Next Step Full Length 10, Bio/Biochem Discretes 3

Session 129

We’re onto our last set of discrete questions for the bio/biochem section of Next Step Test Prep full-length 10.

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[01:23] Question 44

A medical student stubbed her toe and observed that there was a momentary gap between when she realized she had stubbed her toe. And when she felt the onset of pain, she wondered if this might relate to a structural difference in the neurons involved in the baroception versus those involved in nociception. Which of the following is the best hypothesis for explaining this observation?

  • (A) The neurons in the nociception pathway are not myelinated whereas those involved in the baroception pathway are myelinated.
  • (B) The neurons in the nociception pathway have more dendrites than those in the baroception pathway.
  • (C) The neurons in the baroception pathway are not myelinated whereas those involved in the nociception pathway are myelinated.
  • (D) The neurons in the baroception pathway have more dendrites than those in the nociception pathway.

Clara’s insights:

The correct answer here is A.

[04:26] Question 45

GTP is best classified as a member of which class of biological molecules.

  • (A) Amino acids
  • (B) Peptides
  • (C) Nucleotides
  • (D) Nucleic acids

Clara’s insights:

The correct answer is C. Those are typical examples of DNA and RNA. Nucleic acids are polymers of nucleotides. GTP is just one molecule, which is just one molecular unit. It’s not a long string of monomers along a string of nucleotides, like a nucleic acid is. And so GTP is a nucleotide.

Where most people mess this up is we typically think of nucleotides as like the regular A, C, T, G, and those in DNA will only have one phosphate group. But GTP which is not a part of a DNA directly, it has three phosphate groups. To be a nucleotide, a molecule just needs to have at least one phosphate group, 5-carbon sugar, and a nitrogenous base.

So a nucleotide is the individual subunit while nucleic acids are long-chain polymers of nucleotides. So if you’re talking about a long strand of DNA then that would be a nucleic acid but that individual unit like the base or the phosphate, or the sugar, then that would be a nucleotide.

And it’s just like the amino acids and peptides where the amino acids are the individual subunits of the peptides.

Just remember that all those we see all the time – GTP, ATP, etc – are technically nucleotides.

[08:00] Question 46

DCCD is a chemical that blocks the proton core of ATP synthase. If treated with DCCD, which of the following is most likely to decrease in the actively respiring mitochondria of an adult rat cardiac cell?

  • (A) Hydrogen concentration within the intermembrane space.
  • (B) ADP concentration within the matrix.
  • (C) Oxygen consumption
  • (D) Chemiosmotic gradient across the inner membrane

Clara’s insights:

This is easy to mess things up because they’re asking about what’s likely to decrease. So start from the very beginning and sum it up. The first sentence means protons during ATP generation. Basically, we have the electron transport chain and the oxidative phosphorylation that generates ATP. A major step in the phosphorylation is protons moving through protein synthase. As that happens, they make ATP. So this chemical is blocked. As a result, we’re not going to be able to make as much ATP. Eventually, when you stop ATP synthase from functioning, eventually the whole thing will shut down so the electron transport chain will slow and actually stop.

So that’s what’s happening. Now, you just have to make sure you get the right answer and don’t get it backward.

In A, the intermembrane space is where we’ve been pumping protons into that space to build up this major proton gradient. We’ve blocked protons from coming back into the matrix. So what’s happening is that protein concentration in the membrane space is going to get bigger. And so it can’t travel back inside so A is out because it won’t decrease.

For B, it says ADP concentration and then right away, that looks like something is going to increase because ADP is the inverse of ATP.

Oxygen consumption looks tempting because eventually, we’re just not going to be able to run this process at all. And so we’re not going to be using oxygen as our final electron receptor of the ETC.

For choice D, it just refers to the actual proton gradient. It’s a fancy word for saying a gradient of chemicals and those chemicals are protons. So C is the correct answer.

[13:00] Question 47

A couple trying to conceive without using IVF methods would most likely have their highest chance of pregnancy:

  • (A) Early in the follicular phase of the menstrual cycle
  • (B) After the onset of mensies
  • (C) During the luteal phase of the menstrual cycle
  • (D) Midway through the follicular phase of the menstrual cycle

Clara’s insights:

The phases of the menstrual cycle would be two: the follicular phase, the first half where the follicle is just essentially growing and getting nutrients. Then at the end of the follicular phase is the LH surge, which is the ovulation. After the ovulation is the luteal phase. Until ovulation happens, the follicle is actually still not released from the ovary. So we don’t have the egg freely available to be fertilized until after ovulation, which is the luteal phase. So the best answer here is C.

[16:00] Next Step Test Prep

Next Step Test Prep is known for their one-on-one tutoring as well as an amazing course that I’ve actually reviewed at Whatever that you need in your MCAT test prep, the Next Step Test Prep team is there to help you all along the way. Go to Next Step Test Prep and use the promo code MCATPOD to save some money.


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