3. CELLULAR ASPECTS OF ACTIVE TRANSPORT
157
transport of some ionic species. The potential difference can thus be
considered as a direct consequence of the mechanism enabling the cell
to keep its volume constant.
Interesting adaptations are offered by the different categories of
conducting cells (neurons, nerve fibers, electric organs of fishes, etc.)
since they make use of the free energy accumulated as concentration
gradients and electrical potential to produce an electric current. This
aspect of the problem has been considered more extensively in a preceding chapter (Volume II, Chapter 10) (92).
III. Cellular Differentiation and Permeability Characteristics
As shown above, one may thus explain very satisfactorily the origin
of the concentration differences, as well as the potential difference existing across a living membrane, by the permeability characteristics of this
membrane.
Our purpose now is to show that cellular differentiation at the level
of the ionic regulation (permeability characteristics of the membrane)
seems to be the result of physiological radiations of a biochemical system rather than of a true biochemical differentiation of membrane components* (42, 97, 98). In other words, the diversity in the field by permeability would generally be achieved through a spatial rearrangement of
permeability characteristics. This, however, does not exclude discrete
biochemical modification of the membrane in the course of adaptation
to different environmental conditions.
A. AMPHIBIAN SKIN
To illustrate this proposition let us analyze in more detail some observations demonstrated using the isolated amphibian skin as experimental material.
As we have already mentioned, the skin of the frog is the site of an
active transport of Na from the outside solution toward the internal
medium. There is thus an asymmetry as far as the movement of sodium
is concerned, and one may expect the cellular membrane facing outside
to have characteristics different from those of the membranes facing
inward. This is indeed the case: (a) the effect of various compounds on
Na flux or potential difference is dependent on the site of application
(99-102);
(b) pH variations, using a P0 4 buffer, affect the skin differently according to the site of application, optimum values being above
6 in the solution bathing the outside of the skin, while they are above 8
in the solution bathing the inside of the skin (103); (c) cellular mem* It is, however, evident that physiological radiation is dependent on genie control and thus implies a biochemical differentiation at this level.
157
transport of some ionic species. The potential difference can thus be
considered as a direct consequence of the mechanism enabling the cell
to keep its volume constant.
Interesting adaptations are offered by the different categories of
conducting cells (neurons, nerve fibers, electric organs of fishes, etc.)
since they make use of the free energy accumulated as concentration
gradients and electrical potential to produce an electric current. This
aspect of the problem has been considered more extensively in a preceding chapter (Volume II, Chapter 10) (92).
III. Cellular Differentiation and Permeability Characteristics
As shown above, one may thus explain very satisfactorily the origin
of the concentration differences, as well as the potential difference existing across a living membrane, by the permeability characteristics of this
membrane.
Our purpose now is to show that cellular differentiation at the level
of the ionic regulation (permeability characteristics of the membrane)
seems to be the result of physiological radiations of a biochemical system rather than of a true biochemical differentiation of membrane components* (42, 97, 98). In other words, the diversity in the field by permeability would generally be achieved through a spatial rearrangement of
permeability characteristics. This, however, does not exclude discrete
biochemical modification of the membrane in the course of adaptation
to different environmental conditions.
A. AMPHIBIAN SKIN
To illustrate this proposition let us analyze in more detail some observations demonstrated using the isolated amphibian skin as experimental material.
As we have already mentioned, the skin of the frog is the site of an
active transport of Na from the outside solution toward the internal
medium. There is thus an asymmetry as far as the movement of sodium
is concerned, and one may expect the cellular membrane facing outside
to have characteristics different from those of the membranes facing
inward. This is indeed the case: (a) the effect of various compounds on
Na flux or potential difference is dependent on the site of application
(99-102);
(b) pH variations, using a P0 4 buffer, affect the skin differently according to the site of application, optimum values being above
6 in the solution bathing the outside of the skin, while they are above 8
in the solution bathing the inside of the skin (103); (c) cellular mem* It is, however, evident that physiological radiation is dependent on genie control and thus implies a biochemical differentiation at this level.