MCAT Biochem Discretes – beta-oxidation, Krebs cycle & more

Session 75

Cellular respiration, beta-oxidation and some Krebs cycle problems are the topics covered on the MCAT Podcast today. Come learn along and crush your MCAT!

If you’re a nontraditional student, check out The OldPreMeds Podcast. This week, Ryan and Bryan from Next Step Test Prep dive into some biochemistry questions.

First, they’re going to look into the theme around metabolism. A lot of biochem on the MCAT overlaps with Biology, this is one of the areas where the lines between the two get pretty fuzzy.

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[01:30] Voltage and Electrical Reduction

Question 15: Given the role of the reaction in cellular respiration, what is the most likely standard electrical reduction value for the reduction of oxygen to water?

  • (A) +0.02
  • (B) +0.82
  • (C) -0.02
  • (D) -0.82

Bryan’s Insights:

The positive voltage is a favorable thing. When you think about batteries, for example, you have a 9-volt batter, not a negative 9-volt battery. It’s a large positive number that shows a favorable chemical reaction. Inside that 9-volt battery. some chemistry is going on that’s producing 9 volts.

In this case, remember that metabolism starts with oxygen which you breathe in and ends with water, which you sweat, breathe, or pee out. So oxygen to water has to be a favorable thing. It has to something that wants to happen on a chemical level.

So you know the answer has to be one of the positive numbers. Then the answer has to be either (A) or (B).

And because it’s the very end of the process, the way the body changes to get multiple reactions is going towards more and more favorable reactions. Hence, the answer here is (B) since it has the biggest most positive voltage here.

Bryan says you have to recognize oxygen to water as the final step of metabolism. From that alone, you can pick either, answer choices (B) or (D), thinking you need an extreme answer. So it’s either the biggest positive or the biggest negative even if you couldn’t remember the rule about the General Chemistry underlying it.

[04:35] Fat Oxidation

Question 30: Each cycle of mitochondrial beta oxidation liberates a 2-Carbon Acetyl-CoA unit, as well as which, if any other molecule/s:

  • (A) NADH and FADH2
  • (B) NADH only
  • (C) FADH2 only
  • (D) Neither NADH nor FADH2

Bryan’s Insights:

Ryan’s guess is answer choice (C) as something that stands out for him. The correct answer here is (A). This is a pure memorization question for the students. You need to know the key metabolic pathways including glycolysis and the Krebs cycle and beta oxidation to burn fat. And then when you have an even numbered carbon fatty acid, the standard beta oxidation cycle applies. You just chop two carbons off the very tail end of the molecule. And in the process of doing that, you get and NADH and FADH2.

[07:00] Mitochondrial Function

Question 46: DCCD is a chemical that blocks the proton pore of ATP synthase. If treated with DCCD, which of the following is most likely to decrease in the actively respiring mitochondrion in an adult rat cardiac cell?

  • (A) H+ concentration in the internal membrane space
  • (B) ADP concentration in the matrix
  • (C) Oxygen consumption
  • (D) The chemiosmotic gradient across the inner membrane

Bryan’s Insights:

Thinking about the mitochondrial function, there’s the electron transport chain and how it works. What happens if you plugged up the end of the pipe? You block the proton pore at the end in the ATP synthase.

Remember that ATP synthase is actually the molecule that uses oxygen. If you clog that pipe, it doesn’t work anymore so you’re not going to consume any oxygen. Hence, oxygen consumption decreases.

On the other the electron chain pumps protons into the membrane space. And that’s going to go up a lot because you blocked the pipe. And because you blocked that at the end, you can’t take those ADPs and ramp them up to ATP. So the ADP just builds up.

The chemiosmotic gradient also increases because it’s the same thing as (A). If the proton concentration gradient in the internal membrane space is building up, then the overall gradient is building up.

[10:30] Next Step Test Prep

If you’re looking for a full-length MCAT practice test, check out Next Step Test Prep. Based on students’ feedback Ryan has heard is that Next Step’s full-length tests are the most realistic scoring-wise to their real test.

Buy up to ten full-length exams from Next Step and use the promo code MCATPOD to save some money.


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