Fermentation

Some organisms can respire even in the total absence of oxygen through a process called fermentation, of which three types exist: alcoholic fermentation, lactic acid fermentation and mixed acid fermentation. In alcoholic fermentation, cells of roots growing in waterlogged, oxygen-poor soil, and many yeasts (Saccharomyces) more generally, convert pyruvic acid into ethyl alcohol and CO2 in two steps: first, pyruvate decarboxylase (with thiamine pyrophosphate, TPP, and Mg2+) removes a CO2 from pyruvic acid to form acetaldehyde ($2\,CH_3COCOOH \rightarrow 2\,CH_3CHO + 2CO_2\uparrow$); then alcohol dehydrogenase reduces acetaldehyde to ethyl alcohol, using up the $2\,NADH+H^+$ generated earlier in glycolysis and regenerating NAD+ ($2\,CH_3CHO + 2NADH + 2H^+ \rightarrow 2\,CH_3CH_2OH + 2NAD^+$). This pathway underlies several industrial processes: leavening bread, cakes and biscuits in bakeries; producing wine and other alcoholic beverages (made by fermenting the sugars in malted barley or grapes until rising ethanol concentration eventually becomes toxic to the yeast itself, halting fermentation and leaving behind beer or wine); making vinegar and tanning/curing leather; and producing ethanol for gasohol fuel, as used in cars in Brazil. A classic laboratory demonstration uses a Kuhne's fermentation tube -- an upright glass tube with a side bulb -- filled with 10% sugar solution and baker's yeast; as fermentation proceeds, CO2 accumulates and the liquid level in the upright column falls, and adding a pellet of KOH (which absorbs the CO2) causes the level to rise again, directly demonstrating that gas production is due to CO2. Lactic acid fermentation, by contrast, is carried out by some bacteria (such as Bacillus), certain fungi, and vertebrate muscle cells, using a single enzyme (lactate dehydrogenase, with Zn2+) to reduce pyruvic acid directly to lactic acid in one step, without forming acetaldehyde as an intermediate and without releasing any CO2 ($2,CH …