Session 226

Attacking another set of discretes this week with Adam from Blueprint MCAT. We break down different questions about nuclear localization, proteins, and more! Don’t forget to go check out their new flashcards over at MCATflashcards.com. If you would like to follow along on YouTube, go to premed.tv.

Listen to this podcast episode with the player above, or keep reading for the highlights and takeaway points.

[01:13] Self-Study vs. Prep Course

We had a great conversation last week with Hunter about MCAT self-study or prep course to help you decide what you should do and what is right for you. Not everyone needs a course. And at the end of the day, the ultimate decision is yours to figure out what you need. So go check out last week’s MCAT podcast if you haven’t done that yet.

[02:35] MCAT Tips When Jumping to Discretes

The difference between going from either passage to passage or discrete to passage is that you don’t have to worry about moving efficiently through a pretty dense passage. With discretes, it’s not as much to prepare for what is coming as much as it is. Don’t worry about what just happened. Maybe the passage didn’t go great but keep your mindset steady and make sure you’re not worried about anything that went wrong so far.

[03:26] Question 57

Which of the following do NOT have proteins with a nuclear localization signal?

1. E. coli

II. Homo sapiens

III. Fungi

1. Archaea
2. I only
3. III only
4. I and IV only
5. I, III, and IV only

Thought Process:

This is a negation question here. Now, if we decide that E. coli does not have proteins with this signal, then it could still be A, C, or D. It’s important to know what a nuclear localization signal is. But if you’re coming at this question and you don’t know what it is, you can start using context clues. None of these answer options have Roman Numeral II in them. Knowing what Homo sapiens are, we know they have a nuclear localization signal.

So let’s try to distinguish between homosapiens and any of these other Roman numerals. What are the categories that homo sapiens fall in and how do they differ from any of these other types of organisms?

In this case, the nuclear localization signal is a hint where if you don’t have proteins with a nuclear localization signal, then you don’t have a nucleus. So we’re looking for which type of organisms are not going to have a nucleus.

If you have outside information, you know that prokaryotes are not going to have nuclei, whereas eukaryotes will. So really, this is a question of which of the following are prokaryotes? 

And we know that E. coli is a type of bacteria and bacteria are prokaryotes. So Roman numeral I has to be in your answer.Homo sapiens are eukaryotes and so are fungi. Therefore, we shouldn’t see option III there. And then archaea is also a prokaryote which doesn’t have any nucleus. So we should see IV in the answer. And the only answer option that has I and IV is C.

Correct Answer: C

[11:22] Question 58

While the blood is buffered primarily through the equilibrium between carbon dioxide and carbonic acid, coupled with hemoglobin, the blood may also be buffered through other plasma proteins. Which of the following is true?

1. A shift in the pH can alter the tertiary or quaternary structure of the protein, allowing it to buffer the pH by precipitating out of plasma in response to pH shifts.
2. The amino acid residues that make up the protein may act as Brønsted acids or bases, reducing shifts in pH.
3. Plasma soluble proteins have enzymatic function allowing them to sequester hydronium ions from the blood inside membrane-bound organelles in the podocytes lining the capillaries.
4. In the presence of altered pH, any plasma-soluble proteins will undergo either acid- or base-catalyzed cleavage, thus depleting the acid or base causing the disruption to blood pH.

Thought Process:

A – This is wrong because it references precipitating out of the plasma. That’s not what happens with alters in pH. Although pH can denature protein and alter the tertiary and quaternary structure. This first component is correct. However, the second component about precipitating out of the plasma is wrong.

B – Amino acid residues are both basic and acidic side chain groups. We know that side chains could act as Brønsted acids or bases. So this seems reasonable.

C – This is just overcomplication and not true.

D – This is also incorrect. It’s not a physiological phenomenon that we see. Plasma soluble proteins would undergo acid or base catalyzed cleavage. In the presence of ultra pH, our bodies would be in a hefty amount of trouble pretty frequently because we have to be able to moderate those changes in pH.

Correct Answer: B

[17:41] Question 59

Which of the following correctly lists a pair of analogous structures and a pair of homologous structures, respectively?

1. The wing of a bee and the wing of a bird; the wing of a bird and the leg of a bird
2. The wing of a bee and the wing of a bird; the arm of a human and the flipper of a walrus
3. The arm of a human and the wing of a bat; the wing of a bird and the wing of a bat
4. The wing of a bird and the wing of a bat; the wing of a bee and the wing of a bat

Thought Process:

Analogous structures are structures that evolved independently, but they have similar functions. So just like the wing of a bee and the wing of a bird. They evolved independently and they have different evolutionary histories. But those wings serve the same purpose.

Homologous, on the other hand, are structures that have a similar evolutionary history, but might now have different functions. Whereas analogous evolved differently, the function is the same. Homologous is similar evolutionary history, and now, possibly branching out into different functions. And so we’re looking for something that evolved similarly, but might be serving different functions here.

For that reason, a lot of these here are analogous structures – the wing of a bee and the wing of a bird, and the wing of a bird and the wing of a bat. So C and D are both wrong since they’re both analogous and we’re looking for homologous.

And so, we’re left with A and B. So we look for those that have similar evolutionary histories, but possibly, now different functions. And so for that reason, B is the correct answer.

Correct Answer: B

Links:

Meded Media

Blueprint MCAT

The MCAT Podcast Episode 225: Which is Better, MCAT Self-Study or Prep Course?