Fermentation, energy requirement

Under anaerobic conditions, respiration and fermentation processes may proceed to support the energy requirements of the organisms (Figure 2.1). Contrary to respiration, fermentation does not require the participation of an external electron acceptor. In this case, the organic substrate undergoes a balanced series of oxidative and reductive reactions, i.e., organic matter reduced in one step of the process is oxidized in another. 

Battley reports that 23 per cent of the glucose consumed during the fermentation is utilised to produce biomass whilst 77 per cent is utilised to produce the energy required by the cell. He gives the stoichiometry for the anabolic reaction as ... 

Acetate is simultaneously produced and used (oxidized) by the gut, possibly accounting for up to 50% of the gut s energy requirement in ruminants. Propionate and butyrate produced in the rumen and/or hind gut fermentations (nonruminant herbivores) are extensively metabolized by the visceral tissues, so that only insignificant amounts reach the portal circulation. Plasma fatty acids, rather than acetate, are the major energy source for skeletal muscle energy in ruminants and other mammals (53). 

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