Protein Secretion via Endomembrane System

Steps:

• Proteins for secretion have signal peptides directing ribosomes to the rough ER for translation and entry into the ER lumen.
• In the rough ER, proteins fold and undergo initial glycosylation.
• Vesicles bud from the ER and fuse with the Golgi apparatus for further modification and sorting.
• Processed proteins move via secretory vesicles to fuse with the plasma membrane for exocytosis.

Why C is correct:

• Secreted proteins require the rough ER (structure 1) for synthesis and entry, and the Golgi (structure 3) for packaging, as defined by the endomembrane system's vesicular transport pathway.

Why the others are wrong:

• A includes 5 (likely smooth ER or vesicles, unnecessary for core synthesis/processing).
• B includes 4 (possibly nucleus or mitochondria, irrelevant to secretion pathway).
• D omits essential ER and Golgi, relying only on 5 (insufficient for full secretion).

Final answer: C