272
GEORGE G. LATIES
charges of the iron atoms of neighboring cytochrome molecules. On the
other hand, the redox pump as described by Conway (1953) would make
of one of the cytochromes a carrier which would migrate between the
cell surface and the cell interior. Such a model, in which a cytochrome
is a carrier, although questionable in fact, is logical in conception, and
bears some resemblance to the new hypothesis of Lundegárdh (1955a)
in which the existence of a mobile carrier is postulated.
Lundegárdh has recognized the difficulty just pointed out by indicating the necessity of a permeability barrier somewhere along the postulated chain in order to prevent the movement of anions from within the
cell to the outside. Lundegárdh noted that in the presence of selected
inhibitors the salt uptake by wheat roots and the reduction of cytochrome
b therein were simultaneously prevented. The cytochrome oxidase system, however, remained operative. On the basis of these observations
Lundegárdh (1955c) considers the diffusion barrier in some way related
to cytochrome b. His present view (1955a) suggests that anions, acquired
by cytochrome b as previously described, are passed to a carrier, following which the carrier-anion complex penetrates the tonoplast.
Since the cytochrome system is no longer considered by Lundegárdh
to reside at the surface membrane of root cells, no apparent purpose is
served by passing anions through the cytochrome electron ladder before
their combination with a carrier. Furthermore, cytochrome b is intimately associated with cytochrome c and cytochrome oxidase within
the mitochondria, and bears no special spatial relationship to the vacuolar membrane through which accumulation occurs.
Whereas cytochrome b has heretofore been relegated exclusively to the
succinoxidase system, presumably as an intermediate between succinic
dehydrogenase and cytochrome c, it is now considered to be in the direct
path of electron transfer in the oxidation of DPNH. Ironically, cytochrome b is no longer considered in the main path of electron transfer
within the succinoxidase system (Chance and Williams, 1955).
In view of what has been said, it is not surprising that salt absorption
is related to the movement of electrons through cytochrome b, for the
extent of this movement is an indirect measure of the phosphorylative
respiration. The persistence of the cytochrome oxidase activity under
conditions where electrons are not passing through cytochrome b remains
to be explained. Lehninger (1955) has already shown that DPNH can
be oxidized via a pathway which, though terminating in the cytochromecytochrome oxidase system, is nevertheless different from the pathway
of electron transfer associated with the tricarboxylic acid cycle. In the
former instance oxidation is nonphosphorylative and occurs on the surface
of the mitochondria.
GEORGE G. LATIES
charges of the iron atoms of neighboring cytochrome molecules. On the
other hand, the redox pump as described by Conway (1953) would make
of one of the cytochromes a carrier which would migrate between the
cell surface and the cell interior. Such a model, in which a cytochrome
is a carrier, although questionable in fact, is logical in conception, and
bears some resemblance to the new hypothesis of Lundegárdh (1955a)
in which the existence of a mobile carrier is postulated.
Lundegárdh has recognized the difficulty just pointed out by indicating the necessity of a permeability barrier somewhere along the postulated chain in order to prevent the movement of anions from within the
cell to the outside. Lundegárdh noted that in the presence of selected
inhibitors the salt uptake by wheat roots and the reduction of cytochrome
b therein were simultaneously prevented. The cytochrome oxidase system, however, remained operative. On the basis of these observations
Lundegárdh (1955c) considers the diffusion barrier in some way related
to cytochrome b. His present view (1955a) suggests that anions, acquired
by cytochrome b as previously described, are passed to a carrier, following which the carrier-anion complex penetrates the tonoplast.
Since the cytochrome system is no longer considered by Lundegárdh
to reside at the surface membrane of root cells, no apparent purpose is
served by passing anions through the cytochrome electron ladder before
their combination with a carrier. Furthermore, cytochrome b is intimately associated with cytochrome c and cytochrome oxidase within
the mitochondria, and bears no special spatial relationship to the vacuolar membrane through which accumulation occurs.
Whereas cytochrome b has heretofore been relegated exclusively to the
succinoxidase system, presumably as an intermediate between succinic
dehydrogenase and cytochrome c, it is now considered to be in the direct
path of electron transfer in the oxidation of DPNH. Ironically, cytochrome b is no longer considered in the main path of electron transfer
within the succinoxidase system (Chance and Williams, 1955).
In view of what has been said, it is not surprising that salt absorption
is related to the movement of electrons through cytochrome b, for the
extent of this movement is an indirect measure of the phosphorylative
respiration. The persistence of the cytochrome oxidase activity under
conditions where electrons are not passing through cytochrome b remains
to be explained. Lehninger (1955) has already shown that DPNH can
be oxidized via a pathway which, though terminating in the cytochromecytochrome oxidase system, is nevertheless different from the pathway
of electron transfer associated with the tricarboxylic acid cycle. In the
former instance oxidation is nonphosphorylative and occurs on the surface
of the mitochondria.