MCAT Practice Test Sample Questions
Try these free sample questions from the MCAT Practice Test. Each one shows the correct answer and an explanation. The full practice tests covers: Chemical and Physical Foundations of Biological Systems, Biological and Biochemical Foundations of Living Systems, Psychological, Social, and Biological Foundations of Behavior, Critical Analysis and Reasoning Skills (CARS).
Question 1
Noncompetitive inhibition differs from uncompetitive inhibition in that a noncompetitive inhibitor binds to an allosteric site on the enzyme and prevents it from catalyzing a reaction, whereas uncompetitive inhibitors bind to the enzyme-substrate complex and prevent catalysis. Increasing the substrate concentration would have which of the following effects?
- Increasing impact of uncompetitive inhibitor and decreasing concentration of noncompetitive inhibitor
- Decreasing impact of uncompetitive inhibitor and increasing impact of noncompetitive inhibitor.
- Increasing impact of uncompetitive inhibitor
- No effect
Answer: Increasing impact of uncompetitive inhibitor
Explanation: As substrate concentration increases, there will be more binding between the substrate and the enzyme. More enzyme-substrate complex in solution means more opportunity for an uncompetitive inhibitor to bind. Thus, as substrate concentration increases, the impact of an uncompetitive inhibitor would be expected to increase. A: The concentration of a noncompetitive inhibitor isn’t changed by changing the substrate concentration. B, D: As discussed above, the impact of an uncompetitive inhibitor would be increased.
Question 2
A person suffering from severe dehydration and starvation would NOT be expected to have elevated plasma concentrations for which of the following hormones?
- ADH
- Cortisol
- Aldosterone
- Insulin
Answer: Insulin
Explanation: This question asks you to determine the body’s physiological response to the extreme conditions of dehydration and starvation. In such cases, the person would be expected to have low blood pressure and low nutrients (i.e. proteins, carbohydrates, and lipids). As a result, the body will respond by producing hormones that increase blood pressure and mobilize stored nutrients (glycogen, fat, etc.). ADH and aldosterone both increase blood pressure so they would be expected to appear in high plasma concentrations in response to the low blood pressure, eliminating choices A and C. Cortisol would be expected in high plasma to increase carbohydrate, protein, and fat availability, making B false. That leaves answer choice D. Insulin lowers blood glucose levels, which is expected to appear in the blood after a sugary meal. In the case of starvation, the body would respond by increasing blood glucose levels via the production of glucagon.
Question 3
A certain molecule acts by binding to cytochrome oxidase A3, the final enzyme in the electron transport chain. Administration of a large dose of this substance to a human would likely:
- Lead to death due to an inability of the cell to pass electrons to oxygen, thus stopping aerobic respiration and asphyxiating the cells.
- Lead to death due to an inadequate supply of ADP to accept a phosphate group at the ATP synthase enzyme.
- Have no effect as cells would switch which macronutrient they metabolize to circumvent the blocked biochemical pathway.
- Increase the cell’s ATP production as negative feedback would cause the cell to up-regulate anaerobic pathways.
Answer: Lead to death due to an inability of the cell to pass electrons to oxygen, thus stopping aerobic respiration and asphyxiating the cells.
Explanation: The electron transport chain works by taking high-energy electrons from molecules like NADH and passing them through a series of enzymes, ultimately ending with oxygen as the final electron acceptor. If this chain is blocked, aerobic respiration would cease, killing the cell. Eukaryotic cells in humans cannot survive using only anaerobic respiration. Thus (A) is the right answer. Several poisons work in the manner described, most notably cyanide. B: ADP is produced when cells use up ATP to carry out cell functions. Blocking the electron transport chain would have no effect on ADP’s immediate availability. C: As discussed above, this poison would kill the person. D: As discussed above, this poison would kill the person.
Question 4
An enzyme catalyst is kept in a solution buffered to the pH that maximizes activity for those enzymes found in the blood. The enzyme catalyzes a reaction that produces an acid whose pKa is 4.5. In this buffer solution, the acid product will be:
- Primarily deprotonated
- Primarily protonated
- Protonated and deprotonated in roughly equal amounts
- Not formed
Answer: Primarily deprotonated
Explanation: The blood has a pH around 7.4, making it somewhat basic. If this enzyme is in a solution buffered to a point to maximize enzymes found in the blood, then the buffer solution will have a pH of 7.35 to 7.45. When an acid is in a solution whose pH is higher than its pKa, the acid is deprotonated, making choice (A) the correct answer. B: The acid would be protonated if the buffer solution had a pH that was below 4.5 C: The acid would have equal concentrations of the acid and conjugate base form if the buffer had a pH of around 4.5 D: There’s no reason for us to assume the product wouldn’t form.
Question 5
What is the identity of the reducing agent and oxidizing agent, respectively, in the reaction shown below? 2Cr(OH)<sub>3</sub> + 3H<sub>2</sub>O<sub>2</sub> + 4NaOH ? 2Na<sub>2</sub>CrO<sub>4</sub> + 8H<sub>2</sub>O
- Cr(OH)3 and NaOH
- Cr(OH)3 and H2O2
- H2O2 and Cr(OH)3
- H2O2 and NaOH
Answer: Cr(OH)3 and H2O2
Explanation: To answer this question, you must identify the atom that is reduced and the atom that is oxidized. In the reaction above, chromium is oxidized from +3 to +6, which makes chromium hydroxide the reducing agent. Alternatively, oxygen in hydrogen peroxide is reduced from -1 to -2, which makes it the oxidizing agent. Thus, the correct answer is B.