11. HYDROSTATIC PRESSURE
449
creases are accelerated by high pressures, while processes involving
volume increases are slowed.
Pressure effects on solubilities depend on the ionization state of
the dissolved substance, its partial specific volume, and the density
(therefore the compressibility) of the solution. For most salts, solubility increases with increased pressure. Gas solubilities vary more
widely. For example, oxygen solubility is relatively insensitive to pressure, while nitrogen solubility decreases substantially ( Klotz, 1963a,b).
The details of reaction mechanisms critically determine the direction
and magnitude of pressure effects on chemical reaction rates. Theoretical
analyses of these effects have been attempted using both collision theory
and transition state theory. Most recent discussions use the latter approach, in which pressure effects are interpreted as being largely determined by the direction and magnitude of changes in volumes of activation of the reacting substances ( Whalley, 1967).
Changes in reaction rates may result from changes in the concentrations of the reacting substances or from changes in the reaction rate
constant. For reactions in solution, the former effect is usually the most
important at pressures below about 1000 atm. Polymerization reactions
are often particularly sensitive to pressure (Bradley and Munro, 1965).
Activated states in many biologically important reactions are ionized
states. Ionic dissociation in aqueous solutions generally produces a contraction of the surrounding water structure (electrostriction ). Thus most
ionization reactions are speeded by pressure and, in many cases, their
equilibrium constants are increased. The dissociation constants of
carbonic acid in seawater at 22"C, for example, are increased by factors
of 1.84 for K,' and 1.48 for K2' at 654 atm as compared with 1 atm (Culberson et nl., 1967). Dissociation constants for most small ions increase
2-6 times between 1 and 1000 atm (Bradley and Munro, 1965).
D. Prospects
The variety of possible physicochemical effects of pressure, combined
with the diversity of the living world, make it improbable that many
areas of substantial uniformity will be uncovered by studies of the
biological effects of pressure. The fundamental nature of pressure effects
simultaneously make it probable that important effects occur at every
level of biological organization, from the molecular to the populational.
Microorganisms have been most thoroughly studied at the former level,
with results clearly supporting this position ( Fenn, 1967; Morita, 1967).
The most extensive studies of higher organisms, including fishes, have
449
creases are accelerated by high pressures, while processes involving
volume increases are slowed.
Pressure effects on solubilities depend on the ionization state of
the dissolved substance, its partial specific volume, and the density
(therefore the compressibility) of the solution. For most salts, solubility increases with increased pressure. Gas solubilities vary more
widely. For example, oxygen solubility is relatively insensitive to pressure, while nitrogen solubility decreases substantially ( Klotz, 1963a,b).
The details of reaction mechanisms critically determine the direction
and magnitude of pressure effects on chemical reaction rates. Theoretical
analyses of these effects have been attempted using both collision theory
and transition state theory. Most recent discussions use the latter approach, in which pressure effects are interpreted as being largely determined by the direction and magnitude of changes in volumes of activation of the reacting substances ( Whalley, 1967).
Changes in reaction rates may result from changes in the concentrations of the reacting substances or from changes in the reaction rate
constant. For reactions in solution, the former effect is usually the most
important at pressures below about 1000 atm. Polymerization reactions
are often particularly sensitive to pressure (Bradley and Munro, 1965).
Activated states in many biologically important reactions are ionized
states. Ionic dissociation in aqueous solutions generally produces a contraction of the surrounding water structure (electrostriction ). Thus most
ionization reactions are speeded by pressure and, in many cases, their
equilibrium constants are increased. The dissociation constants of
carbonic acid in seawater at 22"C, for example, are increased by factors
of 1.84 for K,' and 1.48 for K2' at 654 atm as compared with 1 atm (Culberson et nl., 1967). Dissociation constants for most small ions increase
2-6 times between 1 and 1000 atm (Bradley and Munro, 1965).
D. Prospects
The variety of possible physicochemical effects of pressure, combined
with the diversity of the living world, make it improbable that many
areas of substantial uniformity will be uncovered by studies of the
biological effects of pressure. The fundamental nature of pressure effects
simultaneously make it probable that important effects occur at every
level of biological organization, from the molecular to the populational.
Microorganisms have been most thoroughly studied at the former level,
with results clearly supporting this position ( Fenn, 1967; Morita, 1967).
The most extensive studies of higher organisms, including fishes, have