29: MCAT Biology Passage Deep Dive

Session 29 This week, we're going to do a deep dive into a full biology MCAT passage as we show you how you can prevent yourself from falling into the trap of some “tricky” questions and ultimately beat the MCAT. [01:34] Beta Oxidation Passage This passage shows the description of the process of beta oxidation, how the body in the mitochondrion metabolizes fat for energy. It also shows a couple of different molecular structures of carnitine and palmitic acid as well as the first three steps of beta oxidation. With the addition of biochemistry to the MCAT, the test raters are going to expect you to be very comfortable with all these different metabolic pathways, not quite to the level that you would learn them in medical school but it still has to be the kind of language you're comfortable reading about. [02:25] Answering "Least" Questions Question #41: Injection of insulin into the bloodstream is least likely to result in which of the following: (A) Increased glycogen synthesis (B) Decreased lipid synthesis (C) Increased deesterification of fatty acids (D) Decreased gluconeogenesis (We know that insulin is a very powerful metabolic hormone so this is not directly coming from the passage itself but just related to the topic of the passage.) [03:19] Bryan's Insights: The question sounds like it's going to require a whole lot of hyper-detailed knowledge of the various metabolic results of insulin. When organizing all the various complicated biochemistry pathways, start with the broadest, most general understanding. Insulin is arguably the most powerful anabolic hormone in the body that involves building up and storing big molecules. Knowing that it's big job is to store energy to build up big molecules, you can usually hack your way through this question and find your way to the right answer. Choice (A) increased glycogen synthesis sounds like something insulin would do since it means more storage. However, a common mistake among students is they'd just pick this answer and move on, forgetting the word "least" in the question. Therefore, choice (A) is already crossed off. Choice (B) decreased lipid synthesis is the right answer since insulin's job is to help us store energy. Since this is a "least" question, answer choice (B) is then the right answer. [05:08] Tips to Prevent Mistakes in  Answering "Least" Questions First, AAMC has written this where they've put all in caps, or italics, words like least, not, except, etc. Second, the MCAT comes with a highlighting function where you can mouse over and highlight words in the passage or in the question. So if you see words like challenge, weaken, not, accept, or least, highlight it in the question itself. This visual reminder will help prevent any mistakes. [06:07] Answering Roman Numeral Questions Question #42: Which steps in Figure 3 are oxidations? (Steps are presented in Roman Numerals. Figure 3 starts with a big long fatty acid tail attached to Coenzyme A.) Step 1 shows a single bond becoming a double bond. Step 2 shows the double bond going away and an OH (hydroxyl group) has now been tagged on where that double bond was. III. Step 3 shows the OH group becomes a carbonyl carbon (C=O). [07:11] Bryan's Insights: There are different ways to answer Roman Numeral questions but do the easiest one first. In this case, going from a hydroxyl group (OH) to a carbonyl carbon (C=O) is fairly obviously an oxidation. It's a classic definition of oxidation set up when there are more bonds. So Step 3 is an oxidation. Let's just eliminate answer choice (A) and if this is far as you can get and you're not really sure about the other steps,t hat's fine. Take whatever progress you can get and make your best guess and move on. Step 1 of the process going from a single bond to a double bond is another example of an oxidation step. So when we reduce or saturate a molecule, we reduce all the double bonds down to single bonds. And if you go the other way, going from a single bond to a double bond would be an oxidation step. So (I) is also an oxidation step. Hence, Steps 1 and 3 are oxidations. This then leads us to eliminate another answer choice, (B), aside from (A) which we've already eliminated. For the two remaining answer choices (C) and (D), we just have to decide whether Step 2 is an oxidation step. And this is a tricky question because a majority of students get this question wrong and think that Step 2 is an oxidation step so they pick the incorrect answer (D) whereas the right answer is (C)  I and III. So you have to look back at Step 2 and figure out why it's not an oxidation. It's tricky in that the molecule starts from a double bond to adding a hydroxyl group. Students simply see the oxygen and they immediately conclude it's oxidation, without recognizing that since the double bond went away, that means one of the carbons got the OH group therefore the other carbon had to have gotten a hydrogen. The double bond did not just disappear so you had to put a hydrogen on there. Hence, this is not an oxidation step but a hydration step. They actually added water across a double bond.sure one carbon got an oxygen but the other carbon got a hydrogen. This is tricky because you really have to think about what happened to the other carbon in that double bond. But ultimately, it's not a trick question because the figure is right there on your screen or handout. [10:50] Final Thoughts You just want to walk into the test being super comfortable with beta oxidation as a metabolic pathway so none of these steps will be something new. It's good to really know your metabolic pathways so you won't be worried about interpreting the figure correctly because it would be very comfortable for you already and you can instead just focus on the exact question asked. Links: Next Step Test Prep (Use the promo code MCATPOD to save some money on their practice exams, tutoring, and their new course.) MedEdMedia Network Specialty Stories Podcast

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