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We are almost done with cellular respiration! We will be doing the same procedure as for
the last two. We'll go over the process of fermentation then I will be asking you questions
that you should know for the exam. If a cell doesn't have access to oxygen, it
can't undergo aerobic cellular respiration. Some cells, such as brewer's yeast and your
muscle cells, can switch over of another process when there is no oxygen. We call this process
fermentation. Fermentation is a way to regenerate ***+ so that glycolysis can continue. If you
don't continue glycolysis, you don't get the two ATPs that are generated by glycolysis.
You may be thinking that the cell would be wasting the NADH is generated during glycolysis,
but NADH doesn't last very long, so it's not like the cell can store it while it waits
for oxygen to show up so it can get respiration online again.
So, here in this diagram, the products of glycolysis, two pyruvate molecules, have an
acid group clipped off. This leaves as CO2. We now have two acetaldehydes. They could
go to the citric acid cycle, but that would generate more NADH and the cell can't use
what it has. The hydrogen from a NADH is placed on the an acetaldehyde, which regenerates
***+. The ***+ can go back to the glycolysis cycle. Once a hydrogen is added to acetaldehyde,
we have ethanol, the most famous fermentation product. This is why non-distilled alcoholic
beverages are bubbly, from the CO2. If we are making bread, we want this CO2 to cause
the bread dough to rise. Otherwise, you have a tortilla or a cracker. During baking, the
ethanol in the bread evaporates off. Your muscles and the bacteria that ferments
dairy products have a slightly different process. Rather than clipping off the acid group from
the pyruvate, the hydrogen is placed direction on the pyruvate from the NADH. This regenerates
***+ and lactic acid. That way, you don't get drunk when you exercise. Also, that's
why yogurt and sour cream aren't fizzy. Once again, for both types of fermentation,
I want you to know the why, the where, the starting substrate or substrates, the energy
used, the energy produced, and the ending product or products.