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do u know about anaerobic respiration ???
Well when there is no oxygen present the NADH can't be used in the ETC, and sence cells constantly need energy and there are a limited amount of NAD+ and NADH in cells it is important that they can restore NADH back to NAD+.
This can be done primarly in two ways: The formation of ethanol or the formation of lactate. In the following I explain how this is done for both ethanol and lactate:
Ethanol is formed from pyruvate in yeast and several other microorganisms. The first step is the decarboxylation of pyruvate. The first step is catalyzed by pyruvate decarboxylase, which requires the coenzyme thiamine pyrophosphate. This coenzyme, derived from the vitamine thiamine (B1), also participates in reactions catalyzed by other enzymes. The second step is the reduction of acetaldehyde to ethanol by NADH, in a reaction catalyzed by alcohol dehydrogenase. This process regenerates NAD+.
Lactate is formed from pyruvate in a variety of microorganisms. The reaction also takes place in the cells of higher organisms when the amount of oxygen is limiting, as in muscle cells during intense activity. The reduction of pyruvate by NADH to form lactate is catalyzed by lactate dehydrogenase. This process too regenerates NAD+.
So to recap:
The regeneration of NAD+ in the reduction of pyruvate to lactate or ethanol sustains the continued process of glycolysis under anaerobic conditions. Both processes is known as fermentations (alcoholic fermentation and lactic acid fermentation).