B2.2 Organelles and compartmentalization

Insulin is a secreted protein, meaning it must leave the cell. Secreted proteins are synthesized by rER-bound ribosomes, not free ribosomes.

As insulin is translated, the growing polypeptide is threaded into the lumen of the rough ER, where it undergoes folding and glycosylation. It is then packaged into transport vesicles.

Transport vesicles carry insulin to the cis face of the Golgi apparatus, where it is further modified, sorted, and packaged into secretory vesicles at the trans face.

The complete correct sequence is: rER-bound ribosome → rER lumen → vesicle → Golgi apparatus → secretory vesicle → exocytosis. This matches the second option.

The question asks for the correct secretory pathway for a protein destined to leave the cell (insulin). The key distinguishing feature is where synthesis begins.

Free ribosomes produce proteins for internal cytoplasmic use, not for secretion. Insulin would never be synthesized on a free ribosome.

Proteins are not made in the nucleus — the nucleus is the site of transcription, not translation. This pathway is biologically impossible.

After synthesis, secretory proteins enter the rER lumen, not the cytoplasm. They also do not travel back to the nucleus.

The remaining option — rER-bound ribosome → rER lumen → vesicle → Golgi apparatus → secretory vesicle → exocytosis — correctly describes the secretory pathway.

Lysosomes maintain an internal pH of approximately 4.5–5.0, which is achieved by proton pumps in the lysosomal membrane. This acidic environment is essential for activating hydrolytic enzymes.

Enzymes have a specific pH optimum at which they function. Lysosomal enzymes are optimally active at acidic pH. The cytoplasm has a pH of approximately 7.2, at which these enzymes would be largely inactive.

The lysosomal membrane acts as a barrier, isolating the acidic interior from the neutral cytoplasm. Even if small amounts of enzyme leaked out, they would be unable to function at cytoplasmic pH.

The correct explanation is that lysosomal hydrolytic enzymes require acidic pH and are inactive at cytoplasmic pH — this is compartmentalization protecting the cell.

The question asks why lysosomes do not damage the cytoplasm — the answer must relate to how the dangerous enzymes are kept under control.

This is the opposite of the truth. Lysosomal enzymes are specifically adapted for acidic pH, not neutral pH.

There is no known mechanism by which the lysosomal membrane destroys enzymes that have escaped. This option describes a fictional process.

While lysosomes do play a role in apoptosis, they are continuously active in normal intracellular digestion. This option is too restrictive and inaccurate.

The correct answer is that lysosomal hydrolytic enzymes require acidic pH (~4.5–5.0) and are inactive at cytoplasmic pH (~7.2) — a classic example of compartmentalization.