cellular respiration and fermentation

asatish
Posts: 8
Joined: Mon Aug 14, 2017 11:23 pm

cellular respiration and fermentation

Postby asatish » Sun Sep 24, 2017 2:58 pm

Quick question:

There was a question in the bio / biochem content review book about what happens when a facultative anaerobe simultaneously undergoes aerobic respiration and fermentation.

I selected the answer that respiration yields more ATP / glucose than fermentation therefore ATP produced by respiration would be greater that by fermentation.

The correct answer was that ATP produced by fermentation may be greater IF many more glucose molecules are processed by fermentation (solution explains 2 glucose via respiration = 72 ATP vs. 100 ATP for 50 molecules via fermentation). This makes sense but I didn't think about it that way so I'm trying to recognize what heuristics I reach for (perhaps incorrectly) and how to catch myself.

My thought process was that cellular respiration would be the preferred metabolic process, so regardless of of how many glucose molecules are present, CR would win out because it has a higher energy yield. Is this problematic because it's making assumptions about the amount of oxygen present or something else I'm not considering?

Thank you!
NS_Tutor_Andrew
Site Admin
Posts: 521
Joined: Mon May 23, 2016 1:47 pm

Re: cellular respiration and fermentation

Postby NS_Tutor_Andrew » Mon Sep 25, 2017 8:21 pm

Hi asatish,

The shortest answer to your question is that it's incorrect to assume that the more energetically efficient form of respiration must be preferred in absolute terms. That assumption seems very logical, but we have to be careful about projecting our assumptions onto biology -- biology is complicated and weird, and the outcome of evolutionary processes might not always seem the most 'logical' or optimal (e.g., what's up with the appendix anyway?). It turns out that the evolutionary history of aerobic metabolism is pretty fascinating, because oxygen was initially a toxin, but the details of the Great Oxygenation Event go beyond the MCAT. In fact, it turns out that yeast actually prefer fermentation even when oxygen is available (https://en.wikipedia.org/wiki/Crabtree_effect). However, that's also not really MCAT-mandatory content. The point of this question is not to apply outside knowledge, but to recognize that the greater efficiency of aerobic respiration doesn't automatically mean that it produces more overall glucose, and that the passage doesn't give us enough information to determine where glucose is being shunted.

Hope this helps and feel free to ask more follow-up questions!
Andrew D.
Content Manager, Next Step Test Prep.
asatish
Posts: 8
Joined: Mon Aug 14, 2017 11:23 pm

Re: cellular respiration and fermentation

Postby asatish » Sat Sep 30, 2017 6:42 pm

Totally forgot to subscribe to this post after submitting it and just came to check on it and saw your response. Thanks Andrew, that's super helpful. :)

shifting gears a little, I also had another question regarding a passage from the AAMC official guide. In short, the question discusses an antibiotic that does not inhibit translation in e coli cells. I managed to recognize that the passage mentioned that e coli had an enzyme that would actually alter the chemical composition of the antibiotic, so it wouldn't work. However, the remaining choices were : use a different antibiotic, or alter temperature by a few degrees. My instinct was "use a different antibiotic" but that seemed too simple, so I reasoned that it is possible that changing the temperature would result in denaturation of the protein and allow the abx to take effect (the correct answer was use a different antibiotic).

What I didn't understand was that that "a moderate increase in temperature will increase translational efficiency but not impact translational inhibition. Since we're generally taught that enzymes are very susceptible to changes in temperature or pH, can you explain how this works?
And are there special cell types or situations we should keep in mind? I feel like I'm making a lot of absolute assumptions, so appreciate any advice on how to avoid those :?

Best,
Anita
NS_Tutor_Andrew
Site Admin
Posts: 521
Joined: Mon May 23, 2016 1:47 pm

Re: cellular respiration and fermentation

Postby NS_Tutor_Andrew » Sun Oct 01, 2017 12:58 pm

Hi Anita,

Glad to have been of help!

Great points re: this Q (AAMC OG B/B 9). I think this is one of those questions where the hardest part is recognizing exactly what the question is asking and phrasing it to yourself in a clear and simple way. This question boils down to saying "chloramphenicol isn't working on these bacteria, how do we fix the problem?" From this point of view, it's pretty easy to eliminate A and B. The easiest way I'd suggest disentangling C and D would be to say that increasing the incubation temperature by a few degrees isn't going to take something that's not working at all and completely kick it into gear. On the other hand, trying another antibiotic would be a great solution.

So that's the simple approach. There's a little bit more going on here, though, if you dig deeper, so let me address that too.

You're 100% right that you should be aware that enzymes are temperature-sensitive, though. To really understand why C is not the best answer, we have to understand a little bit more about the mechanisms that the passage describes. Basically, these bacteria contain cat, which we're told encodes a chloramphenicol acetyltransferase. The MCAT does expect you to infer that the acetylation catalyzed by cat is what inactivates chloramphenicol. So we're faced with a situation where cat is doing its thing and chloramphenicol isn't working. Trying to fix this by tweaking the temperature isn't the best idea. Although the chloramphenicol acetyltransferase can be assumed to be temperature-sensitive, altering the temperature by a few degrees may improve or hinder its function, but it won't change the basics of what it does, and if we change the temperature to the point that it's completely inactivated, we've probably affected a lot of other things in the cell too. Instead, D is a much simpler alternative -- just use another antibiotic! By definition, chloramphenicol acetyltransferase is specific to chloramphenicol, so it shouldn't affect another antibiotic.

Hope this clarifies things! Parenthetically, if you haven't checked them out yet, take a look at our Test and Strategy Review videos (available under "resources"). We have videos—done by Bryan (one of our lead course instructors and VP of MCAT content), Dr. Anthony, and me—on every single AAMC practice passage available to course students (official guide, sample, scored test 1, and scored test 2), as well as on some selected Next Step material. If you ever have a quick question about an AAMC resource, that can be a good place to turn, although you are of course more than welcome to ask follow-up questions here as well!
Andrew D.
Content Manager, Next Step Test Prep.

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