Next Step Full Length 10, Bio/Biochem Passage 10

Session 131

Passage 10 of Bio/Biochem is about a research study looking at the activity of membrane proteins in three different cell types.

Once again, we’re joined by Clara from Next Step Test Prep as we dig into the last batch of the Bio/Biochem passages.

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[03:30] Passage 10

A research team conducted a number of lab experiments to investigate activity in membrane proteins in three different cell types.

Experiment 1: Voltage clamp electrodes were used to measure the transmembrane potential difference of a squid axon and to track it as a function of time. Application of L-glutamate to the cell dendrites resulted in the depolarization of the membrane and the repolarization the opposite direction, as shown in the accompanying chart. (Table 1 says transmembrane potential difference of squid axon over time).

[03:30] Passage 10

A research team conducted a number of lab experiments to investigate activity in membrane proteins in three different cell types.

Experiment 1: Voltage clamp electrodes were used to measure the transmembrane potential difference of a squid axon and to track it as a function of time. Application of L-glutamate to the cell dendrites resulted in the depolarization of the membrane and the repolarization the opposite direction, as shown in the accompanying chart. (Table 1 shows a transmembrane potential difference of squid axon over time).

Experiment 2: Mutated red blood cells were reproduced in a number of cell cultures which were separated into different repositories. Repository A was placed in a 310 milliosmole per kilogram sodium solution. Repository B was placed in distilled water and repository C was placed in a mixture which contained fluorescent transferring complexes, fluorescent concanavalin A complexes, iron 3+ ion, and a dilute concentration of vinblastine.

Each repository was microscopically observed one hour later and 12 hours later. It was later determined that the mutation in the cell cultures affected active transport ion channels. Then the results of the environmental surrounding’s tests are given and we see that culture A after one hour had no effect, culture B had partial lysis, and culture C had slight fluorescence of cell cytoplasms and then they also tell us that after 12 hours, culture A had partial shrinkage of the cell size. Culture B now had complete lysis. And culture C had increased fluorescence of cell cytoplasms. Finally, experiment 3, an extraction from a localized site of a biological specimen which as experiencing an infection due to a prokaryotic pathogen was cryofixated and imaged with electron microscopy. (The resulting image depicts cells and actually, this passage did not give an image so it must be inferred results.)

Depict cells identified by the researchers as neutrophil granulocytes. Several of these cells appeared to abend in various stages of engulfing pathogen cells. The image additionally depicts cells identified as macrophages which appear to abend in various stages of engulfing neutrophil granulocytes.

[06:42] Question 53:

Which transmembrane protein is most likely to have been involved with the repolarization of the transmembrane potential difference in Experiment 1.

  • (A) Sodium potassium passive transport channel
  • (B) Sodium potassium active transport channel
  • (C) Sodium potassium ligand gated channel
  • (D) None of the above

Clara’s insights:

The correct answer here is B. Experiment 2 did mention that this mutation affected active transport ion channels but the real reason the answer is active transport is that the question talks about repolarization, which reestablishes the resting membrane potential. That is always the sodium-potassium pump, which is an active transport channel. And that is what establishes a resting membrane potential back at negative 70 value.

[09:33] Question 54

In Experiment 2, what best explains the consequences of the 310 milliosmoles per kg sodium NA+ solution on the cells in repository A.

  • (A) Partial functioning of the active transport ion channels
  • (B) Bi-concave nature of red blood cell membrane decreasing the cell structure or rigidity
  • (C) Passive flow of ions through transmembrane proteins
  • (D) Outflow of water molecules through aquaporins

Clara’s insights:

310 milliosmoles sound high so that must be hypertonic. And the cells in a hypertonic solution is water wanting to rebalance so it will move from an area where there is low solute in the cell to an area of high solute. And that is exactly why our water molecules will flow out through aquaporins which is answer choice D.

So you can actually get this without using the passage. Although you can also use the passage to help you answer this. If you didn’t know that 310 milliosmoles per kg is hypertonic, you could go back to Table 2 and that where it says that after 12 hours Culture A showed partial shrinkage of cell size. So water must be falling out of the cell that totally fits in with answer choice D.

[13:50] Question 55

Why did the lysis in Experiment 2 occur?

  1. Lack of ability of transmembrane channels to relieve osmotic pressure.
  2. Hydrophilic nature of cell membrane working in combination with the surrounding water

III. Flow of distilled water into the cells

  • (A) I and II
  • (B) I and III
  • (C) II and III
  • (D) I, II, and III

Clara’s insights:

Experiment 2 had three different cell cultures. Culture A is the one we talked about in the previous question and that’s the one where water flowed out of the cell. But there were two other cultures. If you go back to Experiment 2, you can see that complete lysis happens with culture B. So it’s actually asking about a totally different cell culture than the question before.

Lysis means that the cell essentially explodes and that can totally happen when distilled water floats into the cell.

The answer here is B. The outside of the cell membrane is hydrophilic, the inner part with the phospholipid tail is hydrophobic. So the hydrophilic part isn’t wrong that it touches the outside water. But it just has nothing to do with this because the cell membrane is always hydrophilic on the outside. And cells aren’t always lysing.

[17:30] Question 56

One effect that Vinblastine has been shown to have is to induce endocytotic activity in red blood cells. With this in mind, what would be the likely effect if Vinblastine levels were increased in Experiment 2 were to be re-run with a higher Vinblastine concentration in the mixture of surrounding cell culture C.

  • (A) Fluorescence of the cell cytoplasms would occur more rapidly due to increased uptake of fluorescent bonded molecule complexes
  • (B) Fluorescence of the cell cytoplasms would not occur due to increased endocytosis causing more rapid evacuation of fluoresced material
  • (C) No effect as the endocytotic activity is not specific to transfer and/or Concanavalin A but rather has equal effects on various types of surrounding molecules
  • (D) No effect as the increased fluorescence is being caused by the defect in active transport membrane proteins, which prevents them from evacuating fluoresced materials

Clara’s insights:

The correct answer here is A. Endocytotic activity is induced here and endocytosis means bringing cargo ring material into the cell. The passage says that cell culture C was placed in a mixture containing all of these basically fluorescent complexes. So with endocytosis, the cell is floating around with all these fluorescent complexes outside of it. Now, it’s endocytosing them and bringing more fluorescence into the cell.

[22:22] The Triaging Strategy Doesn’t Work!

This was used by some of the other test prep companies for the old MCAT. This means that when you get to a new section, the first thing you should do is click through really quickly and try to see which passages kook hard and which look easy. Then do the easy ones first and then maybe there are some hard ones. And if you don’t have time, you don’t get to those. This was a horrible failure of a method because people may think a question looks hard but turns out the question is really easy. Then they would miss out. So this doesn’t work anymore because the MCAT now shows one question at a time so it just takes forever to click through.

So don’t assume a question is too hard. Secondly, even if you just focus on getting a mental summary of the three experiments. it can make you feel like this is more manageable because you realize it’s really just about three different experiments. You don’t get distracted by all the dense, Concanavaline, random words.

Lastly, you can just flag an answer or mark it and then move on. So if you have time in the end, you can go back to it. But if not, then you weren’t supposed to spend your time on it anyway.

[24:10] Next Step Test Prep

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Next Step Test Prep (Use the promo code MCATPOD to save $50 off their course.)

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