Themes of Plant Biology
Respiration
A supply and demand problem arises among cells when glycolysis produces more NADH than can be utilized or when NAD+ supplies are diminished or oxygen is unavailable. NADH production in glycolysis is a way to dispose of electrons and hydrogen; the NADH needs the electron transport chain with its terminal oxygen acceptor and NAD+ is needed to complete the conversion of PGAL to pyruvate. If the pathway is disturbed, organisms remedy the problem generally in one of two ways.
Lactate fermentation
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Animals, protists, and many bacteria and fungi make lactate and release two molecules of ATP, enough to regenerate some NAD+ and keep glycolysis running (but utilizing only a small portion of the energy of the glucose). Yogurt and cheese makers employ bacteria that respire this way and harvest the tasty byproducts of the reactions.
Alcohol fermentation
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Most plant cells and yeasts (fungi) breakdown pyruvate to acetaldehyde, releasing CO2. The acetaldehyde is then reduced by NADH to ethanol (ethyl alcohol). The CO2 makes bread rise, and ethanol is used by brewers and distillers to make alcoholic beverages of all kinds.
Thermodynamically, this is a poor use of glucose. Over 90 percent of the energy of glucose remains in the two alcohol molecules; fermentation has removed only about 7 percent. The ATP captures about one quarter of that, with the rest released as heat.
