MCAT Biology Discrete Questions – Acetyl-CoA and more

Session 73

Hypoglycemia, the electron transport chain, and beta-oxidation are the subjects covered by Ryan and Bryan on this episode.
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[01:02] Hyploglycemia

Question 16: Which of the following is the most plausible explanation for a patient experiencing hypoglycemia?

  • (A) Accidental self injection of excess insulin
  • (B) Increased gluconeogenesis
  • (C) A four-hour fast following a carbohydrate-rich meal
  • (D) Increased rate of glycogenolysis

Bryan’s Insights:

Ryan goes through is thought process until arriving at answer choice (A) which is the right answer and the right thought process.

The correct answer here is (A) since all the other choices involve putting sugar into your blood, whether you’re making it directly by breaking down the glycogen to make it or just by eating a bunch of carbs.

The MCAT can have “tricky” questions. They can be subtle, but there’s never trick questions. They’re not out to fool you.

So the right answer is always going to stand out in some categorical way or for some categorical reason. In this case, categorically, three of the four answer choices would raise the blood sugar.

[04:00] Fat Oxidation

Question 17: In a metabolic analysis experiment, researchers subjected fatty acid samples to beta oxidation max spec performed on the resulting products would reveal:

  • (A) Glucose
  • (B) Acetyl-CoA
  • (C) Pyruvate
  • (D) Succinate

Bryan’s Insights:

Ryan guesses (B) and it’s the right answer. What Bryan usually tells students is if they’re not sure about biometabolism question is that it’s not bad to guess Acetyl-CoA. Because it’s the taxi cab of the metabolism system. It’s that two carbon group that shuttles energy around in all sorts of different processes.

Beta oxidation is burning fat and making Acetyl-CoA which can be used for energy. All the other choices, glucose and pyruvate are part of carbohydrate metabolism, not fat. And succinate is part of the Krebs Cycle. So it’s not directly part of fat metabolism.

[05:33] All About Electrons and Reduction Potential

Question 27: In what order do electrons move through the ETC (Electron Transport Chain)?

  • (A) From carriers with lower reduction potential to carriers with higher reduction potential
  • (B) From carriers with higher reduction potential to carriers with lower reduction potential
  • (C) From carriers with stronger proton binding capacity to carriers with weaker proton finding capacity
  • (D) From carriers with weaker proton binding capacity to carriers with stronger proton binding capacity

Bryan’s Insights:

Start with the electron transport chain. So there are two answer choices here (C and D) that don’t make any sense – proton binding capacity. It’s not the proton transport chain we’re talking about here but the electron transport chain.

Reduction potential means that the more positive, the more favorable it is. Reduction potential is how much something wants to be reduced. How desperately does it grab onto electrons?

So if you have a really high reduction potential, really big positive number, that means you really want to be reduced. You want electrons. The electron transport chain basically starts out with electrons with really high energy. So you can stick them on a molecule that doesn’t have much in the way of reduction potential. It doesn’t really want the electrons. It’s like a sponge that’s not very absorbed. But those electrons are so high energy that they can just plug it right on there.

And since you’re passing those electrons from one molecule to the next, and in each step along the way, the next molecule down the line really wants those electrons more than the guy in front of them. So you go to higher and higher reduction potentials as you go down the chain. So answer choice (A) is the right answer here. Hence, the reduction potential is the level of electron attractiveness.

[09:10] Next Step Test Prep

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Stay tuned next week for some Psychology questions!


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