Microbes and Metabolism 35oxidised by relinquishing electrons to the cytochromes in the chain thus regener-
ating the oxidised cofactor. In this scenario, pyruvate may enter the TCA cycle
rather than a fermentation route, thus a further mole of ATP is produced at sub-
strate level during conversion of succinyl CoA to succinate via GTP, which then
transfers the terminal phosphate to ATP. In addition, NADH and FADH 2 are pro-
duced during the TCA cycle thus generating up to 15 moles of ATP per mole of
pyruvate. An overall comparison may be made between glycolysis followed by
reoxidation of NADH by fermentation or, alternatively, glycolysis followed by
entry into the TCA cycle and reoxidation of cofactors via the electron transport
chain. Remembering that one mole of glucose generates two moles of pyruvate
during glycolysis, and that the two moles of NADH produced during glycoly-
sis may also be reoxidised by transfer to the electron transport chain and not
through fermentation, the net result is that glucose catabolised by the glycoly-
sis–fermentation route results in the production of two moles of ATP whereas
catabolism by the glycolysis–TCA cycle–electron transport/oxidative phospho-
rylation route produces up to 32 moles of ATP. The figure of 36 was deduced
by Lehninger (1975) but has been revised more recently to reflect the tenets of
the chemiosmotic theory described earlier.
Anaerobic respiration is less efficient than aerobic respiration. Oxidation of
the same amount of cofactor by methanogenesis rather than oxidative phospho-
rylation would produce fewer moles of ATP. Consequently, for a given amount
of ATP production, the flux of glucose through glycolysis followed by fermen-
tation would have to be approximately 16 times greater than through glycolysis
followed by oxidative phosphorylation, and the flux through methanogenesis is
somewhat intermediate. It is the metabolic capability of the organism and the
presence or absence of the appropriate inorganic electron acceptor which deter-
mines the fate of pyruvate on the grounds of energy considerations. On a practical
basis this may explain why anaerobic processes, such as the anaerobic digestion
of sewage sludge and municipal solid waste, are considerably less exothermic
than their aerobic counterparts. For a given quantity of carbon source, an aerobic
process will be able to extract in the order of 10 times the amount of energy than
that generated by an anaerobic process.
Regeneration of NAD+in plants
In addition to the processes discussed above for the production of NADH, plant
mitochondria operate an additional system whereby the required protons are
derived from two molecules of the amino acid glycine. During this mitochon-
drial process, one molecule of molecular oxygen is consumed in the production
of carbon dioxide and the amino acid, serine. The superfluous amino group from
the second glycine molecule is released as ammonia. The glycine molecules
were derived from phosphoglycolate, the metabolically useless product of pho-
torespiration. This subject is very important with regard to plant breeding and