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MCAT Content / AAMC MCAT Biology Question Pack 1 Solutions

AAMC QPack Bio1 [Web]

Biology Question Pack Volume 1: Passage 1

1) First thing I’m doing is deciding whether I need the information in the passage to answer this question, or whether this is a pseudo-discrete, or standalone question. Even though the passage talked about the sodium pump, I’m leaning toward using my external knowledge of the sodium pump. AAMC loves doing this where they’ll ask you a question that’s tangentially related to the passage, but in reality it’s just a standalone question you can answer using your general knowledge. From our content outline we know that the sodium potassium pump is the most common example of active transport on the MCAT. We have two potassium ions moved into the cell, and three sodium ions moved out of the cell. The pump creates gradients in nerve cells, allowing for electrical signals. The pump is also used in the kidneys to maintain ion balance in the body. It also helps maintain blood pressure, and plays a role in cardiac contractions. Which is to say the pump is obviously important throughout the body. We’re going to try and find an answer that matches one of these main functions.

    1. Veins: Veins return deoxygenated blood to the heart. I mentioned the sodium potassium pump is a player in maintaining blood pressure. I’m thinking this is a viable answer choice, but it’s not as direct as the answers we typically see from AAMC.
    2. Loop of Henle: The loop of Henle is a U-shaped tube that consists of a descending limb and ascending limb. It transfers fluid from the proximal to the distal tubule. The descending limb is highly permeable to water but completely impermeable to ions, causing a large amount of water to be reabsorbed. Sodium and potassium are reabsorbed through the walls of the ascending limb. The walls of the thick ascending limb are impermeable to water. And here, the sodium concentration gradient has to be maintained. The fact that the concentration of urine is regulated so closely here, the loop of Henle is going to be our best answer for the time being. Remember we’re looking for the structure where the sodium potassium pump is most active. That’s why I’m liking answer choice B here.
    3. Lungs: The sodium potassium pump can be necessary to regulate fluid in the lungs. Again, the pump is used to create and maintain that concentration gradient. This is more important when you see a lung malady, or excess fluid. I’m still sticking with answer choice B for “most active” and the best answer so far
    4. Bone Marrow: This is not typically a structure that we associate with active sodium potassium pumps. While we may have the pump present, we’re not expecting the same, active pump that we would in nerves, or in the kidneys. I’m still going to stick with our best answer here, answer choice B. We didn’t end up needing the passage much (or at all) to answer this.

2) In other words, we saw swelling in the passage. This comes down to knowing the reason behind the swelling of the cells when they were placed in distilled water. During your initial readthrough you always want to visualize any experimental setup, especially a visual description like this. If you can’t visualize the experiment, you’re not understanding it clearly.
The passage says Reconstituted erythrocytes are formed by bathing erythrocytes in distilled water under controlled conditions. These cells swell, forming pores that release cytoplasmic proteins and ions into the distilled water. What we’re focused on is the reasoning behind the swelling. We know the erythrocytes contain proteins and ions, meaning there is an initial concentration gradient. Osmosis is the diffusion of water across the membrane. The movement of the distilled water can change the cell’s volume, and that’s what’s happening here. After the cell swells, that’s when the pores form and release cytoplasmic proteins and ions. But that is the result of the cell swelling, not the cause. We want an answer choice consistent with the movement of the distilled water into the erythrocyte.

    1. gradient of ions causes water to enter the cells. Right away, I’m liking this answer choice. When I broke down the question, I said the distilled water will cross the membrane of the erythrocyte due to the concentration gradient. That causes the cell to swell, and eventually open pores which allow for the release of cytoplasmic proteins and ions. 
    2. contractile filaments of the cytosol open pores in the plasma membrane. This is the result of the swelling, not the cause of the swelling that we need to explain to answer this question. Answer choice A is a superior answer choice here.
    3. sodium pump transports sodium out of the erythrocytes more rapidly than normal. This is not the cause of the swelling. Sodium leaving the erythrocyte would have the opposite effect. By increasing the sodium concentration outside of the erythrocyte, less water would enter the cell. 
    4. erythrocyte’s DNA produces degradative enzymes. Degradative enzymes do what their name suggests and degrade molecules. Enough enzyme activity can degrade the membrane, but that’s not what we’re looking for here. Breaking down the membrane is not what causes the cell to swell. We stick with answer choice A. This comes from knowing the experimental setup and using that to answer the question.

3) In simple words, we want to explain whether the sodium pump causes action potentials along neurons. This is going to be similar to question 1 because it’s almost like a standalone, or discrete, question. What do we know about action potentials? Neural impulses occur when a stimulus depolarizes a cell membrane, prompting an action potential which sends an “all or nothing” signal. Transmission of a signal within a neuron is carried out by the opening and closing of voltage-gated ion channels. This, in turn, creates an action potential. Once the threshold potential is reached, the neuron depolarizes and opens the voltage-gated calcium channels. When depolarization is finished, the cell resets its membrane voltage back to resting potential. So how does our sodium-potassium pump fit in here? The sodium-potassium pump maintains the resting potential, once it’s established. It’s not the direct cause of the action potentials along neurons. We’re focused on causation here.

    1. No; action potentials result in an increased permeability of the plasma membrane to sodium. I said the sodium pump isn’t the direct cause of action potentials along neurons. Rather the influx of sodium from the opening of the voltage-gated ion channel cause the depolarization.
    2. No; the myelin sheaths of neurons prevent movement of ions across the plasma membranes of the neurons. Based on the initial breakdown of the question, I do think the answer is “no.” But between myelin sheaths we have the nodes of Ranvier that are not myelinated. There is sodium movement down these unmyelinated nodes. Also, the movement doesn’t cause action potentials.
    3. Yes; sodium is transported out of neurons during action potentials. I said the pump does not cause action potentials. Also, sodium is transported out of the cell by the sodium potassium pump, meaning I’m not a fan of answer choice C.  
    4. Yes; action potentials are accompanied by the hydrolysis of ATP. Again, I’m focused on causation. In the overview, I didn’t say the energy from the hydrolysis of ATP was responsible for the influx of sodium, or causing the action potential. That means we’re left with our best remaining answer choice, answer choice A. 

4) We could reference the equation in the passage, but first we’re going to answer this question using 1) our general knowledge and knowing the number of ATP molecules produced per glucose molecule in glycolysis, and 2) how many sodium ions are transported out of the cell by the sodium potassium pump per molecule of ATP used. The answer is as simple as a net production of 2 ATP, which also means 6 total molecules of sodium.
Alternatively, I can look at reaction 1 from the passage. First thing we want to do is recall the number of ATP molecules produced per glucose molecule in glycolysis. For every glucose molecule that undergoes glycolysis, there’s a net production of 2 ATP molecules and 2 NADH molecules. Look at the reaction, and recall that for a single molecule of ATP, three sodium ions are removed from the cell. If we had two ATP molecules instead of a single molecule, that number would be doubled. That means 6 molecules of Na+.

    1. 3 molecules is an incorrect answer. You would get this if you accidentally used a single molecule of ATP.
    2. 6 molecules match the number of molecules we came up with. 
    3. 9 molecules would be the answer if there were 3 ATP molecules instead of 2.
    4. 12 molecules would be the answer if there were 4 ATP molecules. This answer just came down to knowing the content, and specifically glycolysis.

 

Biology Question Pack Volume 1: Passage 2

5) We’re focused on the causation of gastric cancer here, and relating to the proliferating cells. The passage mentions there is a correlation between H. pylori and gastric cancer. But now we want to explain why increased proliferation of mucosal cells would lead to gastric cancer. The reason for this should theoretically be specific mutations, or presence of genes that lead to cancer. We’re expecting these mucosal cells to be responsible in some way. 

    1. genetic mutations occur in proliferating germ cells. This answer choice is insinuating genetic mutations in germ cells themselves is causing the gastric cancer. It sounds similar to what I said during my initial breakdown-we needed mutations in proliferating cells. That does sound like cancer. But we’re expecting mutations in the mucosal cells, not germ cells. We want to focus on the proliferating mucosal cells in the stomach, which means we’ll look for a better answer.
    2. genetic mutations occur in proliferating somatic cells. This answer choice is more reasonable than answer choice A and similar to what I said in the prediction. What are somatic cells? They’re the cells that aren’t reproductive cells in a living organism. In this case, if we have mutations in the rapidly reproducing mucosal cells in the stomach, those are somatic cells. That’s exactly what we expect, so this is better than answer choice A already. 
    3. the immune system fails to recognize bacterial antigens. The author mentions in the passage “antibodies rarely eradicate the infection; evidently, this pathogen has developed effective ways to elude host defenses.” We’re also told that infection by the antigens doesn’t always lead to gastric cancer. This isn’t consistent with the breakdown I did. Answer choice B is still superior.
    4. crowded mucosal cells are likely to remain in interphase. This description does not sound like gastric cancer. Cancer is associated with uncontrolled cell division. Answer choice D mentions cells that are not dividing, which isn’t what I’m looking for. That means I’m left with the best, and most correct answer, answer choice B.

6) This is something the author brings up in the passage, and now we’re asked to explain the statement. That means we’re going to have to understand how antibodies typically work, and why they might not work in a particular environment or against a specific antigen.
Antibodies are produced by the immune system to fight antigens. Antigens by definition will induce the production of antibodies. But why might an antibody not be effective? Antibodies are proteins that respond to specific antigens. Just like other proteins, antibodies will undergo conformational changes and not work properly under extreme conditions. The passage mentioned many individuals will develop antibodies. But keep in mind the author talks about the extreme pH in the stomach. Antibodies are typically not as effective in extreme pH or temperatures.

    1. Antibody proteins may be denatured in the harsh environment of the stomach. This is consistent with the breakdown I just went through. The harsh environment of the stomach includes a very low, acidic pH. Antibodies will experience conformational changes in these situations and not work properly. 
    2. Antibodies are not generally effective against bacteria. This goes against our breakdown and also contradicts what we know about antibodies and bacteria. Antibodies are meant to counteract antigens. Answer choice A is still superior.
    3. H. pylori infection may suppress the activity of the immune system. This answer choice goes against what we read in the passage. The author mentions the immune system is still able to develop antibodies, but the antibodies themselves are not effective. Answer choice A is still the best. 
    4. Antibodies are not secreted from host tissues into extracellular spaces. We know antibodies are secreted from tissues into extracellular spaces. This answer is factually incorrect, so answer choice A remains the best answer choice for this question.

7) The author mentions the differences between strains of H. Pylori. That’s going to be the key to answering this question. The passage says “Genetic studies of H. pylori have identified genes that are expressed in different strains of this bacterium. One gene, vacA, encodes a toxin. Expression of another gene, cagA, leads to inflammation and may be related to the genesis of gastric cancer”. So what does this tell us? Not all H. pylori strains may possess the same genes. In fact, we get two examples of individual genes that are found in specific strains. Both strains seem like they can affect humans, but affect them differently. 

    1. attack different hosts. This is inconsistent with what we just mentioned in our breakdown. Different strains can attack the same host. We see different effects in the same host from different strains. 
    2. express different genes. That sounds very similar to our breakdown and the examples the author presents in the passage. Different strains express different genes like vacA and cagA. Answer choice B is the best option so far.
    3. exhibit different degrees of resistance to antibiotics. There’s no evidence given in the passage about different strains having different degrees of resistance to antibiotics. We’ll still keep answer choice B as our superior answer choice.
    4. exist in either developed or developing countries. Just like answer choice C, the passage does not mention anything about different strains of H. pylori existing in either developed OR developing countries. The only thing the passage mentions is the prevalence rate of H. pylori in general. We’re left with the best answer: answer choice B.

8) This involves going back to the passage and noting the effect of seeing the cagA gene in our H. pylori strain. The passage says says “Expression of another gene, cagA, leads to inflammation and may be related to the genesis of gastric cancer”. So our answer is going to deal with this inflammation, and possible with the beginning of gastric cancer as well. When an inflammatory response is triggered, usually white blood cells will fight any infection or debris. So we’re expecting inflammation followed by a defensive response.

    1. the disruption of host cell enzymatic activity. We’re not expecting a disruption per se, rather we’re expecting an inflammatory response. It’s a fairly straightforward question and a specific answer, so I’m hoping for something a little better.
    2. the disruption of host cell protein synthesis. Again, we’re not expecting a disruption. We want an answer choice that explains what would happen as an inflammatory response. Again, not ideal so we’ll keep looking for a better answer.
    3. the movement of leukocytes into mucosal tissue. This is consistent with the passage and my breakdown. Expression of the gene leads to inflammation in the stomach cells. Leukocytes, or white blood cells, will head to the site of the injury as an inflammatory response. So we’re expecting the presence of the cagA gene to be followed by the movement of leukocytes into the mucosal tissue. Best answer so far, but I still want to look at answer choice D to make sure.
    4. the vasoconstriction of arterioles in the mucosal layer. When we have inflammation, blood vessels will dilate to increase the flow of blood and immune cells to the site. Answer choice D not a good option. I’m going to stick to answer choice C.

9) The author says in the passage “There is a relationship between H. pylori infection and cancer.” But then immediately after says “Infected individuals have a two-fold increased risk of gastric cancer, although >75% of patients with active infections do not develop cancer.” This essentially means less than a quarter of patients with active infections do develop cancer. Most patients have been infected, but they do not actually develop cancer.

    1. do not incorporate bacterial genes in their chromosomes. Make sure to read these answer choices slowly. Note the question says “most do not develop” and now option A says “do not incorporate.” That means the people that are infected by H. pylori won’t develop cancer because bacterial genes don’t incorporate in their chromosomes. The verbiage can get tricky. There’s no mention of this in the passage, but the answer doesn’t explicitly go against the passage either. Let’s see if we can find anything better. 
    2. have robust immune systems that defeat early cancers. Even if a patient has a robust immune system, the author mentions the pathogen has developed effective ways to elude host defenses. This contradicts what the author mentions in the passage, and what I’ve provided in my breakdown. 
    3. eradicate the infection before any tumors develop. This contradicts what we just read from the passage. The passage explicitly says many individuals develop antibodies against H. pylori antigens, these antibodies rarely eradicate the infection. The only mention of eradication in the passage is due to using antibiotics. 
    4. tolerate the infection without developing tumors. This is consistent with our prediction. The passage explicitly says more than 75% of patients with active infections don’t develop cancer. That means out of 100 people that are infected, less than a quarter, or fewer than 25 would actually develop gastric cancer. It also says that antibodies are rarely fully effective to eradicate the disease. Meaning the rest all tolerate the infection and may have adverse effects, but these adverse effects don’t include cancer. This is a superior answer to answer choice A. B and C we already ruled out. We can say most people don’t develop gastric cancer because they tolerate the infection without developing tumors.

 

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