MP 25 : Breaking Down Discrete MCAT Biology Questions

Session 25

In  this episode, Ryan and Bryan break down some fun, independent discrete biology questions on the MCAT.

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There are 59 questions in each of the three different science sections – 15 are discrete while 44 questions are based on passages.

Question #16: Which of the following will cause a blood pH 8.2?

  1. Decreased O2 concentration
  2. Decreased tidal volume
  3. Increased H2O concentration
  4. Increased respiratory rate

Strategies for answering:

Before looking at the answer choices, you should know that normal physiological blood pH is more like 7.35 – 7.4. So if the person has a blood pH of 8.2, that is wildly alkaline. So you're looking for an answer choice that is a higher pH, less acid or more base.

Let's break down down the answer choices:

(a) Decreased O2 concentration – This might be tempting knowing that O2 and CO2 have an inverse relationship in the blood.

(b) Decreased tidal volume – Tidal volume is the natural resting breathing rate without forcibly trying to pull in or exhale more air. So if you breathe less, there is decreased tidal volume. This means that the CO2 building up in your body is not getting exhaled as well as it could since you're holding CO2 back in your body.

For the purposes of blood pH, CO2 is acid. If you decreased the tidal volume, that would actually give you respiratory acidosis that would make your blood pH more acidic. So this is the opposite of what we're looking for.

(c) Increased H2O concentration would not affect the pH because it would dilute the acid and the base at the same time. Adding more water won't  directly change the pH balance itself.

(d) Increased respiratory rate – You start hyperventilating and blowing out a lot of CO2. By literally breathing acid out of your body, what's left behind in your blood is more basic so you're blood will become more basic and your blood pH could be higher. So this answer choice could tie more into the more basic blood pH.

From the perspective of blood pH, CO2 is acid so you're exhaling acid. So if you stop exhaling, you hold your acid in while if you blow out a whole bunch, you blow the acid out and leaving base behind.

Question #17: Patients with excess fat are more likely to require larger therapeutic doses of which vitamin:

  1. Vitamin B1
  2. Vitamin C
  3. Vitamin D
  4. Vitamin B3

Strategies for answering:

This questions is a classic example of “which one of these doesn't belong?” It almost doesn't matter what question is asked.

Looking at all the choices, one thing jumps out right away. You have to know your water-soluble and your fat-soluble vitamins.

Vitamins B and C are water-soluble and fat-soluble vitamins are Vitamins A, D, E and K.

Hence, answer (c) Vitamin D is the only one that stands out.

Question #29: Which of the following is most likely to use a protein channel to cross the eukaryotic cell membrane.

  1. Aldosterone
  2. Ca2+ ion
  3. Oxygen (O2)
  4. Carbon dioxide

Strategies for answering:

This is just like the question about vitamin solubility mentioned before. Aldosterone is a steroid hormone that can just diffuse right across the cell membrane and doesn't need a protein channel to get through a cell membrane.

Gases (oxygen and carbon dioxide) can also just diffuse right across a cell membrane. They don't need a protein channel.

Ions need a protein channel. Hence, answer (c) Calcium needs a protein channel to cross the membrane.

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Transcript

Introduction

Dr. Ryan Gray: If you haven't decided what type of specialty you're going to want to practice, go check out our Specialty Stories Podcast at www.SpecialtyStories.com.

This is The MCAT Podcast, session number 25.

A collaboration between the Medical School Headquarters and Next Step Test Prep, The MCAT Podcast is here to make sure you have the information you need to succeed on your MCAT test day. We all know that the MCAT is one of the biggest hurdles on your premed journey, and this podcast will give you the motivation and information that you need to know to get the score you deserve so you can one day call yourself a physician.

Welcome back to The MCAT Podcast, or welcome if this is your first time, I am glad you are here. The MCAT Podcast is part of the Med Ed Media network which you can find at www.MedEdMedia.com. That's www.MedEdMedia.com.

Today we're going to answer some awesome and fun biology questions, let's dive in.

Alright Bryan, so we're back for more biology fun for the MCAT.

Bryan Schnedeker: Yeah this time we're going to work on some independent discrete questions, so there's no passage. Even if you were tempted to go back and look up a passage, there isn't one. And we talked about this with doing the physics discretes; remember the MCAT, a mile wide but an inch deep. So lots and lots of stuff you have to be comfortable with, but don't over-analyze the questions.

Dr. Ryan Gray: Before we jump into that, the fact that you mentioned the discrete versus passages, is there a specific percentage of question on the test that will be discrete questions like this that don't have passages?

Bryan Schnedeker: Yeah it's the same every time. So there's 59 questions in each of the three different science sections, there's 15 of them that are discrete- sorry, yeah 15 that are discrete, and 44 that are based on passages.

Dr. Ryan Gray: Alright.

Bryan Schnedeker: Yeah so unfortunately as much as the discrete questions are like eating popcorn, they're quick and easy, or like eating pistachios. Right? They're just each individual one is nice, and quick, and easy. There's not very many, right? I mean the majority of them are based on passages.

Dr. Ryan Gray: You get those pistachios though that don't have any crack and they take forever.

Bryan Schnedeker: Yeah and that's when you just pick answer choice C and move on. The pistachio of your life, you don't even bother with it.

Dr. Ryan Gray: I like it.

Discrete Biology Question #1

Bryan Schnedeker: Yeah. Okay, question number sixteen. Which of the following will cause a blood pH of 8.2? So right away before we even look at the answer choices, blood pH of 8.2, you should know that normal physiological blood pH is going to be more like 7.35 or 7.4 give or take. So if the person has a blood pH of 8.2 that's wildly alkaline. I mean at this point the patient is at death's door. But you're just looking for an answer choice then that is a higher pH, so either less acid, or more base. One of those two. So let's check the answer choices. Decreased O2 concentration. Decreased title volume. Increased H2O concentration. Increased respiratory rate. Okay so we have to know a little bit of our physiology here. So start with answer choice A, decreased O2 concentration. Now that might be tempting, because we know that O2 and CO2 have this kind of inverse relationship in the blood, right? The more you breathe, the more O2 you have, the less CO2 you have. So if there's decreased O2, maybe that means something funky about the CO2, maybe that would affect pH. So maybe we leave answer choice A for a moment. Okay next, decreased title volume. So title volume is just that kind of natural resting breathing rate, the volume of air that you suck in and out on each breath without kind of forcibly trying to pull in or exhale more air. So if you breathe shallower, if you breathe less, decreased title volume, let's think about what that means. It means that the CO2 that's building up in your body is not getting exhaled as well as it could. You're holding the CO2 back in your body, and we want to remember that for the purposes of blood pH, CO2 is acid, right? CO2 gets converted by carbonic anhydrases into carbonic acid, which then dissociates into a proton, so that would lower the pH. So if you decrease the title volume, if you breathe really shallow and hold your CO2 in, that would actually give you respiratory acidosis that would make your blood pH go down more acidic. So that's the opposite of what we want. Answer choice C says increased H2O concentration, more watery. That actually wouldn't have any effect on pH at all because it would dilute the acid and dilute the base at the same time. So adding more water doesn't actually change the pH balance itself directly. And then so finally we get D, increased respiratory rate. So if you start hyperventilating, blowing out a lot of CO2. Well if you breathe in and out really, really quickly and blow out all your CO2, you're literally breathing acid out of your body, that would make- by extension of your breathing the acid out, what's left behind in your blood is more basic, so your blood would become more basic, and your blood pH could be higher, so answer choice D could tie in with more basic blood pH. The way I always try to kind of really describe this in an invocative way, get you to think about it is from the perspective of blood pH, you are an acid exhaling monster. You're like Godzilla and every time you exhale, you're exhaling acid. CO2 is acid as far as your blood is concerned. So if you stop exhaling, you're holding your acid in. If you blow out a whole bunch, you're blowing the acid out leaving base behind.

Dr. Ryan Gray: Godzilla. I am Godzilla.

Bryan Schnedeker: Yes.

Dr. Ryan Gray: That's what I took from that.

Bryan Schnedeker: Yup you're breathing out carbonic acid, there you go.

Dr. Ryan Gray: Alright, I like it.

Discrete Biology Question #2

Bryan Schnedeker: Okay number seventeen. Patients with excess fat are more likely to require larger therapeutic doses of which vitamin? Vitamin B1? Vitamin C? Vitamin D? Vitamin B3? Okay this question is a classic example of a game we play on the MCAT; which one of these is not like the others, which one of these doesn't belong? It almost doesn't matter what question they asked you because looking at these choices; vitamin B, vitamin C, vitamin D, vitamin B3, one thing should jump out at you right away. You have to know your water soluble and your fat soluble vitamins. So your B vitamins and your C vitamin are water soluble, and your fat solubles are D, E, A and K. So answer choice C here, vitamin D is the only one that stands out, it's the only fat soluble vitamin here. So that ends up being the right answer, if you can just pick the one that's different based on its solubility.

Dr. Ryan Gray: Nice, I like it. That's the one that popped out in my head because I was going to say something, I was going to cut in and go, ‘Let me see if I don't make a fool of myself and say vitamins A, D, E and K are the fat soluble ones.' That's how I remember. I like the order that you put them in, D-E-A-K, because it makes a word kind of. Deak. I don't know if that's a word.

Bryan Schnedeker: Yeah so that's a mnemonic just personal to me. I went to junior high with a kid whose last name was Deak, and he was kind of a big kid.

Dr. Ryan Gray: Nice.

Bryan Schnedeker: So I remember those as the fat soluble vitamins.

Dr. Ryan Gray: I remember A-D-E-K. I don't know why that stuck in my head but that's how we remembered it.

Discrete Biology Question #3

Bryan Schnedeker: Yeah, absolutely. And let's do one more here. So question number 29. Which of the following is most likely to use a protein channel to cross the eukaryotic cell membrane? So Ryan you asked about making a fool of yourself, do you want to try this one?

Dr. Ryan Gray: No, I don't want to try this one.

Bryan Schnedeker: Okay so again the question is which of the following is most likely to use a protein channel to cross a cell membrane. And our answer choices are aldosterone, calcium 2+ ion, oxygen, and carbon dioxide. O2 oxygen. And so again this is just a solubility question like the vitamin question we looked at before because to get across a cell membrane, aldosterone, a steroid hormone, in fact all steroid hormones can just diffuse right across the cell membrane. They don't need a protein channel to get through a cell membrane. Similarly your gases oxygen, carbon dioxide, very small, non-polar, they can just diffuse right across the cell membrane. They don't need a protein channel. But ions, the famous sodium-potassium pump, or sodium and potassium channels in a neuron would be an example, or calcium channels in the sarcolemma for the muscle. So ions definitely do need a protein channel, and so the right answer here is B, calcium needs a protein channel to cross the membrane.

Dr. Ryan Gray: That's exactly how I would have came to the conclusion, and I would have definitely gotten B right, just to let you know.

Bryan Schnedeker: I'm 100% sure that's right.

Final Thoughts

Dr. Ryan Gray: Alright there you go. Again you can go to www.TheMCATPodcast.com/25 to get the show notes for this specific episode which will give you the passages and the questions that we are talking about in these last few episodes, specifically for today's episode is the biology discrete questions. Again that's www.TheMCATPodcast.com/25.

I hope you enjoyed the episode today. If you did I'd love for you to go rate and review it in iTunes. That's the easiest thing to do, but if you wanted to even show your support even more, go share this with a friend. Bring a friend to next week's MCAT Podcast. Although you technically don't have to bring them, just show them how to subscribe on their phone. I would love you for that. I love you anyway for listening, thank you for that.

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I hope you enjoyed today's episode and join us next week here at The MCAT Podcast and the Medical School Headquarters.

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