Respiration oxidation-reduction balance

These energy-producing reactions are termed respiration processes. They require the presence of an external compound that can serve as the terminal electron acceptor of the electron transport chain. However, under anaerobic conditions, fermentation processes that do not require the participation of an external electron acceptor can also proceed. 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. 

By constraining the fraction of the total FeS that is from SO4- reduction based on isotope mass balance, the amount of CO2 produced from respiration of Fe3+ can then be calculated from the remaining portion of the total organic carbon oxidized. 

There are, however, many enzymes which, because they do not seem to be contained in such cellular particles, are referred to as soluble enzymes . Thus, the chemical reactions whereby the yeast cell converts glucose into ethanol and which seem to be identical with the first sequence of reactions in the normal respiration of plant cells are catalysed by such soluble enzymes. In the living cell such enzymes may, however, function as an organised multi-enzyme system which forms part of the endoplasmic reticulum. The fact that they cannot be obtained in association may merely reflect our inability to preserve the endoplasmic reticulum in cell-free preparations. Thus, in alcoholic fermentation by the yeast cell, which occurs in the absence of oxygen, there is an oxidative reaction which is balanced by a reduction. These both involve dehydrogenase enzymes having the same co-enzyme, nicotinamide adenine dinucleotide. One of these reactions, that catalysed by alcohol dehydrogenase, has already been mentioned (p. 73, eqn. 6). The other reaction... 

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