Next Step Full Length 10, Bio/Biochem Passage 2

Session 120

We tackle passage 2 from Blueprint MCAT (formerly Next Step Test Prep)‘s full-length 10 bio/biochem section. The topic of this passage is all about insulin. Follow along with us.

[02:18] Passage 2

Insulin is one of the smallest protein in the human body with only 51 amino acids. However, its role in maintaining homeostasis is tremendous. Insulin consists of cleaved two chain subunits linked together by a pair of disulfide bonds. The precursor of insulin, proinsulin, is a single chain peptide. Proinsulin is two basic amino acids to create insulin and a small peptide chain byproduct, Chain-C.

Insulin’s action depends completely on its binding to an insulin receptor protein found in most cell membranes. Patients with mutations in the gene that codes for insulin receptors suffer from Donohue Syndrome or leprechaunism, an autosomal recessive disorder characterized by severely stunted growth. The binding of insulin to its receptor sets in motion many cellular signaling pathways. Insulin encourages cellular glucose uptake by translocating GLUT-4 transporter proteins to the cell membrane. It encourages glycolysis, glycogen synthesis, and fatty acid synthesis.

Insulin-like growth factor or IGF is a hormone secreted by the liver upon stimulation by growth hormone. It is highly similar to insulin. It binds to a variety of receptors in order to mediate human growth. Diabetes is a classic disorder characterized by problems with insulin. Type I diabetes is characterized by a lack of insulin production. Type II diabetes is characterized by the insensitivity of cells to insulin, known as insulin resistance despite insulin’s presence.

[04:22] The Danger of Knowing Too Much on the MCAT

The same with CARS, if you have a lot of detailed background information about a certain topic, there can be a danger to it. This is a common problem especially with students who work in certain labs and studies phenomena where they know everything about insulin inside and out. But the questions are going to ask about what MCAT students need to know and whatever is on the passage.

So you have to take this from a perspective of – what would an MCAT student who’s really well-versed in the MCAT content but doesn’t know anything outside of that, what would they know? And don’t go beyond that information.”

[05:15] Question 5

Which of the following will most likely cause low blood glucose levels?

(A) A hormone-secreting tumor in the alpha cells of the pancreas
(B) A hormone-secreting tumor in the beta cells of the pancreas
(C) A lesion to the beta cells of the pancreas
(D) A hormone-secreting tumor in the thyroid gland

Clara’s insights:

B is the correct answer here. The tumor is going to release too much insulin as opposed to a lesion that would impede insulin release.

Know your content right and know that insulin is created in the beta cells, not the alpha cells. Alpha cells make the glucagon. Insulin and glucagon are both in the pancreas.

[08:39] Question 7

Which of the following are possible terminal amino acids for the cleaved peptide chain-C?

(A) Lysine and tryptophan
(B) Arginine and arginine
(C) Histidine and aspartate
(D) Glutamate and aspartate

Clara’s insights:

First, go back to the passage where they first mentioned the peptide chain-C where they talked about the two basic amino acids. You should walk into the MCAT knowing your basic and acidic amino acids. The basic amino acids are arginine, lysine, and histidine. And the only choice here that has two basic amino acids would be answer choice B.

[12:23] Question 8

Which of the following best explains why the insulin receptor mutation described in the passage results in stunted growth?

(A) Since insulin can’t bind insulin receptors, the cells don’t get enough glucose so growth is stunted.
(B) Excess insulin competes for binding sites on IGF or insulin-like growth factor receptors.
(C) This disorder is most likely a side effect of diabetes.
(D) IGF normally binds insulin receptors.

Clara’s insights:

A is the most commonly picked answer here. However, we’re naturally predisposed to try to relate glucose to growth. If cells don’t get enough glucose then we’re not getting enough energy. The thing is glucose doesn’t directly relate to growth. And a lack of getting enough glucose would cause all sorts of problems. There’s no reason it would cause stunted growth more than anything else. But D where IGF normally binds insulin receptors, and if there’s a mutation in the insulin receptors, then you can say IGF is a growth factor so it must be linked to growth. Now, it can’t bind one of these types of receptors to where it binds to resulting in less growth. So D is the more direct answer here.

Clara warns students to not make logical jumps on the MCAT as MCAT is a very direct test. Making a jump between not glucose means not enough growth is a big jump compared to not enough growth factor behind not enough growth is a much smaller jump.

[16:47] Question 9

If the body is producing insufficient insulin, down-regulation of which of the following hormones would help lower plasma glucose concentration?

(A) Cortisol
(B) Testosterone
(C) Prolactin-releasing hormone
(D) Estrogen

Clara’s Insights:

This is a pseudo-discrete question. In fact, it’s a hard step for students to know which is not in the passage so there’s no sense wasting time to go back. So this is something you would be able to answer based on your own knowledge.

Using the process of elimination, B, C, and D are hormones related to the development of the reproductive system or secondary sex characteristics but not related to blood sugar. While A is Cortisol, is a stress hormone which does increase blood sugar. So having down-regulated cortisol would help lower blood sugar.

[20:15] Recognizing Pseudo Discrete Questions

This is an important skill to have and Clara suggests a few things that can help. First, really know your content. Otherwise, you’re going to have no clue sometimes. But if you know content inside and out, as soon as you see hormones, you can see keywords and phrases where you can use your knowledge.

Second, just pay a lot of attention to what the question and even what the answer choices are saying, whether or not they were mentioned in the passage.

[22:22] Blueprint MCAT (formerly Next Step Test Prep)

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