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J.S. Amthor
each nucleotide). (H) Rapid transport of oxaloacetate (OAA) across the
inner membrane of plant mitochondria can occur under physiological conditions. (I) NAD+ can be actively accumulated from an external medium by
plant mitochondria, apparently via a specific NAD+ transporter. (J) Plant
mitochondria readily oxidize malate in the presence of OAA and absence of
pyruvate. (K) Plant mitochondria contain some unique cytochromes and the
composition of common cytochromes can differ among plants and other
organisms. (L) In leaf mitochondria, glycine decarboxylase may account for
as much as half of the matrix protein and rapid glycine decarboxylation, a
component of photorespiration, is possible. (M) Fatty acid oxidation is
generally slow in plant mitochondria. (N) Plants contain the largest, most
complex mitochondrial genome. Many of these "unique" properties confer a
large degree of metabolic flexibility to plants.
4.2.2 Control of Respiration Rate
Respiration rate can be regulated by the amount of respiratory machinery
(enzymes and transporters), the amount of respiratory substrate (carbohydrates and O2), the rate of ATP and NAD(P)H use [ADP and NAD(P)+
regeneration], or the rate of respiratory intermediate use. There has also
been considerable interest in the control of respiration by calcium in plants
(Wellburn and Owen 1991) and mammals (Brown 1992). In rapidly growing
plant cells, the amounts of respiratory machinery or carbohydrate may limit
respiration because the demands for respiratory products (carbon skeletons,
A TP, and reductant) are high and those products are used as rapidly as
they are produced. Moreover, the amount of respiratory machinery in very
young cells may be small, being in a state of construction itself, and the
supply of carbohydrate may be limited by incompletely developed phloem
near growing cells. In mature or slowly growing plant cells, short-term
(seconds to hours) control of respiration rate is probably brought about by
the rate of use of respiratory products, and in particular ATP,2 rather than a
lack of metabolic machinery or carbohydrates (French and Beevers 1953;
Beevers 1961, 1970; Copeland and Turner 1987; Dry et al. 1987; Farrar and
2 Control of respiration rate by ADP availability (use of ATP) can exist with respect to
glycolysis, the TCA cycle, and oxidative phosphorylation. Generally, the term respiratory
control is used to refer to a feedback inhibition of respiratory chain (Fig. 4.3) activity due
to a large proton motive force (Ap) arising when ADP levels are low and oxidative
phosphorylation is slow. Respiratory control has been studied with the use of uncoupling
agents, which not only disengage the link between respiratory chain activity and Ap
and therefore oxidative phosphorylation, but also limit pyruvate and Pi uptake by mitochondria because that transport is driven by Ap (see Figs. 4.2 and 4.3). This may underlie
the observation that uncouplers stimulate glycolysis to a greater extent than they do the
TCA cycle (Wiskich and Dry 1985) and indicates that the capacity of respiration is
underestimated under the influence of uncouplers.
J.S. Amthor
each nucleotide). (H) Rapid transport of oxaloacetate (OAA) across the
inner membrane of plant mitochondria can occur under physiological conditions. (I) NAD+ can be actively accumulated from an external medium by
plant mitochondria, apparently via a specific NAD+ transporter. (J) Plant
mitochondria readily oxidize malate in the presence of OAA and absence of
pyruvate. (K) Plant mitochondria contain some unique cytochromes and the
composition of common cytochromes can differ among plants and other
organisms. (L) In leaf mitochondria, glycine decarboxylase may account for
as much as half of the matrix protein and rapid glycine decarboxylation, a
component of photorespiration, is possible. (M) Fatty acid oxidation is
generally slow in plant mitochondria. (N) Plants contain the largest, most
complex mitochondrial genome. Many of these "unique" properties confer a
large degree of metabolic flexibility to plants.
4.2.2 Control of Respiration Rate
Respiration rate can be regulated by the amount of respiratory machinery
(enzymes and transporters), the amount of respiratory substrate (carbohydrates and O2), the rate of ATP and NAD(P)H use [ADP and NAD(P)+
regeneration], or the rate of respiratory intermediate use. There has also
been considerable interest in the control of respiration by calcium in plants
(Wellburn and Owen 1991) and mammals (Brown 1992). In rapidly growing
plant cells, the amounts of respiratory machinery or carbohydrate may limit
respiration because the demands for respiratory products (carbon skeletons,
A TP, and reductant) are high and those products are used as rapidly as
they are produced. Moreover, the amount of respiratory machinery in very
young cells may be small, being in a state of construction itself, and the
supply of carbohydrate may be limited by incompletely developed phloem
near growing cells. In mature or slowly growing plant cells, short-term
(seconds to hours) control of respiration rate is probably brought about by
the rate of use of respiratory products, and in particular ATP,2 rather than a
lack of metabolic machinery or carbohydrates (French and Beevers 1953;
Beevers 1961, 1970; Copeland and Turner 1987; Dry et al. 1987; Farrar and
2 Control of respiration rate by ADP availability (use of ATP) can exist with respect to
glycolysis, the TCA cycle, and oxidative phosphorylation. Generally, the term respiratory
control is used to refer to a feedback inhibition of respiratory chain (Fig. 4.3) activity due
to a large proton motive force (Ap) arising when ADP levels are low and oxidative
phosphorylation is slow. Respiratory control has been studied with the use of uncoupling
agents, which not only disengage the link between respiratory chain activity and Ap
and therefore oxidative phosphorylation, but also limit pyruvate and Pi uptake by mitochondria because that transport is driven by Ap (see Figs. 4.2 and 4.3). This may underlie
the observation that uncouplers stimulate glycolysis to a greater extent than they do the
TCA cycle (Wiskich and Dry 1985) and indicates that the capacity of respiration is
underestimated under the influence of uncouplers.