STP Biomedical Sciences 3 — Questions and Answers
Question 1: During oxidative phosphorylation, at which complex does electron transport directly drive proton pumping into the intermembrane space?
- Complex II only
- Complexes I, III, and IV (Correct answer)
- Complex V only
- Complexes I and II
Correct answer: Complexes I, III, and IV
Complexes I, III, and IV are proton pumps; Complex II does not translocate protons, and Complex V uses the proton gradient to synthesize ATP.
Question 2: Which cell type is the primary source of IgE antibodies in an allergic response?
- Cytotoxic T cells
- Natural killer cells
- Plasma cells derived from B cells (Correct answer)
- Mast cells
Correct answer: Plasma cells derived from B cells
B cells undergo class switching to IgE production upon IL-4 and IL-13 signaling, then differentiate into IgE-secreting plasma cells.
Question 3: What does the Km value of an enzyme indicate?
- The maximum reaction velocity
- The substrate concentration at half-maximum velocity (Correct answer)
- The total enzyme concentration
- The product inhibition constant
Correct answer: The substrate concentration at half-maximum velocity
Km is the substrate concentration at which reaction velocity is half of Vmax, reflecting the enzyme's affinity for its substrate.
Question 4: Which organelle contains enzymes that generate hydrogen peroxide and subsequently break it down with catalase?
- Lysosome
- Peroxisome (Correct answer)
- Mitochondria
- Smooth ER
Correct answer: Peroxisome
Peroxisomes use oxidases to generate H₂O₂ from fatty acid oxidation and other reactions, then neutralize it with catalase.
Question 5: A researcher observes that a drug increases cAMP levels in cells. Which enzyme is most likely being inhibited?
- Adenylyl cyclase
- Phosphodiesterase (Correct answer)
- Protein kinase A
- Guanylyl cyclase
Correct answer: Phosphodiesterase
Phosphodiesterase degrades cAMP; inhibiting it allows cAMP to accumulate, mimicking the effect of receptor activation.
Question 6: Which DNA repair mechanism corrects base mismatches introduced during DNA replication?
- Nucleotide excision repair
- Base excision repair
- Mismatch repair (Correct answer)
- Homologous recombination
Correct answer: Mismatch repair
Mismatch repair (MMR) recognizes and corrects nucleotide mismatches and small insertion/deletion loops arising during replication.
Question 7: In flow cytometry, what does the FSC (forward scatter) parameter primarily indicate?
- Cell granularity
- Cell surface marker expression
- Cell size (Correct answer)
- Mitochondrial membrane potential
Correct answer: Cell size
Forward scatter correlates with cell size because larger cells deflect more laser light in the forward direction.
STP Biomedical Sciences 3
During oxidative phosphorylation, at which complex does electron transport directly drive proton pumping into the intermembrane space?