needed to maintain the structure of the membrane. Facilitated diffusion has best
been characterized in the processes of sugar transfer into cells, but may playa role
in ion movement, and possibly in the movement of sodium across the outer border
of the epithelial cells of amphibian skin and urinary bladder.
e) Active Transport
Active transport is described by USSING (1960) 'as a transfer wh ich cannot be accounted for by physical forces'. Solute transfers down the sum of gradients of electrical potential, chemical concentration, temperature, pressure and as a result of
'drag' forces accompanying solvent movement do not constitute active transport.
Transport against the sum of such gradients requires the expenditure of energy
by the cell into or out of which solute moves.
To satisfy these criteria is not always an easy matter, for while differences in
chemical concentration are relatively simple to determine in vivo, other contributing factors such as electrical differences may be more difficult to measure.
In addition the transfer of several sub stances across a membrane may be linked
in such a manner as to make it difficult to identify the primary process. Thus, in
membranes that are permeable to both anions and cations, each ion species could
conceivably move down an electrical gradient created by an active transfer of the
other. While it is currently considered unlikely that water is transferred actively
in vertebrates, there are instances, as in the intestine, in which it moves against an
osmotic gradient. Such transfer of water has been linked to active sodium transport,
and may be due to local osmotic and hydrostatic forces that arise in the membrane
as a result of sodium movement; it does not constitute active transport (CURRAN,
1965). A linkage in transport of sodium and other solutes such as sugars and amino
acids has also been described (CRANE, 1965).
Satisfaction of the criteria of active transport, and identification of the primary
process, can often be obtained from information about the electrical conditions
across membranes and the measurement of the transfer of each constituent solute
in the absence of its normal associates. Electrical differences can usually be measured more easily in v itro, and if a difference in potential (p. d.) exists, its sign and
magnitude in relation to the transfer of a cation or anion will often suggest the prime
mover. Thus in the instance of the frog's skin, the inside (corium) may have a p. d.
of 100 mV positive, relative to the outside solution. When the chemical concentrations are identical on both sides of the skin, sodium and chloride move towards the corial side . The charge present there would suggest that the cation is
actively transported, though the possibility of a slower active transport of the anion
in the same direction cannot be excluded without more information. An electrical
p. d. cannot always be detected across membranes (such as the rabbit gall bladder)
that transport sodium and chloride, so that such an analysis is not pos sible; measurements of the movements of the separate ion species with the aid of their radioactive isotopes is then necess ary. Separate measurements of the transfers of each
linked solute may aid the identification of the primary process. Such experiments
can be difficult to perform and interpret, espec ially in cases where one of the solutes
11
been characterized in the processes of sugar transfer into cells, but may playa role
in ion movement, and possibly in the movement of sodium across the outer border
of the epithelial cells of amphibian skin and urinary bladder.
e) Active Transport
Active transport is described by USSING (1960) 'as a transfer wh ich cannot be accounted for by physical forces'. Solute transfers down the sum of gradients of electrical potential, chemical concentration, temperature, pressure and as a result of
'drag' forces accompanying solvent movement do not constitute active transport.
Transport against the sum of such gradients requires the expenditure of energy
by the cell into or out of which solute moves.
To satisfy these criteria is not always an easy matter, for while differences in
chemical concentration are relatively simple to determine in vivo, other contributing factors such as electrical differences may be more difficult to measure.
In addition the transfer of several sub stances across a membrane may be linked
in such a manner as to make it difficult to identify the primary process. Thus, in
membranes that are permeable to both anions and cations, each ion species could
conceivably move down an electrical gradient created by an active transfer of the
other. While it is currently considered unlikely that water is transferred actively
in vertebrates, there are instances, as in the intestine, in which it moves against an
osmotic gradient. Such transfer of water has been linked to active sodium transport,
and may be due to local osmotic and hydrostatic forces that arise in the membrane
as a result of sodium movement; it does not constitute active transport (CURRAN,
1965). A linkage in transport of sodium and other solutes such as sugars and amino
acids has also been described (CRANE, 1965).
Satisfaction of the criteria of active transport, and identification of the primary
process, can often be obtained from information about the electrical conditions
across membranes and the measurement of the transfer of each constituent solute
in the absence of its normal associates. Electrical differences can usually be measured more easily in v itro, and if a difference in potential (p. d.) exists, its sign and
magnitude in relation to the transfer of a cation or anion will often suggest the prime
mover. Thus in the instance of the frog's skin, the inside (corium) may have a p. d.
of 100 mV positive, relative to the outside solution. When the chemical concentrations are identical on both sides of the skin, sodium and chloride move towards the corial side . The charge present there would suggest that the cation is
actively transported, though the possibility of a slower active transport of the anion
in the same direction cannot be excluded without more information. An electrical
p. d. cannot always be detected across membranes (such as the rabbit gall bladder)
that transport sodium and chloride, so that such an analysis is not pos sible; measurements of the movements of the separate ion species with the aid of their radioactive isotopes is then necess ary. Separate measurements of the transfers of each
linked solute may aid the identification of the primary process. Such experiments
can be difficult to perform and interpret, espec ially in cases where one of the solutes
11