rate of such transfer, and this will be influenced by such factors as its thickness,
porosity and chemical composition. Apart from its concentration and kinetic
energy, the nature of the diffusing molecule itself will affect its rate of diffusion,
due to its size (in inverse proportion) and, in biological systems, interactions (such
as solubility) with the interspersed barrier.
Measurements of the influx and outflux of molecules across membranes, with
the aid of isotopes, have revealed that such transfers are not always strictly related
to differences in electro-chemical activity between the two sides. Active transport
is not necessarily involved in such circumstances. The ' uphill' and 'downhill' exchange of solutes, such as sodium, may be tightly coupled to each other so that
a molecule moving towards one side of a membrane is directly exchanged for another at the opposite side so that no net flux results. Thus, reducing the concentration of the solute (by replacing it with an impermeant one) on the outside
will similarly reduce the rate of its flux from the inside. Ideally, on a simple electrochemical basis, the outflux should not change in these circumstances. This effect
has been called 'exchange diffus ion' (USSING, 1947; KEYNES and SWAN, 1959). Solutes involved in this process would appear to interact with the membrane in some
manner. This is usuall y conceived as involving the participation of a 'carrier' that
moves passivel y in either direction across the membrane but onl y when combined
with the appropriate solute. Influx and outflux are thus tightly coupled to each
other and are the same in each direction. Ideally they do not follow classical electrochemical gradients. This process plays an important role in controlling the permeability of the gills of some fish to sodium.
b) Osmosis
When two solutes of different concentrations are separated by a barrier which prevents (semipermeable), or strongly restricts, the movement of the solute, the solvent will flow from the side with the lower to that of th e higher solute concentration. This flow of solvent is called osmosis. The application of an opposing
pressure will impede the movement of the solvent. If the fluid on one side is pure
solvent then the pressure that must be applied to the opposing solution in order
to stop net water transfer is called its osmotic pressure. A pressure of 22.4 atmospheres is required to prevent movement of water acro ss a semipermeable membrane
int o a 1 molal (or osmolal) solution (1 M or Osm.).
The osmotic movement of water can be viewed either as its molecular diffusion
or as a 'bulk' flow in a continuous phase.
(i) Molecular diffusion. The solute is seen as reducing the number of favourable
collisions of solvent molecules with the dividing membrane, by interacting with
them and /or lowering their molecular concentration. Hydrostatic pressure may
then influence the movement of the solvent molecules by altering their kinetic
energy (HARRIS, 1960).
(ii) 'Bulk flow' of fluid may result from hydrostatic forces that arise in narrow
channels, or 'pores' , through which the solvent flows as a continuous phase. These
forces arise in such channels at the junction of the membrane and solution (see
MAURO, 1965).
9
porosity and chemical composition. Apart from its concentration and kinetic
energy, the nature of the diffusing molecule itself will affect its rate of diffusion,
due to its size (in inverse proportion) and, in biological systems, interactions (such
as solubility) with the interspersed barrier.
Measurements of the influx and outflux of molecules across membranes, with
the aid of isotopes, have revealed that such transfers are not always strictly related
to differences in electro-chemical activity between the two sides. Active transport
is not necessarily involved in such circumstances. The ' uphill' and 'downhill' exchange of solutes, such as sodium, may be tightly coupled to each other so that
a molecule moving towards one side of a membrane is directly exchanged for another at the opposite side so that no net flux results. Thus, reducing the concentration of the solute (by replacing it with an impermeant one) on the outside
will similarly reduce the rate of its flux from the inside. Ideally, on a simple electrochemical basis, the outflux should not change in these circumstances. This effect
has been called 'exchange diffus ion' (USSING, 1947; KEYNES and SWAN, 1959). Solutes involved in this process would appear to interact with the membrane in some
manner. This is usuall y conceived as involving the participation of a 'carrier' that
moves passivel y in either direction across the membrane but onl y when combined
with the appropriate solute. Influx and outflux are thus tightly coupled to each
other and are the same in each direction. Ideally they do not follow classical electrochemical gradients. This process plays an important role in controlling the permeability of the gills of some fish to sodium.
b) Osmosis
When two solutes of different concentrations are separated by a barrier which prevents (semipermeable), or strongly restricts, the movement of the solute, the solvent will flow from the side with the lower to that of th e higher solute concentration. This flow of solvent is called osmosis. The application of an opposing
pressure will impede the movement of the solvent. If the fluid on one side is pure
solvent then the pressure that must be applied to the opposing solution in order
to stop net water transfer is called its osmotic pressure. A pressure of 22.4 atmospheres is required to prevent movement of water acro ss a semipermeable membrane
int o a 1 molal (or osmolal) solution (1 M or Osm.).
The osmotic movement of water can be viewed either as its molecular diffusion
or as a 'bulk' flow in a continuous phase.
(i) Molecular diffusion. The solute is seen as reducing the number of favourable
collisions of solvent molecules with the dividing membrane, by interacting with
them and /or lowering their molecular concentration. Hydrostatic pressure may
then influence the movement of the solvent molecules by altering their kinetic
energy (HARRIS, 1960).
(ii) 'Bulk flow' of fluid may result from hydrostatic forces that arise in narrow
channels, or 'pores' , through which the solvent flows as a continuous phase. These
forces arise in such channels at the junction of the membrane and solution (see
MAURO, 1965).
9