Question 1

Which of the following best describes the primary function of the rough endoplasmic reticulum (rough ER) in eukaryotic cells?

Accepted Answer

Protein synthesis and initial glycosylation

Question 2

A liver cell is exposed to a hydrophobic drug. Which process in the smooth ER will most likely increase the drug's solubility for excretion?

Accepted Answer

Hydroxylation by cytochrome P-450 monooxygenases

Question 3

Which equation correctly represents the hydroxylation reaction catalyzed by cytochrome P-450 in drug detoxification?

Accepted Answer

$$NAD(P)H + H^+ + O_2$ \rightarrow ROH + NAD(P)^+ + H_2O$$

Question 4

A mutation in the gene encoding glucose-6-phosphatase would most directly affect which process in the smooth ER?

Accepted Answer

Glycogen breakdown and glucose release

Question 5

Which of the following is a key difference between the stationary cisternae model and the cisternal maturation model of Golgi trafficking?

Accepted Answer

In the cisternal maturation model, cisternae are transient and change identity over time

Question 6

Which of the following tags is most likely to retain a protein in the ER until it is fully assembled?

Accepted Answer

RXR

Question 7

A muscle cell's sarcoplasmic reticulum is specialized for which function?

Accepted Answer

Calcium ion storage and release

Question 8

Which of the following best explains why cells producing steroid hormones have extensive smooth ER networks?

Accepted Answer

Smooth ER is the site of steroid biosynthesis

Question 9

During co-translational import into the ER, which sequence is recognized by the signal recognition particle (SRP)?

Accepted Answer

An N-terminal ER signal sequence

Question 10

Which of the following is TRUE regarding the function of molecular chaperones such as BiP in the ER?

Accepted Answer

They facilitate proper folding of polypeptides and prevent aggregation

Question 11

A protein destined for the lysosome is synthesized in the rough ER. Which sequence of organelles does it follow before reaching its final destination?

Accepted Answer

Rough ER → Golgi apparatus → lysosome

Question 12

Which enzyme catalyzes the committed step in cholesterol synthesis in the smooth ER?

Accepted Answer

Hydroxymethylglutaryl-CoA reductase (HMG-CoA reductase)

Question 13

Which of the following best describes the role of flippases in membrane biosynthesis?

Accepted Answer

They transfer phospholipids from the cytosolic to the lumenal leaflet of the ER membrane

Question 14

A protein with the C-terminal sequence KDEL is found in the Golgi. What is its most likely fate?

Accepted Answer

It will be retrieved and returned to the ER

Question 15

Which process is responsible for balancing the flow of lipids toward the plasma membrane during anterograde transport?

Accepted Answer

Retrograde transport from Golgi to ER

Question 16

Which of the following is a plausible consequence of a defect in the SRP receptor on the ER membrane?

Accepted Answer

Ribosome-mRNA-polypeptide complexes cannot dock to the ER for co-translational import

Question 17

Which of the following best explains the role of start-transfer and stop-transfer sequences in the insertion of integral membrane proteins?

Accepted Answer

They determine the orientation and number of transmembrane domains

Question 18

A mutation causes the RXR retention tag of the NMDA receptor to be unmasked prematurely. What is the most likely cellular consequence?

Accepted Answer

NMDA receptor subunits will be retained in the ER until fully assembled

Question 19

Which of the following organelles is NOT part of the endomembrane system?

Accepted Answer

Mitochondria

Question 20

Which protein is responsible for packaging proteins with retrieval tags into vesicles for return to the ER?

Accepted Answer

COPI

The Endomembrane System: Structure, Function, and Trafficking in Eukaryotic Cells

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