MCAT Biochemistry Quiz

Once you’re familiar with the MCAT Biochemistry section, take this quiz to test your readiness.

Question 1

Which of the following statements about enzyme kinetics is FALSE?

A. An increase in the substrate concentration (at constant enzyme concentration) leads to proportional increases in the rate of the reaction.

B. Most enzymes operating in the human body work best at a temperature of 37°C.

C. An enzyme–substrate complex can either form a product or dissociate back into the enzyme and substrate.

D. Maximal activity of many human enzymes occurs around pH 7.4.

Answer 1

A: Most enzymes in the human body operate at maximal activity around a temperature of 37°C and a pH of 7.4, which is the pH of most body fluids. In addition, as characterized by the Michaelis–Menten equation, enzymes form an enzyme–substrate complex, which can either dissociate back into the enzyme and substrate or proceed to form a product. So far, we can eliminate choices (B), (C), and (D), so let’s check choice (A). An increase in the substrate concentration, while maintaining a constant enzyme concentration, leads to a proportional increase in the rate of the reaction only initially. However, once most of the active sites are occupied, the reaction rate levels off, regardless of further increases in substrate concentration. At high concentrations of substrate, the reaction rate approaches its maximal velocity and is no longer changed by further increases in substrate concentration.

Question 2

Which two polysaccharides share all of their glycosidic linkage types in common?

A. Cellulose and amylopectin

B. Amylose and glycogen

C. Amylose and cellulose

D. Glycogen and amylopectin

Answer 2

D: Glycogen and amylopectin are the only polysaccharide forms that demonstrate branching structure, making them most similar in terms of linkage. Both glycogen and amylopectin use α-1,4 and α-1,6 linkages. Cellulose uses β-1,4 linkages and amylose does not contain α-1,6 linkages.

Question 3

Which of the following is correct about fat-soluble vitamins?

I. Vitamin E is important for calcium regulation.

II. Vitamin D protects against cancer because it is a biological antioxidant.

III. Vitamin K is necessary for the posttranslational introduction of calcium-binding sites.

IV. Vitamin A is metabolized to retinal, which is important for sight.

A. III only

B. I and II only

C. III and IV only

D. II, III, and IV only

Answer 3

C: Vitamin A is metabolized to retinal, which is important for sight. Vitamin D is metabolized to calcitriol, which is important for calcium regulation. Vitamin E is made up of tocopherols, which are biological antioxidants. Vitamin K is necessary for the introduction of calcium binding sites, such as during the posttranslational modification of prothrombin.

Question 4

Topoisomerases are enzymes involved in:

A. DNA replication and transcription.

B. posttranscriptional processing.

C. RNA synthesis and translation.

D. posttranslational processing.

Answer 4

A: Topoisomerases, such as prokaryotic DNA gyrase, are involved in DNA replication and mRNA synthesis (transcription). DNA gyrase is a type of topoisomerase that enhances the action of helicase enzymes by the introduction of negative supercoils into the DNA molecule. These negative supercoils facilitate DNA replication by keeping the strands separated and untangled.

Question 5

Which of the following is true of diffusion and osmosis?

A. Diffusion and osmosis rely on the electrochemical gradient of only the compound of interest.

B. Diffusion and osmosis rely on the electrochemical gradient of all compounds in a cell.

C. Diffusion and osmosis will proceed in the same direction if there is only one solute.

D. Diffusion and osmosis cannot occur simultaneously.

Answer 5

A: The movement of any solute or water by diffusion or osmosis is dependent only on the concentration gradient of that molecule and on membrane permeability.