MCAT Practice Questions: Biology

MCAT Practice Questions: Biology

The MCAT will present you with 10 passages on biology and biochemistry topics, and ask 4-7 questions about each passage. The questions will address the four skills listed, although not every passage will require you to use each skill. You will also be presented with 15 discrete questions that are not associated with passages. These will also be designed to test both your science knowledge and application of that knowledge based on these four skills. You can find more details on what you need to know about the overall structure of the MCAT here.

The biology/biochem section of the MCAT is scored on a curved scale of 118-132, with the median score of all test takers set at 125. There is no specific number of right or wrong questions that corresponds to a given scaled score; instead, each test administration is curved according to its level of difficulty and the performance of the test-takers on that day. The score for this section of the test is combined with the other three sections to give an overall score ranging from 472 to 528.


MCAT Practice Questions: Biology

A patient presents to the emergency room with an asthma attack. The patient has been hyperventilating for the past hour and has a blood pH of 7.52. The patient is given treatment and does not appear to respond, but a subsequent blood pH reading is 7.41. Why might this normal blood pH NOT be a reassuring sign?

A. The patient’s kidneys may have compensated for the alkalemia.

B. The normal blood pH reading is likely inaccurate.

C. The patient may be descending into respiratory failure.

D. The patient’s blood should ideally become acidemic for some time to compensate for the alkalemia.

Answer and Explanation

The correct answer is:

When a patient with an asthma attack does not respond to treatment and has been hyperventilating for over an hour, he or she may become fatigued and may not be able to maintain hyperventilation. In this case, the patient begins to decrease his or her breathing rate and is not receiving adequate oxygen. By extension, carbon dioxide is trapped in the blood, and the pH begins to drop. Despite the fact that this pH is normal at the moment, this patient is crashing and may start demonstrating acidemia in the near future. While the kidneys should compensate for alkalemia, this is a slow process and would not normalize the blood pH within an hour; further, adequate compensation by the kidneys would actually be a reassuring sign, eliminating choice (A). There is no evidence to believe the measurement was inaccurate, eliminating choice (B). Finally, after treatment, the patient should return to a normal blood pH with adequate ventilation and would not be expected to overcompensate by becoming acidemic, eliminating choice (D).


Suppose that in a mammalian species, the allele for black hair (B) is dominant to the allele for brown hair (b), and the allele for curly hair (C) is dominant to the allele for straight hair (c). When an organism of unknown genotype is crossed against one with straight, brown hair, the phenotypic ratio is as follows:

25% curly black hair

25% straight black hair

25% curly brown hair

25% straight brown hair

What is the genotype of the unknown parent?


B. bbCc

C. Bbcc

D. BbCc

Answer and Explanation

The correct answer is:

In this dihybrid problem, a doubly recessive individual is crossed with an individual of unknown genotype; this is known as a test cross. The straight- and brown-haired organism has the genotype bbcc and can thus only produce gametes carrying bc. Looking at the F1 offspring, there is a 1:1:1:1 phenotypic ratio. The fact that both the dominant and recessive traits are present in the offspring means that the unknown parental genotype must contain both dominant and recessive alleles for each trait. The unknown parental genotype must therefore be BbCc. If you want to double-check the answer, you can work out the Punnett square for the cross BbCc × bbcc:


MCAT Strategy Tips

Although the designers of the MCAT will provide physiological facts and numbers, that’s not what you’re expected to know before taking the exam. Your MCAT practice will require being able to see the same concepts that you learned in your undergraduate pre-med classes in a very defined and isolated environment—applied in a foreign scope to integrated sciences.

The interplay of scientific disciplines (i.e., how your knowledge of physics or chemistry informs your understanding of how an organ works) is paramount to your success both on the new MCAT and as a future physician. The human body is a network of interdependent physical, chemical, and biological processes, and the MCAT measures your ability to make those connections.

Knowing formulas and reactions is necessary for success on the MCAT, but will in no way be sufficient. Success will only come with practice, so here are a couple of tips:

Tip 1: When studying a “physical” science, think through all of its biological applications.

For example, let’s use reduction and oxidation. We know that LEO the Lion says GER (the Loss of an Electron is Oxidation and the Gain of an Election is Reduction). Therefore, when a metal is losing or gaining electrons, it’s either getting oxidized or reduced, respectively.

Don’t stop there. The ReDox that occurs in the Electron transport chain, with NADH losing an electron to the ETC and getting—you guessed it—oxidized, is the exact same concept. NAD+ is the product of the oxidation of NADH, just as Ag+ is the oxidized product of Ag. It’s the exact same science. So, don’t get thrown off by the fact that you learned it in two different places.

Tip 2: There are a finite number of scientific facts.

There is some truth in the claim that most of biology is rooted in chemistry and physics. Take proteins for example. We think of them as biological molecules because they serve such a prominent role in the body. In reality, they are nothing more than a very specific structural arrangement of carbon, nitrogen, oxygen, hydrogen, and sometimes sulfur.

The way in which they are bonded to each other is through a bonding orbital—just like the ones you learned about in general chemistry. They fold into specific shapes because of attractions and repulsions of the amino acids in their sequence. Those are the same attractions and repulsions seen in chemistry. The proteins themselves are coded from RNA, which is coded from DNA. RNA and DNA are just chemical molecules with the same properties you learned about in chemistry.