Higher Plant Respiration and Its Relationships to Photosynthesis
75
substrate such as glucose. It is possible, even probable, that mitochondria
(and chloroplasts) originated from free-living aerobic eubacteria that invaded a primitive eukaryote, leading to a permanent symbiotic relationship
(Lehninger 1965; Margulis 1970; Douce 1985; Sitte and Eschbach 1992).
4.2.1 Unique Properties of Plant Respiration
and Mitochondrial Metabolism
While respiratory biochemistry is similar among eukaryotes, higher plant
respiration differs from that of many other organisms in notable ways (see,
e.g., Ikuma 1972; Palmer 1979; ap Rees 1985; Douce and Neuburger 1989).
For example: (A) in plants, sucrose is a major product of photosynthesis
and a primary form of carbon translocated between organs, and it is often
appropriate to take sucrose (rather than, say, glucose) as the starting point
of respiration. (B) In addition to the nonreversible conversion of fructose
6-P to fructose 1,6-P 2 by phosphofructokinase, many plants contain PPj -
requiring fructose 6-P 1-phosphotransferase that catalyzes the same reaction,
but reversibly and using PPj (forming PJ rather than ATP (forming ADP).
(C) Plant succinate-CoA ligase phosphorylates ADP rather than GDP (but
see Weitzman 1987). (D) The rate of mitochondrial O2 uptake per unit
protein can be faster in plants than in animals. (E) Rotenone-resistant electron transport (bypass of complex I) is possible in many plants. (F) Rapid
cyanide-resistant O 2 uptake is possible in plants. (G) Plant mitochondria can
oxidize cytosolic NADH and NADPH (a separate dehydrogenase exists for
Fig. 4.3. Higher plant mitochondrial respiratory chain (drawn sizes and shapes are
arbitrary). Complexes I, II, III, and IV are not known to differ greatly in plants and other
organisms. Cytochrome c (Cyt c) is a peripheral protein linking complexes III and IV.
The matrix-facing NADH dehydrogenase (NADH DR) other than complex I is insensitive
to rotenone and is not known to exist in animals. That rotenone-resistant NADH dehydrogenase may actually be a second UQ binding site on complex I (within the membrane)
that is not coupled to proton pumping (SooIe et al. 1990, 1992). The alternative pathway
refers to electron transport from ubiquinol (reduced ubiquinone) to O2 via the alternative
oxidase (Alt ox). Succinate and fumarate are intermediates of the TCA cycle. The NADH
and NADPH dehydrogenases facing the intermembrane space are found in plants and
fungi but not mammals although mammals can oxidize cytosolic reducing equivalents
indirectly via metabolite shuttles. Compared to the dehydrogenases facing the intermembrane space, succinate (Complex II) may have preferential access to the alternative
oxidase, perhaps due to some spatial "association," or more simply, a shorter diffusionpath length between the two (Day et al. 1991). Oxidative phosphorylation is presumably
driven by the movement of protons from the intermembrane space to the mitochondrial
matrix via Fl' Fo ATPases passing through the inner membrane (Nicholls and Ferguson
1992). The outer membrane pores are large enough to be freely permeable to ATP and
NAD(P)H. The plant mitochondrial outer membrane is discussed by Mannella (1985) and
the inner membrane by Douce (1985). Proposed "respiratory" electron transport in
chloroplasts in the dark apparently does not pump protons (e.g., Singh et al. 1992) - a
major function of mitochondrial electron transport - so it is not similar to this figure
75
substrate such as glucose. It is possible, even probable, that mitochondria
(and chloroplasts) originated from free-living aerobic eubacteria that invaded a primitive eukaryote, leading to a permanent symbiotic relationship
(Lehninger 1965; Margulis 1970; Douce 1985; Sitte and Eschbach 1992).
4.2.1 Unique Properties of Plant Respiration
and Mitochondrial Metabolism
While respiratory biochemistry is similar among eukaryotes, higher plant
respiration differs from that of many other organisms in notable ways (see,
e.g., Ikuma 1972; Palmer 1979; ap Rees 1985; Douce and Neuburger 1989).
For example: (A) in plants, sucrose is a major product of photosynthesis
and a primary form of carbon translocated between organs, and it is often
appropriate to take sucrose (rather than, say, glucose) as the starting point
of respiration. (B) In addition to the nonreversible conversion of fructose
6-P to fructose 1,6-P 2 by phosphofructokinase, many plants contain PPj -
requiring fructose 6-P 1-phosphotransferase that catalyzes the same reaction,
but reversibly and using PPj (forming PJ rather than ATP (forming ADP).
(C) Plant succinate-CoA ligase phosphorylates ADP rather than GDP (but
see Weitzman 1987). (D) The rate of mitochondrial O2 uptake per unit
protein can be faster in plants than in animals. (E) Rotenone-resistant electron transport (bypass of complex I) is possible in many plants. (F) Rapid
cyanide-resistant O 2 uptake is possible in plants. (G) Plant mitochondria can
oxidize cytosolic NADH and NADPH (a separate dehydrogenase exists for
Fig. 4.3. Higher plant mitochondrial respiratory chain (drawn sizes and shapes are
arbitrary). Complexes I, II, III, and IV are not known to differ greatly in plants and other
organisms. Cytochrome c (Cyt c) is a peripheral protein linking complexes III and IV.
The matrix-facing NADH dehydrogenase (NADH DR) other than complex I is insensitive
to rotenone and is not known to exist in animals. That rotenone-resistant NADH dehydrogenase may actually be a second UQ binding site on complex I (within the membrane)
that is not coupled to proton pumping (SooIe et al. 1990, 1992). The alternative pathway
refers to electron transport from ubiquinol (reduced ubiquinone) to O2 via the alternative
oxidase (Alt ox). Succinate and fumarate are intermediates of the TCA cycle. The NADH
and NADPH dehydrogenases facing the intermembrane space are found in plants and
fungi but not mammals although mammals can oxidize cytosolic reducing equivalents
indirectly via metabolite shuttles. Compared to the dehydrogenases facing the intermembrane space, succinate (Complex II) may have preferential access to the alternative
oxidase, perhaps due to some spatial "association," or more simply, a shorter diffusionpath length between the two (Day et al. 1991). Oxidative phosphorylation is presumably
driven by the movement of protons from the intermembrane space to the mitochondrial
matrix via Fl' Fo ATPases passing through the inner membrane (Nicholls and Ferguson
1992). The outer membrane pores are large enough to be freely permeable to ATP and
NAD(P)H. The plant mitochondrial outer membrane is discussed by Mannella (1985) and
the inner membrane by Douce (1985). Proposed "respiratory" electron transport in
chloroplasts in the dark apparently does not pump protons (e.g., Singh et al. 1992) - a
major function of mitochondrial electron transport - so it is not similar to this figure