RESPIRATION RATE IN PLANTS
271
Accordingly, as electrons are passed from the substrate to oxygen via the
cytochrome system, anions are thought to move in the opposite direction, so as to satisfy the transient net positive charge of the metal component of the iron-porphyrin enzymes. As Lundegárdh says, "the absence of free anions acts as a specific inhibitor of cytochrome oxidase,
quite as effective as CN or CO." By the same token, the salt respiration
is explained by Lundegárdh on the basis of the removal of that inhibition
by addition of salt.
Lundegárdh (1953a) visualizes the accumulation mechanism as follows: "The cytochrome system may be figured as a potential ladder'
along which electrons move from dehydrogenase systems to oxygen. The
molecular skeleton of the system hereby functions as a metallic conductor. . . . Similar to the conditions in an electric battery electrons will
flow through the system only if an external circuit is closed between the
poles. Electrolytes in the immediate surroundings of the cytochrome system constitute the outer circuit. Electrons are thus transported from the
negative pole (dehydrogenases) to the positive (oxygen) and an equivalent quantity of anions is moved in the opposite direction."
The inherent fallacy of Lundegárdh's hypothesis is expressed in the
above description. To pursue the analogy of a battery, an external circuit must be closed because the slightest directional movement of electrons internally would produce a potential difference between the poles
which would oppose further current flow. In the "potential ladder"
described above, no such potential difference is established, for as
already remarked by Ussing (1949), for each electron moved through
the ladder, a proton from the substrate correspondingly moves through
the solution to give, in the net, electrical neutrality. In other words,
a redox potential ladder does not represent an electrostatic potential
ladder and, in fact, the latter does not exist during electron transport.
Thus the redox pump as described by Lundegárdh depends upon an
electrostatic gradient in time, produced by the consecutive alternation
of charges of contiguous metal prosthetic groups within a catenary
system. Since adjacent cytochromes within the respiratory chain are
firmly attached to one and the same cellular structure (the mitochondrion), it is questionable whether they are free to interact by collision
(see Chance, 1953). In any event, an anion would be held to the ironporphyrin complex by an undiscriminating electrostatic charge, and it is
not to be expected that such an anion would be specifically transferred
in the manner of an enzymatically mediated group transfer. Thus, if a
given anion is not handed on directly to the next positive charge, it must
temporarily become part of the surroundings. The anions in the surroundings can only hope to shuttle between the alternating positive
271
Accordingly, as electrons are passed from the substrate to oxygen via the
cytochrome system, anions are thought to move in the opposite direction, so as to satisfy the transient net positive charge of the metal component of the iron-porphyrin enzymes. As Lundegárdh says, "the absence of free anions acts as a specific inhibitor of cytochrome oxidase,
quite as effective as CN or CO." By the same token, the salt respiration
is explained by Lundegárdh on the basis of the removal of that inhibition
by addition of salt.
Lundegárdh (1953a) visualizes the accumulation mechanism as follows: "The cytochrome system may be figured as a potential ladder'
along which electrons move from dehydrogenase systems to oxygen. The
molecular skeleton of the system hereby functions as a metallic conductor. . . . Similar to the conditions in an electric battery electrons will
flow through the system only if an external circuit is closed between the
poles. Electrolytes in the immediate surroundings of the cytochrome system constitute the outer circuit. Electrons are thus transported from the
negative pole (dehydrogenases) to the positive (oxygen) and an equivalent quantity of anions is moved in the opposite direction."
The inherent fallacy of Lundegárdh's hypothesis is expressed in the
above description. To pursue the analogy of a battery, an external circuit must be closed because the slightest directional movement of electrons internally would produce a potential difference between the poles
which would oppose further current flow. In the "potential ladder"
described above, no such potential difference is established, for as
already remarked by Ussing (1949), for each electron moved through
the ladder, a proton from the substrate correspondingly moves through
the solution to give, in the net, electrical neutrality. In other words,
a redox potential ladder does not represent an electrostatic potential
ladder and, in fact, the latter does not exist during electron transport.
Thus the redox pump as described by Lundegárdh depends upon an
electrostatic gradient in time, produced by the consecutive alternation
of charges of contiguous metal prosthetic groups within a catenary
system. Since adjacent cytochromes within the respiratory chain are
firmly attached to one and the same cellular structure (the mitochondrion), it is questionable whether they are free to interact by collision
(see Chance, 1953). In any event, an anion would be held to the ironporphyrin complex by an undiscriminating electrostatic charge, and it is
not to be expected that such an anion would be specifically transferred
in the manner of an enzymatically mediated group transfer. Thus, if a
given anion is not handed on directly to the next positive charge, it must
temporarily become part of the surroundings. The anions in the surroundings can only hope to shuttle between the alternating positive