Looking at MCAT Biochemistry Discrete Questions

Session 35

This week, we break down some Next Step MCAT biochemistry discrete questions to help you understand what you need to know for your test day.

[01:40] Question #17:

Which of the following is not considered an organic acid?

Answer choices:

(a) Folic acid
(b) Carbonic acid
(c) Ascorbic acid
(d) Citric acid

How to Answer the Question

This is just a nomenclature question. You need to identify what is considered organic and not organic. Looking back at the organic chemistry you did many years ago, you probably remember the definition of what makes a molecule an organic molecule. Usually, we would think of it as carbon. The presence of carbon denotes that it’s an organic molecule.

But remember the exceptions to that. Not every molecule is organic just because it has carbon in it. In fact, the MCAT is going to expect you to remember two particular exceptions, and we’re going to mention a third one.

Carbon dioxide (CO2) has carbon but is not an organic molecule.

Carbonic acid (H2CO3) has carbon in it but it’s not considered an organic molecule.

Any kind of carbon alloy with a metal (ex. steel, which is an alloy of iron and carbon) is not considered organic.

Carbonic acid (H2CO3) is not considered an organic acid. Folic acid, Ascorbic acid, Citric acid are all organic biochemical molecules used in various pathways in the body.

Not every molecule is organic just because it has carbon in it.Click To Tweet

[03:45] Question #44:

What’s the net charge on a phenylalanine molecule at pH1?

Answer choices:

(a) -1
(b) 0
(c) +.5
(d) +1

How to Answer the Question

Biochemistry on the MCAT means a lot of different things, but the two biggest things would be amino acids and enzymes. Every official AAMC practice test includes a whole mess of questions about amino acids and enzymes.

In this case, we’re asked about phenylalanine, which is an amino acid, and you are expected to know about the zwitterionic nature of amino acids. This means at a really low pH, you have to protonate everything. At a physiologically neutral pH, some stuff would be protonated and some stuff would be deprotonated. And at a really high pH, you have to deprotonate everything.

The test expects you to know all the amino acid side chains, so for phenylalanine in particular, the side chain is a phenyl ring attached to a CH2 (like alanine). So phenylalanine has a benzyl functional group, which is a CH2 and a phenyl ring. This means it’s not going to change by pH. The side chain is not going to pick up or lose any protons depending on pH.

The MCAT expects you to know all the amino acid side chains.Click To Tweet

At a pH1, which is very highly acidic, the only functional groups to think about are just the amino and acid parts of an amino acid. So the amino group (NH2) at a really super acidic pH is going to be NH3+ (so +1). And the acid part (COOH) is also going to be protonated. So the COOH is neutral.

So you’ve got COOH neutral, NH3+1, and the phenylalanine side chain which is also neutral. This gives us a total net charge of +1. So the answer here would be (d) +1 charge.

[Related episode: Looking at Amino Acid MCAT Questions.]

[05:55] Final Thoughts on These MCAT Biochemistry Questions

This comes down to knowing all your amino acids. You should be able to go into test day knowing your amino acids so well that if someone gave you a blank sheet of paper and asked you to draw all 20 from memory, including the 3-letter and 1-letter abbreviations, you can do it instantly without a moment’s hesitation.

Every MCAT has at least one or two “freebie” questions if you know the structures of your amino acids.

Every MCAT has at least one or two 'freebie' questions if you know the structures of your amino acids.Click To Tweet