STP Biomedical Sciences 3

Question 1

During oxidative phosphorylation, at which complex does electron transport directly drive proton pumping into the intermembrane space?

Accepted Answer

Complexes I, III, and IV

Answer

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?

Accepted Answer

Plasma cells derived from B cells

Answer

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?

Accepted Answer

The substrate concentration at half-maximum velocity

Answer

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?

Accepted Answer

Peroxisome

Answer

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?

Accepted Answer

Phosphodiesterase

Answer

Phosphodiesterase degrades cAMP; inhibiting it allows cAMP to accumulate, mimicking the effect of receptor activation.

(STP) Scientist Training Programme Practice Test