Blueprint Diagnostic Bio/BioChem Passage 5

Session 282

Jason and I dissect passage 5 from the Biology/Biochemistry section of the Blueprint diagnostic exam. Don’t forget to get this for free at 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.

[01:22] Tips When Jumping to the Next Passage

Jason reminds students that as they move on to the next passage, just start to find the right information in the passage. Then access parts of their memory that correspond to the content that the passage is talking about.

[02:37] Passage 5 (Questions 24 – 28)

Paragraph 1

The human digestive tract is fundamentally a long tube running right through the body, with dedicated sections that are capable of digesting material put in at the proximal end and extracting any useful components from it, then expelling the waste products at the terminal end.

Notes:

For Jason, when a passage starts telling him something that he already knows, then he’s in good shape for that passage. Of course, you have to be on the lookout for anything that might be new or anything that might be specific to this passage or the questions, or anything that might be a little bit unexpected.

He adds that there’s nothing in this paragraph that is testable, except for maybe the association of proximal and terminal ends of the digestive tract. But even then that’s just a vocabulary thing. And that’s not anything scientifically specific to this passage.

[05:13] Paragraph 2

The small intestine is the site where most of the chemical and mechanical digestion is carried out, and where virtually all of the absorption of useful materials is carried out. The intestines also have a smooth muscle wall with two layers of muscle. This enables the generation of rhythmical contractions that force products of digestion through the intestine. To allow for absorption and secretion to take place, specialized zones known as macula communicans exist between the muscle cells. These directly connect the cytoplasm of two cells, which allows various molecules and ions to pass freely between cells.

Notes:

Now, we start to get into some new stuff where it’s a little bit more specific with the small intestine. They’re also giving us some specific functions here. We can highlight “chemical and mechanical digestion” even though that sounds familiar because we want to be able to spot that quickly. You can highlight smooth muscle as well, again something we recognize, but we want to be able to go back to it quickly.

The new stuff is at the end of this paragraph. So we get a new term here, macula communicans, which is simply a spot where communication happens. And it’s actually just molecules and ions passing freely between the cells.

[08:20] Paragraph 3

In the large intestine, these connections allow for the movement of ions and fluids. For example, if material is too solid, it cannot move fluidly and can result in blockage. To prevent this, Cl– ions are secreted into the lumen. Cations and water will trail the chloride anions into the lumen by passing through the intercellular space between the cells. This method of transport is in contrast to typical transcellular transport, where the substances travel through the cell.

Notes:

Things you can highlight here include: “To prevent this, Cl- ions are secreted into the lumen,” which tells us the what and the why. So chlorine ions are secreted to prevent a lack of water.

And we’re told that the effect is that “cations and water will trail the chloride anions into the lumen by passing through the intercellular space between the cells.” And so, if we move a whole bunch of negative charges from one place to another, we can expect some positive charges to be attracted to that and follow. And where there’s more stuff, there’s more water.

Figure 1 Electrolyte transport in the colonic epithelium

Notes:

Things you can highlight here include: “To prevent this, Cl- ions are secreted into the lumen,” which tells us the what and the why. So chlorine ions are secreted to prevent a lack of water.

[13:11] Paragraph 4

Diseases of the intestines are often due to chronic inflammation, which can arise from imbalances in the regulation of fluids and solute movement. Elevated levels of enzymes like thymidine kinase are often used to predict the risk of cancer in the colon.

Notes:

The MCAT loves to take a system that breaks that works perfectly normally, and break it. Then they ask, what’s going to happen? And here, they’re talking about diseases.

When you’re reviewing your content, and you look at normal physiology, a good way of reviewing that normal physiology is to analyze the things that could happen. Be able to think through those things and not just read the content.

'Students spending too much time and content and not understanding how question-writers think and how they write their questions and how they write their answers.'Click To Tweet

[17:11] Question 24

In which of the following cell processes does thymidine kinase most likely play a role?

A.Osmosis

B.Mitosis

C.Translation

D.Conjugation

Thought Process:

This passage is ultra-specific here, in terms of what thymidine kinase is implicated in, which is to predict the risk of cancer in the colon. Kinase has a very specific definition when it comes to enzymes. It is responsible for moving phosphate groups around. And so, we’re talking very specifically about unregulated cell growth and division. Therefore, B is the correct answer.

Correct Answer: B

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[24:10] Question 25

Studies show that macula communicans are common but not found in tissues such as the gallbladder and skeletal muscle. One possible explanation for this could be that these organs are:

A.not under autonomic nervous control.

B.not involved in digestion of materials.

C.not involved in secretion of materials.

D.not involved in absorption of materials.

Thought Process:

This question isn’t about anything from the passage. It’s not really about macula communicans. And it turns out once we get into it, it’s not really even about skeletal muscle, because skeletal muscle doesn’t have any of these properties.

Skeletal muscle is under somatic nervous control. So this question could very well be rephrased as –what doesn’t the gallbladder do?

And we know that the gallbladder is under autonomic nervous control. It is involved in digestion. And it secretes stuff so that we can do digestion. And so, the only answer that makes any sense in terms of being something that is the intersection of the gallbladder and skeletal muscle is D.

Correct Answer: D

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Jasons says that you have to know what’s on the table for the passage and the question. Know what’s important to think about for the questions. Then all you have to do is put the right information together and you get the answer. It’s that simple, but it’s not easy as well.

[28:29] Question 26

In patients with a certain medical condition, the intercellular path described in the passage is damaged in the large intestine, causing lower levels of transport. What change would be expected?

A.Increased Cl- in the lumen of the colon

B.Decreased Cl- in the lumen of the duodenum

C.Decreased waste fluidity in the colon

D.Increased waste fluidity in the colon

Thought Process:

A – We just said that’s where this intracellular path is for. It’s to increase those ions there. Hence, this is not going to happen.

B – If you first take a look at this, this could be the correct answer since it says decreased Cl- in the lumen which is the opposite of answer choice A. But it says “duodenum” which is part of the small intestine. Therefore, this is a red herring that the MCAT wants you to pick out this answer. And so, we throw this one out.

C & D – When there’s decreased fluidity in the colon, it is not mushy enough and so this is what we’re looking for. On the other hand, increased waste fluidity is what’s going to happen when everything is working. Hence, D is out and C is the correct answer.

Correct Answer: C

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[34:15] Question 27

Which molecule is least likely to be able to travel via the macula communicans pathway?

A.Insulin

B.Cl-

C.Na+

D.Lysine

Thought Process:

This is a game of potentially which one is the odd one out. And insulin is not a small thing. It’s a large molecule compared to the other three above.

Then if you take a look at the passage, it says “ions and fluids in the large intestine.” Then in the small intestine, it would be molecules and ions.

Both of these say ions, so B and C are out right off the bat. So now, we’re left with Lysine and Insulin.

Lysine is a small molecule that has a charge and can use this pathway as well. Insulin is a peptide hormone made up of amino acids. It’s a protein. So if lysine can’t get through, insulin certainly can’t get through.

Correct Answer: A

[38:07] Question 28

The muscle of the dual-layered system described in the passage is:

A.striated.

B.multi-nucleated.

C.regulated by the parasympathetic nervous system.

D.innervated by the somatic nervous system.

Thought Process:

We highlighted very early on that this is smooth muscle. And so we just need to know what smooth muscle is. We know that it’s not a somatic nervous system which is skeletal muscle.

Striated is the opposite of smooth muscle so we take this one out. Smooth muscle and cardiac muscle are both mononucleated. And so, answer choices, A, B, and D are wrong.

Here, knowing the difference between parasympathetic and sympathetic is important for being able to eliminate it, especially if you’re not as comfortable with muscles. Sympathetic is fight or flight and parasympathetic is rest and digest.

Correct Answer: C