MCAT Concepts You Probably Misunderstand

Premeds and med students are highly intelligent. They usually have strong grades in their science classes.

Is it possible that most premeds and med students misunderstand some very basic science concepts tested by the MCAT? Worse, if highly intelligent premeds and med students misunderstand these concepts, do the content and question writers of the large prep companies (who are mainly premeds and med students) also misunderstand these concepts.

Though it is my experience that every real MCAT contains science errors, I can promise you that the real MCAT has a good sense of exactly which concept you think you understand but don’t.

Here are some examples of subtly false statements that you probably believe to be true. (Some of these statements are taken directly from the published materials of other prep companies.)

False statement: Breaking the high energy bonds of ATP releases energy.

Correction: No bond releases energy when broken. All bonds absorb energy when broken.

False statement: “The force of friction is a type of force that opposes the movement of objects.” (Kaplan Physics and Math Review 2023-2024 page 20)

Correction: Friction does not oppose the motion of objects. Friction opposes the relative motion between two surfaces. Friction is often in the same direction of motion as the object upon which it acts.

False statement: Only 1.5% of the human genome is occupied by genes.

Correction: Though the definition of a gene is somewhat ambiguous, the 1.5% represents only a portion of the genes in the human genome, specifically exons of protein coding genes. It does not represent the introns of these genes. More significantly, most genes don’t code for proteins. About 28% of the human genome is occupied by noncoding genes (tRNA, rRNA, special function RNAs) and the introns of protein coding genes. There are also genes of transposons that are protein-coding and commonly called genes. Transposons occupy nearly half of the genome.

False Statement: Adding water to a buffered aqueous solution won’t change the pH.

Correction: This error results from a misapplication of the Henderson-Hasselbalch equation and a fundamental misunderstanding of the concept of an ideally dilute solution. Assuming 25°C, adding water to a buffered solution will move the solution toward a pH of 7.

False statement: When K is greater than 1, a reaction is exergonic.

Correction: This error is made by the AAMC on their published free practice exam in the BIO section, question 31. It arises from a confusion between ?G and ?G°. If the equilibrium constant of a reaction is greater than 1, the reaction has a negative ?G°, but that is in no way the same thing as ?G. To have a negative ?G (to be an exergonic reaction), Q must be greater than K.

False statement: An irreversible reaction cannot run backwards.

Correction: On a molecular scale, all reactions are reversible. However, assuming the thermodynamic concept of irreversibility, a physiologically irreversible reaction is nothing more than a reaction that is unlikely to experience conditions that would create a negative Gibbs free energy for the reverse reaction. With such an irreversible reaction, if conditions creating a negative Gibbs energy change for the reverse reaction were to be reached, the reaction would run backwards.

False statement: “…and every Bronsted-Lowry acid is also a Lewis acid.” (Kaplan 2023-2024 Chemistry page 367)

Correction: HCl (and many other acids) is a Bronsted-Lowry acid that is not a Lewis acid. A Lewis acid accepts a pair of electrons. HCl does not. The proton on HCl is a Lewis acid.

False statement: Alcohol is a functional group.

False statement: A nucleophile and a base are the same thing.

False statement: Weak acids have strong conjugate bases.

False statement: Energy is defined as the capacity to do work.

False statement: Pressure has a direction.

False statement: Osmotic pressure is real pressure.

The list of fundamental MCAT science concepts that are misunderstood is long. So long, you could fill an entire set of MCAT prep books with them. In fact, I did just that.

Knowing the facts is only the first step toward a great MCAT score. Understanding the concepts behind the facts means MCAT mastery.

Jonathan Orsay

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