Breaking Down a Metabolic Pathway MCAT Passage

Session 29

This week, we’re going to do a deep dive into a full biology MCAT passage dealing with a key metabolic pathway, beta-oxidation.

We’re going to show how you can prevent yourself from falling into some traps set by some tricky questions and ultimately beat the MCAT.

Listen to this podcast episode with the player above, or keep reading for the highlights and takeaway points.

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[01:34] How Well Do You Need to Know Metabolic Pathways for the MCAT?

This passage describes an important metabolic pathway—the process of beta-oxidation, how the body metabolizes fat for energy in the mitochondrion. It also shows a couple of different molecular structures of carnitine and palmitic acid, along with the first three steps of beta-oxidation.

With the addition of biochemistry to the MCAT in 2015, the test makers are expecting you to be very comfortable with these different metabolic pathways. The level of detail on the MCAT is not quite the same as you’ll learn in medical school, but it still has to be the kind of language you’re comfortable reading about.

Know your metabolic pathways coming into the MCAT.Click To Tweet

[02:25] Answering “Least” Questions

Question #41: Injection of insulin into the bloodstream is least likely to result in which of the following:

(A) Increased glycogen synthesis
(B) Decreased lipid synthesis
(C) Increased de-esterification of fatty acids
(D) Decreased gluconeogenesis

In order to answer this, we need to know that insulin is a very powerful metabolic hormone. This is not directly coming from the passage itself but it is foundational knowledge related to the topic of the passage.

[03:19] Bryan’s Insights:

The question sounds like it’s going to require a whole lot of hyper-detailed knowledge of the various metabolic results of insulin. When organizing all the various, complicated biochemistry pathways, start with the broadest, most general understanding.

Insulin is one of the most powerful anabolic hormones in the body. You need to know that for the MCAT.Click To Tweet

Insulin is arguably the most powerful anabolic hormone in the body. It is involved in building up and storing big molecules. Knowing that its main job is to store energy to build up big molecules, you can probably hack your way through this question and find your way to the right answer.

Choice (A), increased glycogen synthesis, sounds like something insulin would do since it means more storage. However, a common mistake among students is they’d just pick this answer and move on, forgetting the word “least” in the question. Therefore, choice (A) is already crossed off.

Choice (B), decreased lipid synthesis, is the right answer since this is something insulin would not trigger. insulin’s job is to help us store energy, but this option is decreasing energy storage. Since this is a “least” question, answer choice (B) is the right answer.

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[05:08] Tips to Prevent Mistakes in Answering “Least” Questions

First, the AAMC will put in caps, or italics, words like least, not, and except.

The AAMC will typically capitalize or italicize words like 'least,' 'not,' and 'except.' Pay attention to them.Click To Tweet

Second, the MCAT comes with a highlighting function where you can mouse over and highlight words in the passage or in the question. So if you see words like challenge, weaken, not, accept, or least, highlight them in the question itself. This visual reminder will help prevent any mistakes.

[06:07] Answering Roman Numeral MCAT Questions

Question #42: Which steps in Figure 3 are oxidations?

(Steps are presented with Roman numerals. Figure 3 starts with a big, long fatty-acid tail attached to Coenzyme A.)

I. Step one shows a single bond becoming a double bond.
II. Step two shows the double bond going away and an OH (hydroxyl group) has now been tagged on where that double bond was.
III. Step three shows the OH group becoming a carbonyl carbon (C=O).

[07:11] Bryan’s Insights:

There are different ways to answer Roman numeral questions, but do the easiest one first. In this case, going from a hydroxyl group (OH) to a carbonyl carbon (C=O) is fairly obviously an oxidation. It’s a classic example of oxidation when you see more bonds. So Step 3 is an oxidation.

Let’s just eliminate answer choice (A), and if this is far as you can get and you’re not really sure about the other steps, that’s fine. Eliminate as many incorrect answers as you can, make your best guess, and move on.

Eliminate as many incorrect answers as you can, make your best guess, and move on.Click To Tweet

Step 1 of the process going from a single bond to a double bond is another example of an oxidation step. So when we reduce or saturate a molecule, we reduce all the double bonds down to single bonds. And if you go the other way, going from a single bond to a double bond would be an oxidation step. So (I) is also an oxidation step.

Hence, Steps 1 and 3 are oxidations. This then leads us to eliminate another answer choice, (B), on top of (A), which we’ve already eliminated.

For the two remaining answer choices, (C) and (D), we just have to decide whether Step 2 is an oxidation step. And this is a tricky question. A majority of students get this question wrong. They think that Step 2 is an oxidation step, so they pick the incorrect answer, (D), whereas the right answer is (C), I and III.

You have to look back at Step 2 and figure out why it’s not an oxidation. It’s tricky in that the molecule starts from a double bond before adding a hydroxyl group. Students simply see the oxygen and they immediately conclude it’s oxidation. But that’s incorrect.

You need to realize that since the double bond went away, that means one of the carbons got the OH group and, therefore, the other carbon had to have gotten a hydrogen. The double bond did not just disappear, so you had to put a hydrogen on there.

Hence, this is not an oxidation step but a hydration step. They actually added water across a double bond. Sure, one carbon got an oxygen, but the other carbon got a hydrogen.

This is tricky because you really have to think about what happened to the other carbon in that double bond. But ultimately, it’s not a trick question because the figure is right there.

[10:50] Final Thoughts on This MCAT Passage

You want to walk into the MCAT being super comfortable with beta-oxidation as a metabolic pathway. Then none of these steps will be new to you.

It’s good to really know your metabolic pathways for the MCAT. You don’t want to be worried about interpreting the figure correctly. If you’re very comfortable with the pathways already, you can instead just focus on the exact question asked.

It's good to really know your metabolic pathways for the MCAT.Click To Tweet

Links and Other Resources

Check out my book about the MCAT, co-written with Blueprint MCAT (formerly Next Step Test Prep): The Premed Playbook: Guide to the MCAT.
Related episode: Biochemistry on the MCAT: Breaking Down a Passage.
Related episode: Last-Minute MCAT Tips Leading Up to Test Day.
Need MCAT Prep? Save on tutoring, classes, and full-length practice tests by using promo code “MSHQ” for 10% off Next Step full-length practice tests or “MSHQTOC” for $50 off MCAT tutoring or the Next Step MCAT Course at Blueprint MCAT (formerly Next Step Test Prep)!