AP Biology: Unit 2, Cell Structure & Compartmentalization (Drill 5)

Question 1. The researcher observes that partially glycosylated proteins accumulate in the rough ER after brefeldin A treatment. Which of the following best explains this observation in terms of intracellular transport mechanisms?

• A) Brefeldin A causes ribosomes to detach from the rough ER, halting synthesis of proteins destined for secretion.
• B) Without vesicle transport, proteins are redirected through retrograde transport back to the nucleus for degradation.
• C) Proteins synthesized into the ER lumen cannot exit without vesicle-mediated anterograde transport to the Golgi, so they accumulate when that pathway is blocked. ✓
• D) Brefeldin A inhibits the signal recognition particle, preventing proteins from entering the ER lumen in the first place.

Explanation: Proteins enter the ER lumen co-translationally and depend on COPII vesicles for anterograde transport to the Golgi. Brefeldin A inhibits vesicle budding from the ER, blocking this step, so proteins are trapped in the ER lumen. Ribosomes are not caused to detach by brefeldin A (A). Retrograde transport moves material from Golgi back to ER -- not proteins back to the nucleus (B). Brefeldin A targets vesicle budding from the ER, not the SRP-mediated targeting step (D).

Question 2. The passage describes how the endomembrane system maintains distinct compartments with specialized conditions. A student argues that cells could carry out all reactions in a single aqueous compartment without loss of function. Which of the following aspects of the endomembrane system described in the passage best refutes this claim?

• A) The Golgi apparatus provides a physical scaffold that keeps organelles in their correct positions, rather than modifying, sorting, and packaging proteins for use elsewhere in the cell or export as needed.
• B) Reactions such as glycosylation in the ER lumen and detoxification in the smooth ER require distinct chemical environments that cannot coexist in a single compartment without interfering with one another. ✓
• C) Compartmentalization increases the number of ribosomes available for protein synthesis.
• D) Without separate compartments, vesicle transport between organelles would be impossible.

Explanation: The passage explicitly describes compartments serving distinct functions -- the rough ER for protein synthesis and entry into the secretory pathway, the smooth ER for lipid synthesis and detoxification, and the Golgi for further modification. These require different conditions (pH, enzyme sets, ion concentrations) that would be incompatible in a single compartment. A describes structural positioning, not chemical incompatibility. C is incorrect -- compartmentalization does not increase ribosome number. D is circular reasoning.

Question 3. The smooth ER is particularly abundant in liver cells. Which of the following best explains this observation?

• A) Liver cells require large amounts of ATP produced by the smooth ER.
• B) Liver cells must synthesize large quantities of ribosomal RNA.
• C) Liver cells carry out extensive lipid synthesis and detoxification of compounds such as alcohol and drugs. ✓
• D) Liver cells are responsible for producing secreted proteins such as antibodies.

Explanation: The smooth ER is the site of lipid synthesis and detoxification reactions, both of which are primary functions of liver cells. The smooth ER does not produce ATP (A). Ribosomal RNA synthesis occurs in the nucleolus (B). Secreted proteins are produced on the rough ER, not the smooth ER, and antibodies are made by plasma cells, not liver cells (D).

Question 4. A researcher inhibits the SNARE proteins required for vesicle fusion at the Golgi. Which of the following steps in the secretory pathway is directly disrupted?

• A) Synthesis of the protein on ribosomes attached to the rough ER
• B) Entry of the newly synthesized protein into the ER lumen
• C) Delivery of ER-derived vesicles and their contents into the Golgi apparatus ✓
• D) Packaging of finished proteins into secretory vesicles at the trans-Golgi network

Explanation: SNARE proteins mediate membrane fusion between vesicles and their target membranes. Inhibiting SNAREs at the Golgi prevents ER-derived vesicles from fusing with the cis-Golgi, blocking cargo delivery. Ribosome-mediated synthesis (A) and co-translational entry into the ER lumen (B) are upstream and do not require Golgi SNAREs. Packaging at the trans-Golgi network (D) is downstream and would only be affected secondarily once cargo fails to arrive.

Question 5. A mutation eliminates the signal sequence on a protein normally destined for the ER lumen. Which of the following is the most likely outcome?

• A) The protein is synthesized on the rough ER but is misfolded, recognized by ER quality control machinery, and degraded in the ER lumen.
• B) The protein is synthesized by free ribosomes in the cytoplasm and remains in the cytosol, bypassing the endomembrane system entirely. ✓
• C) The protein is synthesized normally but is routed to the lysosome instead of the ER lumen.
• D) Translation does not initiate because ribosomes require a signal sequence to begin synthesis.

Explanation: The signal sequence is recognized by the signal recognition particle (SRP), which docks the ribosome to the rough ER. Without the signal sequence, the SRP is never recruited, the ribosome translates as a free ribosome in the cytoplasm, and the protein is deposited in the cytosol. Choice A is a strong distractor -- misfolding and ER-associated degradation are real quality control mechanisms, but they apply to proteins that have already entered the ER lumen. Without a signal sequence, the protein never enters the ER, so ER quality control is never engaged. C incorrectly implies the protein still enters the secretory pathway. D is incorrect -- the signal sequence is not required for translation initiation.