Today, we go over passage 4 in the Chem/Phys section from Blueprint’s diagnostic. Tune in to hear the expert question breakdown. We’re joined by Pooja from Blueprint MCAT. 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.
[01:52] Tips for Moving On to the Next Passage
Pooja shares this little ritual that she does every three passages where she looks away from the screen. Because staring at the screen for 90 minutes straight makes her eyes tear up.
Then she takes three deep breaths. Every time she breathes in and out, she clenches her fists, closes her eyes, and doesn’t think about anything except for contracting. Then as she exhales, she tries to release everything so she can move on to the next passage feeling relaxed.“If a passage seems hard, there's a good chance that either the skill will work in your favor, or the questions aren't going to be asking a ton about it.” Click To Tweet
Pooja says she noticed that a lot of times in the MCAT, they pair easier passages with harder, more intensive content-based questions. And they pair harder passages with more basic questions that don’t rely a ton on understanding or at least have answer choices that are conducive to elimination.
[04:50] Passage 4 (Questions 19 – 22)
Wnt proteins are secreted signal molecules that act as local mediators to control many aspects of development. Wnts are unusual as secreted proteins in that they have a fatty acid chain covalently attached to their N-terminus, which increases their binding to cell surfaces. Wnts can activate the intracellular signaling pathway, the Wnt/β-catenin pathway. The Wnt/β-catenin pathway acts by regulating the proteolysis of the protein β-catenin, which functions in gene regulation. An experiment was performed to test the effect of β-catenin phosphorylation on the rate of degradation.
You don’t need to know about Wnt proteins. One of the biggest things to remember is to stay rooted in what is MCAT-relevant. And if there are unfamiliar terms that keep getting introduced, just highlight them. And know that you can go back to that paragraph if it shows up right in a question.“Stay rooted in what is MCAT-relevant.” Click To Tweet
Specific things to highlight here are Wnt proteins and secreted signal molecules. We’re told about something that makes it unique. And the reason why it’s unique is that it’s MCAT-relevant. Also, highlight “fatty acid chain” and the consequence of it being a fatty acid chain, which is that it “increases their binding to cell surfaces.”
The rest of this paragraph gets into the bio/biochem sphere of things. You can probably highlight the name of the pathway so you can go back to it if you need it. But don’t spend a lot of time on it.
Towards the end of this paragraph, we see that “an experiment was performed.” This suggests that the rest of the paragraph is probably going to be talking about the experiment. This then means we need to shift the way we’re thinking about this passage now.
You’re no longer just focusing on the content, but also on the bare basics of what an experiment involves. Then here, you can probably highlight the effect of β-catenin phosphorylation and the rate of degradation.
[09:11] Paragraph 2
Mouse cell cultures were treated with radioactive P-labeled phosphate (32PO43-) ions in order to determine if the degraded β-catenin was phosphorylated or not. At various intervals, 5 mL of the cell solution were collected and fractionated with the aid of centrifugation to separate cytoplasmic contents from cellular nuclei. The separated mixtures were then assayed for the presence of 32P. Assay results are summarized in Figure 1.
Figure 1 Results of radioactive phosphate assay (Note: Radioactivity is normalized to total protein content in the sample)
There’s a little bit more information that we get here. You don’t have to spend a ton of time on it. But just be able to recognize what’s being shown here, and in what types of questions.
Based on the caption and the preliminary scan, there’s some sort of contrast in cytoplasmic levels of P-labeled phosphate where you see a drop.
This paragraph is talking about the methods. And so, if there are any questions about the experiment, you know you can go back to paragraph two.
Things you could highlight here are radioactive P-labeled phosphate ad centrifugation to separate cytoplasmic contents from cellular nuclei.
[11:51] Paragraph 3
The degradation of cytoplasmic β-catenin depends on a large protein degradation complex, which binds β-catenin and keeps it out of the nucleus while promoting its degradation. Intact β-catenin accumulates and translocates to the nucleus. Once in the nucleus, β-catenin binds to its receptors, displaces the co-repressor Groucho, and acts as a coactivator to stimulate the transcription of Wnt target genes. In the absence of Wnt signaling, most of a cell’s β-catenin is located at cell-cell junctions, where it is associated with cadherins, which are transmembrane adhesion proteins. The β-catenin in these junctions helps link the cadherins to the actin cytoskeleton. Any β-catenin not associated with cadherins is rapidly degraded in the cytoplasm.
Figure 2 Mechanism of intact β-catenin
This paragraph is overwhelming. When you first start reading a paragraph, everything will seem really important. And that’ll be the classic scenario where students will find themselves highlighting the entire text.
And so, recognize there are specific chunks of information provided in this paragraph, which could help you break it up and figure out what to highlight accordingly.
The first piece of information that’s present is the idea of degradation complexes and it’s talking about what’s going to influence degradation.
And so, it could be helpful to highlight “degradation complex” and “keeps it out of the nucleus.” Then you could also highlight “Intact β-catenin accumulates and translocates” because that’s going to capture that second piece of that information. It tells you that when it’s inside the nucleus, there isn’t as much degradation going on.
The next piece of text to highlight is the name Groucho so that if there’s a question that asks about it, you’ll know exactly where to look. You can also probably highlight “acts as a coactivator” to remind you that β-catenin is acting as a coactivator and Groucho is our co-repressor.
Finally, towards the end of that paragraph, we get another piece of information about degradation, which ties in with the beginning of that paragraph.
[17:42] Question 19
Which of the following is most likely to be the precursor of the group found linked to the N-terminus of a Wnt protein?
A – We have a bunch of NH2s linked to a carboxylic acid group. We can cross this out since it doesn’t seem very familiar to what’s present on the MCAT.
B – This has a bunch of carbon hydrogen chains and an alkyl chain bonded to an O bonded to another carbon. It tells you that it’s a specific functional group, which is an ether.
C – This is the structure of benzene. So we throw this one out.
D – It’s an alkyl chain bonded to a carboxylic acid group.
Fatty acids are just straight up long alkyl chains that terminate in a carbolic acid group. Therefore, D is the correct answer because it’s one of the simplest structures of a fatty acid that you can see. The reason why it’s a precursor, and not necessarily the actual group, is because you can modify it to add a bunch of other stuff to it.
But the backbone of the structure is going to be that alkyl chain with a carboxylic acid towards the end. And that’s going to leave us with D is our answer.
Correct Answer: D
[22:21] Question 20
32P has a very short half-life of 14 days and decays into sulfur as shown below.
This nuclear reaction is an example of:
B.beta plus decay.
C.beta minus decay.
Beta plus decay and positron emission are the same thing so B and D are out. Then using the principle of opposites, you can get to C as your answer.
Pooja says that the best way to best prepare for this kind of question is to memorize what happens during each one.
In answer choice A, we have the emission of what’s known as an alpha particle. An alpha particle is the same thing as a helium nucleus (two protons, two neutrons). Therefore, for alpha decay, our mass number would go down by four, and our atomic number would go down by two. And we can tell that because two protons and two neutrons, so four things in total are getting emitted.
The three types of decay you need to know are alpha decay, beta minus decay, and beta plus decay. Then there’s gamma emission, which is one of the easiest things to recognize, because there’s actually no change in atomic mass or atomic number.
Correct Answer: C
[26:59] Question 21
A scientist concludes that the phosphorylated β-catenin is the form that is degraded. Why do the experimental results support this conclusion?
A.The levels of nuclear 32P decrease faster and remain lower than 32P levels in the cytoplasm.
B.The levels of nuclear 32P increase over time while cytoplasmic 32P levels decrease.
C.Cytoplasmic and nuclear levels of 32P both decrease over time.
D.Cytoplasmic levels of 32P remain constant.
For answer choice B, we’re told that levels of nuclear 32 P increase over time, which isn’t true because it decreases and eventually gets to zero. Although the logic of B could make sense, because it doesn’t match what Figure 1 shows, then it has to be wrong.
Answer choice C is like a classic where they both decrease over time. But that doesn’t explain what the question is asking us to explain. So we can eliminate C for that reason.
A is going to be correct since intact β-catenin has the unphosphorylated one and the phosphorylated β-catenin is going to be what’s degraded.
Correct Answer: A
[35:52] Question 22
In vivo, PO43- molecules utilize proton-coupled transport mechanisms to enter the cell. Which statement explains why this is necessary?
A.PO43- is hydrophilic.
B.PO43- is hydrophobic.
C.The cell membrane is hydrophilic.
D.The secondary active transport mechanism conserves ATP.
This is another explanation type of question so you need an answer that logically explains why something is possible.
D – This is a true statement. But it doesn’t really explain why you need to have that transport in the first place. When you want something to get into the cell, there’s going to be transport facilitated by proteins, or there’s going to be simple diffusion.
The answer we need to choose here is we’re going to have to explain why inorganic phosphate can’t just travel through the cell.
C – The whole thing about the cell membrane, biologically, is that it’s a phospholipid bilayer. It’s hydrophilic on the outside, hydrophobic on the inside. And to say that the cell membrane is hydrophilic would be contradicting what we know about science. So it’s wrong.
B – If it was hydrophobic, it would be able to travel through the little phospholipid tails in the middle and go through.
Hydrophobic things are the types of compounds that can passively diffuse through a membrane. Hydrophilic are the ones that require some sort of transport protein.Therefore, A is the correct answer here.
Another thing to remember is that anything that’s charged is going to be hydrophilic more so than hydrophobic.
Correct Answer: A