Download 9-5 fermentation reading KEY

KEY - IB Bio Year 1 - Reading Questions for 9.5, Fermentation, pp 177-179
(You are not responsible for the last two paragraphs of p. 178)
Note: In the IB learning targets, "Anaerobic respiration" = fermentation. In Neil, "anaerobic respiration" is something
else entirely! In Neil, fermentation = fermentation!
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What is fermentation and why do (some) cells do it? Fermentation means tacking an extra step or two onto
glycolysis so that cells can make ATP in the absence of oxygen.
Compare alcoholic & lactic acid fermentation on the basis of
a. Biochemistry – in alcoholic fermentation, two steps turn pyruvate into a waste molecule (ethanol). In
lactic acid fermentation, one step turns pyruvate into a waste molecule (lactic acid). In both cases, the
sole purpose of wasting pyruvate like this is because in the process NADH is converted into NAD+.
Normally this happens in the ETC. But without oxygen, the ETC can't run. Cells must have NAD+ in
order to keep glycolysis going. And if glycolysis goes, cells can harvest a miniscule amount of ATP (2
per glucose) through substrate level phosphorylation. This could be the difference between life and
death for the cell.
b. Sample organisms – alcoholic fermentation: baker's yeast. Lactic acid fermentation: human muscle
cells, or, lactic acid-producing bacteria.
c. Significance for humans – Fermented foods aided humankind originally because it meant that food
could be stored longer w/o spoilage. We also liked the taste and the texture – bread, wine, beer, etc. In
the case of lactic acid fermentation, the foods produced include yogurt, pickles, sauerkraut and kimchee.
In terms of our muscles, an out-of-shape athlete produces so much lactic acid in intense exercise that
s/he will be sore after their exertion. (Fortunately, the lactic acid produced goes to the liver to be
disposed of properly.)
Describe the differences between obligate and facultative anaerobes, and give examples of each: An
obligate anaerobe cannot do respiration and are, in fact, poisoned by oxygen. Bacteria in a genus called
Clostridium are obligate anaerobes. You might be familiar with C. botulinum, an obligate anaerobe that may
be trapped in the canning process of food. If it is, it will make botulinum toxin (botulism poisoning) and
release some gases. Thus, we never want to eat food from a can that looks puffy and distorted. A
facultative anaerobe can switch between respiration and fermentation if oxygen supplies dwindle.
Baker's yeast are facultative anaerobes.
Concept Check Q 9.5 # 2 "What If…?" – answer in text.