If you’re preparing for the Medical College Admission Test (MCAT), you are going to want to consult the experts. These selections from Kaplan’s MCAT Question of the Day series can help you sharpen your skills as you prepare to begin your potential journey into medical training.
The three questions below come from MCAT’s biological and biochemical foundations of living systems section.
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Question 1: Which of the following is/are function(s) of NADPH in the cell?
I. Antimicrobial resistance via bacterial destruction by bleach within lysosomes.
II. Involvement in the production of the precursors to steroid hormones.
III. Functional carriage of energy across organelle membranes for use within the mitochondria
A. I only.
B. I and II only.
C. II and III only.
D. I, II and III.
The correct answer is B.
NADPH has three primary functions: involvement in biosynthesis of lipids and cholesterol (the precursor to steroid hormones), production of bactericidal bleach in the lysosomes of certain white blood cells, and maintenance of a supply of reduced glutathione for protection against free radical damage. Energy carriage is an important function of NADH, not NADPH.
Question 2: After a brief period of intense exercise, the activity of muscle pyruvate dehydrogenase is greatly increased. This increased activity is most likely due to:
A. Decreased ADP.
B. Increased acetyl-CoA.
C. Increased NADH/NAD+ ratio.
D. Increased pyruvate concentration.
The correct answer is D.
Kaplan explains why: In most biochemical pathways, only a few enzymatic reactions are under regulatory control. These often occur either at the beginning of pathways or at pathway branch points. The pyruvate dehydrogenase (PDH) complex controls the link between glycolysis and the citric acid cycle, and decarboxylates pyruvate (the end product of glycolysis) with production of NADH and acetyl-CoA (the substrate for the citric acid cycle).
After intense exercise, one would expect PDH to be highly active to generate ATP. ADP levels (choice A) should be high because ATP was just burned by the muscle. Acetyl-CoA (choice B) is an inhibitor of PDH, causing a shift of pyruvate into the gluconeogenesis pathway. A high NADH/NAD+ ratio (choice C) would imply that the cell is already energetically satisfied and not in need of energy, which would not be expected in intensely exercising muscle.
Question 3: Which of the following correctly indicates the response of CD8+ T-cells when activated?
A. Secretion of cytotoxic chemicals.
B. Causing isotype switching.
C. Presentation of antigens.
D. Activation of B-cells.
The correct answer is A.
Kaplan explains why: CD8+ T-cells are largely responsible for the cytotoxic immune response. By releasing toxic chemicals into virally infected cells, CD8+ T-cells are able to kill these cells in an effort to contain viral infections. Isotype switching refers to changes in the isotype of antibody produced, which is not caused by CD8+ cells, eliminating choice B. Antigens are presented by macrophages, dendritic cells, certain epithelial cells, and some B-cells, eliminating choice C. B-cells are not activated by cytotoxic T-lymphocytes, eliminating choice D.