6.5.2  Energy generation

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Figure 6.30 Simplified view of processes involved in carbon gain and generation of respiratory energy. CO2 assimilated by chloroplasts is used to produce carbon-rich compounds (photoassimilates) that are subsequently exported to the cytosol and mitochondria. CO2 is then lost during breakdown of these carbon-rich compounds by glycolysis and mitochondrial respiration. Release of CO2 and uptake of O2 by mitochondria are coupled to production of usable energy (ATP, NADH). Carbon skeletons (necessary for protein synthesis) are also produced during mitochondrial respiration (Original drawing courtesy Owen Atkin)



Figure 6.31 Pathways of electron transport on the inner membrane of plant mitochondria. Electrons from NADH or FADH2 are transferred to the ubiquinone pool (UQ) via complexes I and II, respectively. Electrons can then be transferred to O2 via either the alternative pathway or via complex IV in the cytochrome pathway. Energy is ultimately conserved as ATP whenever electrons pass via complex I, complex III or complex IV In contrast, energy is lost as heat when the complex I bypass or alternative pathway is engaged. The alternative pathway can be inhibited by salicylhydroxamic acid (SHAM), whereas the cytochrome pathway is inhibited by cyanide (Original drawing courtesy Owen Atkin)

Photoassimilate is used to generate respiratory products needed for plant growth (Figure 6.30). Carbon is exported from chloroplasts to the cytosol and mitochondria, and used to generate ATP, redox equivalents (in particular NADH) and carbon skeletons via glycolysis, mitochondrial tricarboxylic acid (TCA) activity and mitochondrial electron transport. Generation of these respiratory products necessitates CO2 loss during glycolysis and passage of metabolites around the TCA cycle. Mitochondria subsequently facilitate electron transport from NADH or FADH2 to ubiquinone (Figure 6.31). From there, electrons can be transferred via the cytochrome pathway to complexes III and IV, ultimately reducing O2 to H2O. Complex I, complex III and complex IV are all coupled to proton trans-location and thus ATP synthesis. However, when electrons go via the NADH dehydrogenase step (rotenone resistant), via succinate dehydrogenase (complex II) or via the alternative oxidase pathway, protons are not translocated and thus ATP is not synthesised. Engagement of these non-phosphorylating pathways will result in loss of energy as heat with little accompanying yield of ATP. Heat generation by the Arum lily spadix (see Feature essay 2.2) is an extreme case, where a non-phosphorylating pathway causes thermogenesis.

Conceivably, plants which contrast in RGR also differ in the degree to which they engage alternative versus cytochrome pathways, but definitive evidence is still lacking. Existing estimates of electron partitioning between the alternative and cytochrome pathways based on respiratory inhibitors such as cyanide and salicylhydoxamic acid (SHAM) (Figure 6.31 legend) are ambiguous (see Millar et al. 1995a; Hoefnagel et al. 1995). Nevertheless, theoretical implications of alternative versus cytochrome pathway engagement can be calculated. Fast- versus slow-growing species would differ in the efficiency of ATP generation. An efficient, fast-growing species could generate 32–36 molecules of ATP for each molecule of glucose that enters glycolysis provided all the electrons pass through complex I to the ubiquinone pool and then 100% go via the cytochrome pathway. Less ATP is produced (i.e. 32 molecules) if glycolytic NADH is used for cytosolic reduction processes whereas more ATP is produced (i.e. 36 molecules) if glycolytic NADH goes to ATP production in mitochondria. By contrast, in an inefficient slow-growing species, diversion of 70% of electrons in the ubiquinone pool to the alternative oxidase (with only 30% passing via the cytochrome pathway) would result in only 16–18 molecules of ATP being generated per molecule of glucose.

Variations in engagement of the alternative oxidase (or other non-phosphorylating pathways) could thus have a significant impact on ATP generated per mole of CO2 released during respiration. Slower respiration in fast-growing species (e.g. the herb in Figure 6.29) could be due in part to increased efficiency of energy generation due to greater engagement of the cytochrome pathway.