Section Bank: Biological and Biochemical Foundations of Living Systems: Passage 1

1) First thing we want to note is the test-maker says “based on the information in the passage.” That doesn’t always mean we have to flip back to the passage. You want to read the passage properly and thoroughly for a reason, and that’s to make sure you fully grasp the big picture. For this specific question, we can flip back to the passage and see what the author mentions about the cross-bridge cycle and possible suppression.

We have part of our passage here where the author talks about the CBC. When you go back to the passage like this, I don’t want you re-reading the entire passage or even entire paragraphs. I’m going to walk you through the important parts here, just to be thorough. When you’re actually practicing or taking the exam, going back to the passage (unless it’s for specific details you don’t want to memorize) can be a waste of time. The author says, “calmodulin….activates an enzyme that phosphorylates amino acid residue 19 of the myosin light chain (LC20). Phosphorylation of LC20 is required to activate the myosin head, which binds to actin. This myosin–actin interaction forms the basis of the cross-bridge cycle (CBC).” There’s phosphorylation of LC20. Phosphorylation is done by a protein kinase; we’re adding a phosphate group to a molecule. How would we suppress CBC? By inhibiting, preventing, or reversing this phosphorylation. Answering this question boils down to knowing our enzyme types. A kinase phosphorylates a molecule, while a phosphatase dephosphorylates a molecule. That means our ideal answer is going to be the opposite of a kinase, or a phosphatase.

1. Phosphorylase. Phosphorylase enzymes catalyze the addition of a phosphate group from an inorganic phosphate. We want an answer that focuses instead on dephosphorylation. This answer choice contradicts our breakdown, and what the author told us in the passage.
2. Kinase. This is the opposite of our breakdown of the question, and would actually increase phosphorylation of LC20. Increasing phosphorylation would increase CBC, not suppress CBC. Again, another contradiction. The author mentions phosphorylation actually activates the myosin head, which binds to actin. That interaction forms the basis of the CBC, not suppresses CBC. Both answer choices A and B are, for our purposes, equally incorrect.
3. Phosphatase. This answer choice matches our breakdown. We said a phosphatase dephosphorylates a molecule. That dephosphorylation wouldn’t allow for the activation of the myosin head, or the binding to actin. The enzyme that most likely would suppress CBC is a phosphatase, that matches our prediction. We can eliminate answer choices A and B because both directly contradicted the passage and our question breakdown.
4. Synthase. Synthase enzymes are a type of lyase: There’s a breakdown of a molecule to form two different molecules. This could theoretically prevent activation of the myosin head, but this doesn’t directly address the reasoning given in the passage. Our passage specifically mentions phosphorylation of LC20 is required to activate the myosin head, which binds to actin. By finding an enzyme that focuses on dephosphorylation specifically, we have our best answer. We can eliminate answer choice D, and keep our superior answer, answer choice C. The type of enzyme most likely to suppress CBC is phosphatase. 

2) No tricky wording here, we’re simply relating the results in Table 1 with VSM function and cytoskeletal dynamics. 

We have three different artificially adjusted internal pressures: 10, 80, and 120 millimeters of mercury. We have dependent variables listed on the left side: arterial diameter, percent phosphorylated LC20 and calponin, and G-actin amounts.

Arterial diameter decreases with increased pressure. That corresponds to a decrease in resistance, meaning flow stays relatively consistent.

Percent phosphorylated LC20 and calponin. We see a significant increase in both variables as we go from internal pressure of 10 to 80 millimeters of mercury. When we get to 120, there’s a small increase in LC20 percentage, but calponin percentage stays the same. 

G-actin amount. We have a general decrease from lower internal pressure to higher internal pressure. But there’s reversible polymerization of G-Actin into F-actin filaments. Meaning, that decrease in G-actin amount likely means we have an increase in F-actin amount.

1. Vasoconstriction is associated with an increase in the ratio of F-actin to G-actin. Vasoconstriction is a decrease in arterial diameter, or going from a lower internal pressure to higher internal pressure. That also corresponds to a decrease in G-actin amount, but we said increase in F-actin amount. That means the ratio of F-actin to G-actin is higher when we have vasoconstriction. This answer choice sounds good for now, but we always want to be thorough and look at the other answer choices.
2. F-actin levels decrease as phosphorylation of LC20 increases. Again, we expect F-actin amounts to increase going from left to right in our table, even though it doesn’t explicitly say so. We have an increase of phosphorylation of LC20 from left to right, we also have an increase in F-actin levels from left to right. This answer choice contradicts Table 1 from the passage, so we can eliminate answer choice B.
3. Vasodilation does not affect phosphorylation levels of calponin. Vasodilation happens from right to left in the table, as we decrease internal pressure. As there’s vasodilation, we actually see a decrease in the phosphorylation levels of calponin. We can’t establish the phosphorylation levels are unaffected. We can eliminate answer choice C as well because it contradicts Table 1. 
4. Arterial diameter reduction is always dependent upon increased calponin phosphorylation. Arterial diameter varies with changes in internal pressure. But not always with calponin phosphorylation. Look at internal pressure of 80 millimeters of mercury to 120 millimeters of mercury. We have no change in calponin phosphorylation, but we do have arterial diameter reduction. We can say arterial diameter reduction isn’t always dependent upon increased calponin phosphorylation. This answer choice is too extreme, so we can eliminate answer choice D. We’re left with our correct answer, answer choice A: Vasoconstriction is associated with an increase in the ratio of F-actin to G-actin.

3) This is another answer that is going to come from information in the passage. We’ll note what the author says about latrunculin B and how those effects will affect data in Table 1.

We have a part of our 3^rd paragraph from the passage here. The author says Actin depolymerization can be induced by the drug latrunculin B. Depolymerization would be the breakdown of polymerized actin into monomers of globular actin. Essentially more G-actin, and less F-actin. Not much else we need to dissect there, let’s look at Table 1 below.

Question stem asks specifically about 120 millimeters of mercury. What do we expect to happen at that internal pressure specifically? We’d expect the direct results to be a G-actin amount that is greater. We have depolymerization, like we just talked about, which means polymerized actin is broken down into monomers of globular actin, or G-actin. So, our answer is going to be something consistent with increased G-actin levels.

1. increased phosphorylated LC20 levels. This answer choice is not something we talked about during the breakdown. Phosphorylated LC20 levels depend more on calcium ions and calmodulin, at least for the sake of this passage. Also, as we see increased G-actin levels, we eventually have lower phosphorylated LC20 levels. Not exactly an inverse relationship, but something to consider.
2. decreased arterial diameter. This answer choice contradicts our breakdown. From a general trend perspective, we expect an increased G-actin amount to correspond to increased arterial diameter, not decreased. We can eliminate answer choice B for being a direct contradiction. Answer choice A presented a relationship that we didn’t explicitly identify, but we can still hold on to answer choice A because it didn’t directly contradict our prediction like option B.  
3. decreased F-actin levels. This answer choice is consistent with our breakdown. Our prediction was focused on G-actin amounts, not F-actin levels, but what did we say during the breakdown? Depolymerization would be the breakdown of polymerized actin into monomers of globular actin. That means more G-actin, and decreased F-actin levels. We can now eliminate answer choice A because answer choice C is a superior and more direct answer choice.
4. increased phosphorylated calponin levels. This answer choice is similar to answer choice A. When we have increased G-actin levels, we have decreased phosphorylated calponin levels. That’s the opposite of this answer choice. We can eliminate answer choice D. We’re left with our correct answer, answer choice C: decreased F-actin levels.

4) Essentially, we’re relating the myogenic response with sympathetic stimulation. We’ll do an overview of stimulation of the sympathetic nervous system, and relate to myogenic response. Meaning we’ll focus on blood flow, and varying vessel diameter.

The sympathetic nervous system controls the body’s automatic response to danger. Normally that entails increased heart rate, slowing digestion, and something we’re interested in today: moving blood flow to the heart, muscles, and also the brain. In the muscles we have vasodilation so we can maximize the blood making it to the muscles in our fight or flight response. 

But, if we have increased heart rate and blood is moving toward the brain, we have to be careful. That’s where the myogenic response comes in. We don’t want blood flow to be too high, so we compensate. We can increase resistance to account for the higher pressure. That ensures the flow remains in a safe, steady range-and doesn’t get too high.

1. equalizes blood pressure in cerebral resistance arteries and the aorta. Besides the heart, we expect the aorta to have the highest blood pressure of anywhere in the body. Cerebral resistance arteries likely can’t handle that same blood pressure, so equalizing blood pressure is unreasonable. 
2. moderates blood flow to the brain under high pressure. This answer choice sounds like our prediction and what we’ve covered in the passage. The brain can’t have the same increased blood flow coming in as the muscles and other organs. That’s why the myogenic response works to increase resistance of blood flowing to the brain, despite the higher blood pressure. We can hold on to this answer choice and eliminate answer choice A for being unreasonable.
3. enables cerebral resistance arteries to locally vasodilate. This answer choice is the opposite of our breakdown. We said we expect the arteries to vasoconstrict to add resistance, not vasodilate. If these arteries vasodilated, we’d put the brain at risk because of this sudden, increased blood flow.
4. redirects blood circulating in the brain to organs in the abdominal cavity. This answer choice also contradicts our breakdown. Stimulation of the sympathetic nervous system means slowing digestion and decreasing the amount of blood directed toward organs in the abdominal cavity. We can also eliminate answer choice D, so we’re left with our correct answer, answer choice B: The myogenic response moderates blood flow to the brain under high pressure.

5) To attack this question, we’ll focus on G-actin and F-actin, and actin polymerization. The only mention the author made in the passage about polymerization is actin polymerization: monomers of G-actin polymerize into F-actin. What structural component are we dealing with? F-actin can actually be described as microfilaments, or actin filaments. Essentially this is a standalone, content question that’s asking us to identify microfilaments are made of actin proteins.

1. Microtubules. This answer choice seems like a tool the author uses to trick us. Microtubules sound similar to our breakdown, and have some functions similar to microfilaments. But Microtubules are hollow tubes composed of tubulin proteins, not actin. They are longer and wider than microfilaments. They help the cell transport materials within itself and resist shape changes. Not quite what we’re looking for, but we can keep comparing with the remaining answer choices.
2. Microfilaments. We mentioned microfilaments are thin protein fibers made up of actin proteins; their fundamental role is to absorb tension. They’re the thinnest part of the cytoskeleton. This is what the author was talking about in the passage, so we’re going to keep this answer choice for now. We can eliminate answer choice A because it was a distractor. Answer choice A was only presented because it sounds like microfilaments.
3. Intermediate filaments. Intermediate filaments have multiple different proteins, and not predominantly actin like our microfilaments. Keratin is an example of a protein that makes up intermediate filaments. These intermediate filaments contribute to cellular structural elements. Answer choice B is still our superior, and most direct answer choice, so we can eliminate answer choice C.
4. Thick filaments. Thick filaments deal with muscle contraction, and are associated with myosin, not actin. The two work together in contraction, but we’re looking for an answer choice that focuses on actin specifically. We can eliminate this answer choice as well, so the correct answer, and the most direct answer, is answer choice B: Microfilaments.

6) Let’s flip back and see what the author mentions about the amino acid. We’ll use our general knowledge to identify the amino acid, and match it to its structure.

The passage says Calcium ions bind the cytosolic protein calmodulin, which activates an enzyme that phosphorylates amino acid residue 19 of the myosin light chain (LC20). Key thing we want to point out is we have phosphorylation. Phosphorylation is a form of protein modification and regulation. Which amino acids are phosphorylated? For the sake of the MCAT: serine and threonine. Sometimes we include tyrosine. We’re thinking of amino acids with hydroxyl groups in their side chains, so our answer will likely be serine or threonine. This comes down to knowing your amino acid structures, if you’re not familiar with these structures, you may want to spend some time reviewing. 

Answer choice A shows (Glycine)

Answer choice B shows (Valine)

Answer choice C shows (Isoleucine)

Answer choice D shows (Serine)

Only one answer choice matched either serine or threonine, and it was the only one answer choice had that hydroxyl group in its side chain. We can eliminate answer choices A-C and we’re left with our correct answer, answer choice D: Serine.

 

Section Bank: Biological and Biochemical Foundations of Living Systems: Passage 2

7) To answer this question, we can revisit Figure 1 in the passage and see what body weight trends we should be expecting. Germ free (GF) condition mean the gut doesn’t become colonized, and no normal production of short chain fatty acids. In other words, no typical activation of GPCRs, and specifically GPCR43.

We have Figure 1 here, and we want to break down the results of the experiment. We can make a broad conclusion from the passage that regardless of which diet we look at in the experiment, the GPCR43 deficient mouse is heavier than its wildtype counterpart. Said differently, the wildtype mouse weighs less than its GPCR43 deficient counterpart that ate the same diet. Meaning we’re expecting the wildtype mice to be lighter, given all other conditions are the same.

One more wrinkle we have to introduce. The question stem mentioned we’re also looking at a germ-free condition where the gut doesn’t become colonized. That means no activation of GPCR43-so even the wildtype genotype that’s in the germ-free condition, should yield the same results as the GPCR43 deficient mice. We’re expecting the wildtype genotype in the conventional condition to be the lowest body weight.

1. Answer choice A looks off right away. We have a higher body weight in the wildtype. We said the wildtype mouse weighs less than its GPCR43 deficient counterpart that ate the same diet. Let’s keep comparing to the remaining options.
2. Answer choice B is consistent with our breakdown. Wildtype has a lower body weight, and we mentioned something about this experiment specifically. The question stem mentioned we’re also looking at a germ-free condition where the gut doesn’t become colonized. That means no activation of GPCR43, and even the wildtype in the germ-free condition will have a higher body weight. We can keep this answer choice and eliminate answer choice A because it contradicted Figure 1 and our breakdown. 
3. Answer choice C shows the wildtype genotype corresponding to a lower body weight. But we’re going to focus on the germ-free condition specifically. We said germ-free means no activation of GPCR43 in the wildtype. That body weight shouldn’t be that low. We eliminate answer choice C as well. It contradicts the results in the passage, and what we were told in the question stem.
4. Answer choice D is similar to answer choice A. Wildtype typically has a lower body weight. This is showing us the opposite. This answer choice also contradicts our breakdown of the question and passage, so we can eliminate answer choice D. We’re left with our correct answer, answer choice B.

8) To answer this question, we’re going to cover the conditions in the transmembrane domain, and review the properties of amino acid side chains. We want an amino acid that LEAST likely is found in the transmembrane domains; always be careful with the verbiage.

Transmembrane domains are the regions of a protein that are hydrophobic. This becomes just a content question. The amino acids least likely to be found in one of the transmembrane domains is any hydrophilic amino acid. If we can’t narrow it down to a single answer choice, we can try and find 3 that are hydrophobic, and the remaining answer choice is the odd-one out. We can eliminate answer choices for being hydrophobic. 

1. Aspartic acid. Aspartic acid is a very hydrophilic amino acid. We want an amino acid that’s least likely found in the transmembrane domains, which are hydrophobic. This answer choice matches what we’re looking for already. Let’s quickly go through the other options.
2. glycine. Glycine is fairly neutral, so we’re still sticking with our hydrophilic, best answer: answer choice A. We can eliminate answer choice B for being inferior to answer choice A.
3. tryptophan. Tryptophan is slightly hydrophobic, so it would fit in nicely in the transmembrane domains. This contradicts our prediction, and doesn’t correctly answer the question. We can eliminate answer choice C. 
4. Phenylalanine. Phenylalanine is also slightly hydrophobic, so we have another answer that doesn’t correctly answer our question. We can eliminate answer choice D and stick with the only amino acid listed that’s hydrophilic, answer choice A: Aspartic acid.

9) Let’s recall some broad details from the passage. The leanest mice were the wild type mice that expressed GPCR43. Heaviest mice were the GPCR43 deficient mice. In general, the mice that were fed the high fat diet all got heavier. All of the mice here are being fed a high fat diet, so we’re predicting anything close to wildtype would be the leanest.

1. Mice treated with antibiotics. This implies we’re killing the microorganisms in the gut. In the passage we found that catabolism of dietary fiber by gut microbiota produces short chain fatty acids. That eventually leads to activation of GPCRs. If we don’t have activation of GPCR43, we expect the mice to be heavier. 
2. Mice treated with a GPCR43 antagonist. An antagonist would oppose the effects of GPCR43. That means we have a similar answer to answer choice A. Both correspond to the opposite of our wildtype, and correspond to heavier mice. 
3. Mice in which GPCR43 is overexpressed in WAT. Overexpression of GPCR43 would be exactly what we’re looking for. Overexpression of GPCR43 would account for the high fat diet. GPCR43 deficient mice are heavier according to the passage. Wildtype mouse weighs less than its GPCR43 deficient counterpart that ate the same diet. Meaning presence of GPCR43 equals less body weight. We can eliminate answer choices A and B because both contradict our breakdown and the answer we’re looking for.
4. Mice treated with a drug that inhibits the generation of intracellular second messengers. Without the generation of intracellular second messengers, we don’t see the effects of GPCR43. That’s similar to answer choice B, so we can also eliminate answer choice D. We’re left with our correct answer, answer choice C: Mice in which GPCR43 is overexpressed in WAT

10) To answer this question, let’s pull up Figure 2 and review what we saw in the passage, then we’ll go through our answer choices.

We have Figure 2 here and we’re going to rely on what you saw in your readthrough of the passage. In the presence of insulin, acetate decreases glucose uptake in wildtype mice, but we don’t see that same suppression in the GPCR43 deficient mice. In the absence of insulin, we don’t see any differences in the groups. The big difference is on the right side where we have the presence of both insulin and acetate. 

1. Acetate suppresses insulin-mediated glucose uptake in WT adipocytes, but not Gpcr43–/– adipocytes. This answer choice is consistent with our breakdown. We’re focused on insulin-mediated glucose uptake, which is on the right side of our figure. Note the suppressed uptake in the wildtype adipocytes compared to the GPCR43 deficient adipocytes in the presence of acetate. We can hold on to this answer choice for now and see if any other answer choices are superior.
2. Acetate stimulates insulin-mediated glucose uptake independently of GPCR43 expression in adipocytes. This is the opposite of our breakdown. Acetate actually suppresses insulin-mediated glucose uptake. We can see that clearly on the right side of Figure 2. Glucose uptake is less in the wildtype adipocytes vs. the GPCR43 deficient adipocytes in the presence of acetate. We can eliminate answer choice B for contradicting what we see in Figure 2.
3. Insulin suppresses glucose uptake in the presence of acetate in WT adipocytes, but not Gpcr43–/– adipocytes. Insulin isn’t suppressing glucose uptake. Note the 4 bars on the left side of the figure and the 4 on the right side. Difference there is insulin. Insulin increases glucose uptake. We can eliminate answer choice C also because it contradicts the figure.
4. Insulin stimulates glucose uptake in WT adipocytes, but not Gpcr43–/– adipocytes. We see the same amount of glucose uptake in 3 of the 4 adipocytes on the right side of figure 2. The only outlier is the wildtype in the presence of both insulin and acetate. This answer choice also contradicts our figure so we can eliminate answer choice D. We’re left with our correct answer, answer choice A.

11) To answer this question, we’ll have to remember the results in Figure 2, and where GPCR43 is expressed in the body. First thing we want to establish is GPCR43 is expressed in white adipose tissue, and not in muscle. Next thing we want to remember are the results from Figure 2: in the presence of insulin, acetate decreases glucose uptake in wildtype mice, but we don’t see that same suppression in the GPCR43 deficient mice. Also note, the muscle cells are going to be GPCR43 deficient-we only have GPCR43 expressed in white adipose tissue. That means our ideal answer: In white adipose tissue, we’ll have less Akt phosphorylation in the presence of acetate.

We can compare all of our answer choices at once. We expect Akt phosphorylation levels to be equal in muscle. That means we can eliminate answer choices C and D right away. 

We also expect less Akt phosphorylation in the presence of acetate. We can eliminate answer choice A. We’re left with the only answer choice that’s consistent with our breakdown, answer choice B.

12). To answer this question, we’ll note what antibiotics would do to these mice, and what that does to our experimental results. Antibiotics mean no gut microorganisms. That also means no activation of GPCRs. The author explained that relationship earlier in the passage. So essentially, we’re going to see the same results in the wildtype mice as we’re used to seeing in the GPCR43 deficient mice. That typically means increased body weight, and no effects from the presence of acetate. 

1. increased plasma butyrate levels. This is the opposite of what we said in the breakdown of the question. We said antibiotics means no effects of gut microorganisms. That means no catabolism of dietary fiber, or production of butyrate and acetate. This contradicts our breakdown and the passage. 
2. decreased body mass. Another answer choice that contradicts our breakdown. We said we’re seeing the same results as we’re used to seeing in the GPCR43 deficient mice, which is an increase in body weight, not a decrease. Neither answer sticks out at this point, so we’ll keep answer choices A and B for now.
3. Answer choice C says increased volume of adipocytes. This answer choice is consistent with our breakdown. Antibiotics means no activation of GPCRs, and ultimately, we said that leads to increased body weight. Increased body weight is the result of the increased volume of adipocytes. We can eliminate answer choices A and B now-both contradicted our breakdown and the passage. 
4. decreased insulin sensitivity in adipocytes. We just mentioned in our breakdown of answer choice A: antibiotics means no effects of gut microorganisms. That means no catabolism of dietary fiber, or production of butyrate and acetate. No acetate means insulin sensitivity is normal, and not decreased. We saw that in Figure 2 in the passage. We can eliminate answer choice D and we’re left with our correct answer, answer choice C: increased volume of adipocytes. 

 

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