180
ERNEST SCHOFFENIELS
To initiate the discussion of the model, it may be appropriate to recall
that the phenomenon of osmosis appears to be considered from two
different points of view. The classical view as typified by the work of
Starling and Landis and in more recent years by Ussing, holds that
osmosis is a mass flow of the solvent through the pores of the membrane,
arising when a mole fraction difference of the solvent exists by virtue of
the presence of a substance impermeable or less permeable to the barrier.
The other point of view argues exclusively for the diffusion of the
solvent, that is a molecular-molecular random drift (151). In the experiments carried out on amphibian skin, it is possible to measure simultaneously the net flux of water arising under the influence of an osmotic
gradient as well as the influx using labeled water. Knowing the influx,
one may calculate the permeability coefficient to water since
If we calculate now the permeability coefficient to water from the net
flux value
It appears that the value of P 2 is nearly 16 times that for Pi, as shown by
the results of Ussing on the toad skin (152).
This is also true with nonbiological systems, as demonstrated by the
experiments of Mauro, in 1957, working with a collodion membrane.
Here again, under the influence of a hydrostatic pressure, he was able to
demonstrate that the diffusion component of the solvent flux is 1/730 of
the total flux (153).
One is forced therefore to conclude that any pressure difference,
osmotic or hydrostatic, applied, gives rise to a transfer of water which is
predominantly nondiffusional in nature.
When dealing with living membranes the alternative to the active
transport of water hypothesis has been the acceptance of pores in the
membrane. If it is so, the flow of water in a porous membrane should
produce a drag effect on the solute molecules permeating the membrane
via the pores. It has thus been concluded from experiments carried out
with the isolated toad skin (Bufo bufo) (154) that the flux ratio for
acetamide and thiourea is very near unity, indicating that these molecules are not subjected to active transport. After application of neurohypophyseal hormone, the flux ratio is still unity, except if there is an
osmotic gradient across the skin. In the latter case, the deviation from
unity is generally taken as indicating a drag effect of the solvent on the
solute molecules.
As pointed out independently by Pappenheimer (155) and Ussing
M in = Pic w(0)
(9)
A w = Μ[ n — M out — P2C W ( 0 ) — p2C w (i) = P2(C W ( 0 ) ~~ C w (i)) (10)
ERNEST SCHOFFENIELS
To initiate the discussion of the model, it may be appropriate to recall
that the phenomenon of osmosis appears to be considered from two
different points of view. The classical view as typified by the work of
Starling and Landis and in more recent years by Ussing, holds that
osmosis is a mass flow of the solvent through the pores of the membrane,
arising when a mole fraction difference of the solvent exists by virtue of
the presence of a substance impermeable or less permeable to the barrier.
The other point of view argues exclusively for the diffusion of the
solvent, that is a molecular-molecular random drift (151). In the experiments carried out on amphibian skin, it is possible to measure simultaneously the net flux of water arising under the influence of an osmotic
gradient as well as the influx using labeled water. Knowing the influx,
one may calculate the permeability coefficient to water since
If we calculate now the permeability coefficient to water from the net
flux value
It appears that the value of P 2 is nearly 16 times that for Pi, as shown by
the results of Ussing on the toad skin (152).
This is also true with nonbiological systems, as demonstrated by the
experiments of Mauro, in 1957, working with a collodion membrane.
Here again, under the influence of a hydrostatic pressure, he was able to
demonstrate that the diffusion component of the solvent flux is 1/730 of
the total flux (153).
One is forced therefore to conclude that any pressure difference,
osmotic or hydrostatic, applied, gives rise to a transfer of water which is
predominantly nondiffusional in nature.
When dealing with living membranes the alternative to the active
transport of water hypothesis has been the acceptance of pores in the
membrane. If it is so, the flow of water in a porous membrane should
produce a drag effect on the solute molecules permeating the membrane
via the pores. It has thus been concluded from experiments carried out
with the isolated toad skin (Bufo bufo) (154) that the flux ratio for
acetamide and thiourea is very near unity, indicating that these molecules are not subjected to active transport. After application of neurohypophyseal hormone, the flux ratio is still unity, except if there is an
osmotic gradient across the skin. In the latter case, the deviation from
unity is generally taken as indicating a drag effect of the solvent on the
solute molecules.
As pointed out independently by Pappenheimer (155) and Ussing
M in = Pic w(0)
(9)
A w = Μ[ n — M out — P2C W ( 0 ) — p2C w (i) = P2(C W ( 0 ) ~~ C w (i)) (10)