How are proteins exported from the cell?

Proteins are exported from the cell primarily through exocytosis, which is part of the secretory pathway. In this process, proteins synthesized in the endoplasmic reticulum (ER) are transported to the Golgi apparatus for further processing and sorting. Once these proteins are properly modified and packaged into vesicles, they travel to the plasma membrane.

At the plasma membrane, vesicles containing these proteins fuse with the membrane, allowing the proteins to be released outside the cell. This route ensures that proteins are exported in a regulated and controlled manner, allowing cells to secrete various substances, such as hormones, neurotransmitters, and enzymes, into the extracellular environment.

The alternative options do not adequately describe the mechanism of protein export. Passive diffusion does not apply as proteins are generally too large and polar to cross the lipid bilayer without assistance. Endocytosis involves the internalization of substances into cells rather than their export. Direct passage through the cell membrane is not feasible for intact proteins due to their size and structure, which prevents them from simply diffusing across the lipid bilayer. Thus, exocytosis represents the accurate mechanism by which proteins are exported from the cell.