-
port, digestion, and excretion of inorganic and organic sub-
stances as well as maintenance of body temperature. The
unique properties of water are due to its structure.
Hydrogen Bonding
Water (H
O) is a hydride of oxygen in which the highly
electronegative oxygen atom attracts the bonding elec-
trons from two hydrogen atoms. This leads to polar H-O
bonds in which the hydrogen atoms have a slightly positive
charge (<5+) and the oxygen atom has a slightly negative
charge (<5 j (Figure 1-1). Water molecules have a rela-
tively high dipole moment because of the angle (104.5°)
of the H-O-H bond and the polarity of the bonds. Neigh-
boring liquid water molecules interact with one another to
form an extensive lattice-like structure similar to the struc-
ture of ice. The intermolecular bonding between water
molecules arises from the attraction between the partial
negative charge on the oxygen atom and the partial positive
charge on the hydrogen atom of adjacent water molecules.
This type of attraction involving a hydrogen atom is known
as a
hydrogen bond
(Figure 1-2).
Hydrogen bonds contain a hydrogen atom between two
electronegative atoms (e.g., O and N). One is the formal
hydrogen donor; the other is the hydrogen acceptor. The
amount of energy required to break a hydrogen bond (bond
energy) is estimated to be 2-5 kcal/mol (8.4-20.9 kJ/mol)
in the gas phase. Covalent bonds have bond energies of
50-100 kcal/mol (209^118 kJ/mol). The cumulative effect
of many hydrogen bonds is equivalent to the stabilizing
effect of covalent bonds. In proteins, nucleic acids, and
water, hydrogen bonds are essential to stabilize overall
structure. In ice, each water molecule forms a hydrogen
bond with four other water molecules, giving rise to a rigid
tetrahedral arrangement (Figure 1-2). In the liquid state,
water maintains a tetrahedrally coordinated structure over
short ranges and for short time periods.
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