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 step in drug detoxification catalyzed by cytochrome P-450 enzymes?

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

During membrane biosynthesis, how are newly synthesized phospholipids distributed between the two leaflets of the ER membrane?

Accepted Answer

By enzymes called phospholipid translocators (flippases) that transfer phospholipids to the lumenal side.

Question 8

A protein is encoded by a nuclear gene and is constitutively secreted from cultured HeLa cells. In which compartment must this protein be transiently found?

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 the cotranslational import of a secretory protein?

Accepted Answer

The signal recognition particle (SRP) binds to the ER signal sequence on the nascent polypeptide.

Question 12

Which of the following is NOT a function of the rough endoplasmic reticulum?

Accepted Answer

Synthesis of steroid hormones

Question 13

A mutation in the gene encoding a flippase enzyme would most likely disrupt which cellular process?

Accepted Answer

Distribution of phospholipids between the two leaflets of the ER membrane

Question 14

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

Accepted Answer

It is exported to the cytosol and degraded by proteasomes.

Question 15

Which of the following is TRUE regarding glycosylation of proteins in the endomembrane system?

Accepted Answer

Initial steps occur on the cytosolic side of the ER membrane, and further processing occurs in the ER lumen and Golgi apparatus.

Question 16

A cell that produces large amounts of digestive enzymes for export would be expected to have an abundance of which organelle?

Accepted Answer

Rough endoplasmic reticulum

Question 17

Which of the following best explains why the ER is considered the primary source of membrane lipids in eukaryotic cells?

Accepted Answer

Fatty acids are synthesized in the cytoplasm and incorporated into the ER membrane, where phospholipids are assembled.

Question 18

Which of the following is a function of the Golgi complex?

Accepted Answer

Sorting and packaging proteins for transport to their final destinations

Question 19

A protein destined for the lysosome is synthesized on the rough ER. Which feature ensures it reaches the correct destination?

Accepted Answer

A specific targeting tag, such as a mannose-6-phosphate group, directs it to the lysosome.

The Endomembrane System: Structure, Function, and Protein Trafficking

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