RESPIRATION RATE IN PLANTS
221
organic phosphate is returned as well. When ATP is not being utilized,
the second container will remain full, the return path of ADP to the first
container will be blocked, and respiration will stop. It should be emphasized that the barrier in the return path of ADP in Fig. 3 has been
introduced simply as a means of making the respiration rate dependent
on the rate of utilization of ATP in this imperfect physical analogy. In
actual fact the respiration rate depends only on the concentration of
ADP, and the processes of ATP formation and utilization need in no way
be separated.
When phosphorylation takes place, as shown in Fig. 2, as a consequence of the direct oxidation of a substrate molecule, the process is
termed substrate-level phosphorylation. Phosphorylation accompanying
fermentation and glycolysis is of this type, as is that which attends the
first step of pyruvate and α-ketoglutarate oxidation. The mechanism of
substrate-level phosphorylation has been thoroughly reviewed by Slater
(1953a), Gunsalus (1954), and Anfinsen and Kielly (1954). In the examples given, the receptor for the hydrogen atoms removed from the
substrate is the oxidized form of the coenzyme DPN. As may be seen in
Fig. 1, hydrogen atoms are in turn removed from the reduced form of
DPN and passed to oxygen down a catenary electron-transport chain,
comprising one or more flavoproteins, cytochromes, and cytochrome oxidase. Phosphorylation occurs during this movement of electrons and,
not surprisingly, is called electron-transport phosphorylation. A measure
of the extent of phosphorylation associated with the transfer of a pair
of hydrogen atoms from substrate to oxygen is the ratio (Ρ/0): μ atoms
phosphate estrified/μ atoms oxygen consumed. For example (Fig. 1)
the P/0 ratio for α-ketoglutarate oxidation is 4.0, for succinate oxidation
is 2.0. The average for the oxidations of the tricarboxylic acid cycle is 3.0.
The mechanism of electron-transport phosphorylation remains to be
elucidated. However, the intimate relationship between oxidation and
phosphorylation applies as well to electron transport as to substratelevel phosphorylation. There have been a number of convincing demonstrations of the regulatory control imposed upon oxidations within the
respiratory chain by concomitant phosphorylations. Cross et al. (1949)
have described the effect of phosphate depletion on the oxidations of the
tricarboxylic acid cycle. Rabinovitz et al. (1951) and Lardy and Wellman (1952) have described the manner in which these oxidations are
dependent upon the level of phosphate acceptor and the prevalence of
phosphate-transferring enzyme systems. Lardy (1952) has reviewed the
dependence of rate regulation on the coupling of oxidation and phosphorylation.
Slater (1953b) has formulated a generalized scheme (equations 1 to
221
organic phosphate is returned as well. When ATP is not being utilized,
the second container will remain full, the return path of ADP to the first
container will be blocked, and respiration will stop. It should be emphasized that the barrier in the return path of ADP in Fig. 3 has been
introduced simply as a means of making the respiration rate dependent
on the rate of utilization of ATP in this imperfect physical analogy. In
actual fact the respiration rate depends only on the concentration of
ADP, and the processes of ATP formation and utilization need in no way
be separated.
When phosphorylation takes place, as shown in Fig. 2, as a consequence of the direct oxidation of a substrate molecule, the process is
termed substrate-level phosphorylation. Phosphorylation accompanying
fermentation and glycolysis is of this type, as is that which attends the
first step of pyruvate and α-ketoglutarate oxidation. The mechanism of
substrate-level phosphorylation has been thoroughly reviewed by Slater
(1953a), Gunsalus (1954), and Anfinsen and Kielly (1954). In the examples given, the receptor for the hydrogen atoms removed from the
substrate is the oxidized form of the coenzyme DPN. As may be seen in
Fig. 1, hydrogen atoms are in turn removed from the reduced form of
DPN and passed to oxygen down a catenary electron-transport chain,
comprising one or more flavoproteins, cytochromes, and cytochrome oxidase. Phosphorylation occurs during this movement of electrons and,
not surprisingly, is called electron-transport phosphorylation. A measure
of the extent of phosphorylation associated with the transfer of a pair
of hydrogen atoms from substrate to oxygen is the ratio (Ρ/0): μ atoms
phosphate estrified/μ atoms oxygen consumed. For example (Fig. 1)
the P/0 ratio for α-ketoglutarate oxidation is 4.0, for succinate oxidation
is 2.0. The average for the oxidations of the tricarboxylic acid cycle is 3.0.
The mechanism of electron-transport phosphorylation remains to be
elucidated. However, the intimate relationship between oxidation and
phosphorylation applies as well to electron transport as to substratelevel phosphorylation. There have been a number of convincing demonstrations of the regulatory control imposed upon oxidations within the
respiratory chain by concomitant phosphorylations. Cross et al. (1949)
have described the effect of phosphate depletion on the oxidations of the
tricarboxylic acid cycle. Rabinovitz et al. (1951) and Lardy and Wellman (1952) have described the manner in which these oxidations are
dependent upon the level of phosphate acceptor and the prevalence of
phosphate-transferring enzyme systems. Lardy (1952) has reviewed the
dependence of rate regulation on the coupling of oxidation and phosphorylation.
Slater (1953b) has formulated a generalized scheme (equations 1 to