In ethanol fermentation. One glucose molecule breaks down into two pyruvates (1). The energy from this exothermic reaction is used to bind inorganic phosphates to ADP and convert NAD+ to NADH. The two pyruvates are then broken down into two Acetaldehyde and give off two CO2 as a waste product (2). The two Acetaldehydes are then converted to two ethanol by using the H+ ions from NADH; converting NADH back into NAD+ (3).
<a href="//commons.wikimedia.org/wiki/User:Davidcarmack" title="User:Davidcarmack">David B. Carmack Jr.</a>
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<span class="int-own-work" lang="en">Own work</span>