Question 1

Which of the following best describes the primary function of the endomembrane system in eukaryotic cells?

Accepted Answer

It coordinates the synthesis, processing, and trafficking of proteins and lipids within the cell.

Question 2

A liver cell exposed to high levels of a hydrophobic drug will likely show an increase in which organelle, and why?

Accepted Answer

Smooth ER, because it contains cytochrome P-450 enzymes that hydroxylate drugs to increase their solubility.

Question 3

Which of the following reactions represents the hydroxylation of a drug by the cytochrome P-450 system in the smooth ER?

Accepted Answer

$$\mathrm{RH + NAD(P)H + H^+ + O_2$ \rightarrow ROH + NAD(P)^+ + H_2O}$$

Question 4

A mutation that inactivates the enzyme glucose-6-phosphatase in liver cells would most directly impair which process?

Accepted Answer

Breakdown of glycogen to release free glucose into the blood.

Question 5

Which of the following best explains why muscle cells have a specialized form of smooth ER called the sarcoplasmic reticulum?

Accepted Answer

To store and release calcium ions required for muscle contraction.

Question 6

Which enzyme catalyzes the committed step in cholesterol biosynthesis and is targeted by statin drugs?

Accepted Answer

Hydroxymethylglutaryl-CoA reductase (HMG-CoA reductase)

Question 7

Which of the following statements about membrane biosynthesis in the ER is correct?

Accepted Answer

Phospholipids are synthesized on the cytosolic side and then transferred to the lumenal side by flippases.

Question 8

A protein destined for secretion from a eukaryotic cell is most likely to be transiently found in which of the following compartments?

Accepted Answer

Anterograde transition vesicles

Question 9

Which of the following best describes the role of the signal recognition particle (SRP) during cotranslational import?

Accepted Answer

It recognizes the ER signal sequence and directs the ribosome to the ER membrane.

Question 10

Which of the following is a key difference between anterograde and retrograde transport in the Golgi apparatus?

Accepted Answer

Anterograde transport moves materials toward the plasma membrane, while retrograde transport returns vesicles to the ER.

Question 11

Which of the following steps occurs first during N-linked glycosylation of a protein in the ER?

Accepted Answer

Attachment of the oligosaccharide to a dolichol phosphate on the cytosolic side of the ER membrane.

Question 12

A mutation that prevents the function of flippases in the ER membrane would most directly affect which process?

Accepted Answer

Distribution of phospholipids between the cytosolic and lumenal leaflets of the ER membrane.

Question 13

Which of the following best describes the fate of a misfolded protein in the ER?

Accepted Answer

It is exported to the cytosol for degradation by proteasomes.

Question 14

Which of the following is NOT a function of the rough ER?

Accepted Answer

Synthesis of steroid hormones.

Question 15

A protein contains a specific amino acid sequence that directs it to the lysosome. This sequence is an example of:

Accepted Answer

A protein tag

Question 16

Which of the following best explains why the ER and Golgi apparatus are considered functionally linked?

Accepted Answer

Proteins and lipids synthesized in the ER are further processed, sorted, and packaged in the Golgi for their final destinations.

Question 17

Which of the following best describes the signal hypothesis for cotranslational import?

Accepted Answer

Polypeptides destined for the ER contain an N-terminal signal sequence that directs the ribosome to the ER membrane.

Question 18

Which of the following is a direct consequence of an imbalance between anterograde and retrograde transport in the endomembrane system?

Accepted Answer

Depletion or accumulation of membrane components at the plasma membrane.

Question 19

Which of the following best describes the role of molecular chaperones in the ER?

Accepted Answer

They facilitate the folding and assembly of newly synthesized polypeptides.

The Endomembrane System: Structure, Function, and Protein Trafficking

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