Biochem Chapter 8 Test

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bbiebelberg
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Joined: Thu Jan 09, 2020 8:28 pm

Biochem Chapter 8 Test

Post by bbiebelberg » Wed Feb 19, 2020 12:45 pm

Hello! I did pretty poorly on the passage-based questions in the Biochemistry end of chapter 8 test. I think I have a pretty strong understanding of the fundamentals of the CAC and ETC, but I think I had a hard time following the experiment in the passage and interpreting the implications of certain findings. I also definitely did not know that cis-aconitate is the precursor to isocitrate, so that probably led me astray.

I think one difficulty I also sometimes have, especially with the more dense experiments like this, is associating the various findings. For example, it seemed like we needed to integrate findings about levels of CAC intermediates and cell wall differences.

I guess I'm wondering if you have any advice for interpreting a passage like this, and if you think this passage is on par with what is typical of MCAT, or would you say this is on the harder side?

Thanks.
NS_Tutor_Mathias
Posts: 616
Joined: Sat Mar 30, 2019 8:39 pm

Re: Biochem Chapter 8 Test

Post by NS_Tutor_Mathias » Thu Feb 27, 2020 12:13 pm

Hey there!

Usually in a passage like this, you want to wrap your head around two entirely different pieces of information that you are provided. Difficulty wise, this is not unrepresentative of a fairly normal B&B passage - however, you're probably not used to the intimidation factor of having figures and alphabet salad thrown at you. That is most of what is unusual about this. The experimental design is in itself not particularly complicated.

1. What relationships does the background information describe?

Diagramming a set of relationships, who activates what and how they relate is the best way to do this. For the real thing, you'll almost certainly want to do a majority of these diagrams in your head or in very minimal text form. But take your time while doing practice passages and be thorough. The more often you've thoroughly understood an idea, the easier it will be to get a good hang of a similar idea later.

In this case that would something like this:

Code: Select all

GBM -> Highly glycolytic -> Produces large amounts of lactic acid -> must be transported out of the cell for high rate of glycolysis to continue (le Chatelier etc)
2. How did the experimental design attempt to test a suspected relationship?

In this section, you just want to play close attention to experimental design and realize that every element of experimental design likely plays a purpose - even control design is highly important!

In the first figure, we see that cell cultures were performed in on fluorescent GBM cells , each identical except for the presence of different carbohydrates and an inhibitor of the transporter for lactic acid in one group. You would assume that a higher rate of glycolysis would lead to greater growth of the cell culture, and therefore you would expect the groups provided with substrates for glycolysis to grow the most.

In the second figure, oxygen use of different cell cultures was measured. This time, we have a media-only negative control, a positive control of cells cultured in the presence of some nutrient medium, then nutrient medium+ACCA and finally nutrient medium+ACCA+digitonin. The passage tells us that digitonin would allow ACCA to cross plasma membranes more readily and thereby presumably reach the mitochondrial membrane too. This should prompt us to think about whether we know anything that would be affected by a monocarboxylate transporter that is important for mitochondrial respiration (we do: Pyruvate).

Finally, you put all that together and make some predictions:
1. In figure 1: An MCT inhibitor would be expected to decrease growth rate
2. In figure 2: An MCT inhibitor reaching the mitochondrial outer membrane should inhibit the CAC and thereby the ETC by limiting supply of pyruvate

Then we check the figures to see if these predictions line up, if anything unexpected happened and whether our positive controls acted more or less right. The answer to all the above is usually yes, but the MCAT does place some degree of emphasis on interpreting unexpected results, even if a vast majority of the research passages presented will be finding exactly what they set out to find.
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