RESPIRATION RATE IN PLANTS
283
tissue or organ. Where respiration has been found to consist of both a
cyanide-resistant and a cyanide-sensitive fraction, it was the respiratory component which by indirect evidence was presumed to be phosphorylative in nature that was in each case inhibited by low concentrations of cyanide. Where the respiration was heterogeneous but entirely
cyanide-sensitive, there was some indication that phosphorylative metabolism was associated with that fraction of the respiration which was
mediated by cytochrome oxidase. In many instances, the transfer of
electrons through the cytochrome system was found to be restricted
by the phosphorylative process. The performance of cellular work, or
the artificial uncoupling of phosphorylation from oxidation by chemical
agents, removed this restriction and thereby stimulated the respiration.
Although the oxidations of the tricarboxylic acid cycle are most often
accompanied by oxidative phosphorylation, and although electron transfer attending the operation of the cycle commonly terminates in the
cytochrome-cytochrome oxidase system, the association is not obligatory.
It has been shown, for example (Young and Conn, 1956), that in avocado mitochondria the transfer of hydrogen atoms from component
members of the tricarboxylic acid cycle to molecular oxygen may take
place via glutathione, ascorbate, and ascorbic oxidase in sequence. This
pathway is not phosphorylative, and does not traverse the cytochrome
system. On the other hand, at least several oxidative systems which
terminate in cytochrome oxidase are neither phosphorylative nor necessarily associated with the tricarboxylic acid cycle. Such systems include
both mitochondrial and nonparticulate antimycin Α-insensitive DPNH
oxidase of potatoes (Hackett, 1956b). Distinct from both of the preceding cases is the nonphosphorylative microsomal electron pathway which
terminates in the cyanide-insensitive and carbon-monoxide-insensitive
cytochrome b 3 (Martin and Morton, 1955, 1956).
The evidence to date strongly suggests that oxidative phosphorylation
is a prerequisite to the performance of cellular anabolic processes. Oxidative phosphorylation, furthermore, has been found under normal
conditions to occur only in association with intramitochondrial antimycin
Α-sensitive electron transport, and with the succinoxidase system. The
locus of antimycin A action is taken to be between cytochromes b and c
or, more broadly, between the pyridine nucleotides and cytochrome c
(see Chance and Williams, 1955). Thus, oxidative phosphorylation is
primarily a concomitant of the operation of the tricarboxylic acid cycle
under conditions where electron transfer terminates in cytochrome oxidase. Although it cannot be said with certainty that apart from glycolysis
the energy of respiration is made available exclusively by the cytochrome-
283
tissue or organ. Where respiration has been found to consist of both a
cyanide-resistant and a cyanide-sensitive fraction, it was the respiratory component which by indirect evidence was presumed to be phosphorylative in nature that was in each case inhibited by low concentrations of cyanide. Where the respiration was heterogeneous but entirely
cyanide-sensitive, there was some indication that phosphorylative metabolism was associated with that fraction of the respiration which was
mediated by cytochrome oxidase. In many instances, the transfer of
electrons through the cytochrome system was found to be restricted
by the phosphorylative process. The performance of cellular work, or
the artificial uncoupling of phosphorylation from oxidation by chemical
agents, removed this restriction and thereby stimulated the respiration.
Although the oxidations of the tricarboxylic acid cycle are most often
accompanied by oxidative phosphorylation, and although electron transfer attending the operation of the cycle commonly terminates in the
cytochrome-cytochrome oxidase system, the association is not obligatory.
It has been shown, for example (Young and Conn, 1956), that in avocado mitochondria the transfer of hydrogen atoms from component
members of the tricarboxylic acid cycle to molecular oxygen may take
place via glutathione, ascorbate, and ascorbic oxidase in sequence. This
pathway is not phosphorylative, and does not traverse the cytochrome
system. On the other hand, at least several oxidative systems which
terminate in cytochrome oxidase are neither phosphorylative nor necessarily associated with the tricarboxylic acid cycle. Such systems include
both mitochondrial and nonparticulate antimycin Α-insensitive DPNH
oxidase of potatoes (Hackett, 1956b). Distinct from both of the preceding cases is the nonphosphorylative microsomal electron pathway which
terminates in the cyanide-insensitive and carbon-monoxide-insensitive
cytochrome b 3 (Martin and Morton, 1955, 1956).
The evidence to date strongly suggests that oxidative phosphorylation
is a prerequisite to the performance of cellular anabolic processes. Oxidative phosphorylation, furthermore, has been found under normal
conditions to occur only in association with intramitochondrial antimycin
Α-sensitive electron transport, and with the succinoxidase system. The
locus of antimycin A action is taken to be between cytochromes b and c
or, more broadly, between the pyridine nucleotides and cytochrome c
(see Chance and Williams, 1955). Thus, oxidative phosphorylation is
primarily a concomitant of the operation of the tricarboxylic acid cycle
under conditions where electron transfer terminates in cytochrome oxidase. Although it cannot be said with certainty that apart from glycolysis
the energy of respiration is made available exclusively by the cytochrome-