protein secretory pathway
/ PROH-teen sih-KREE-tor-ee PATH-way /
How does a protein made deep inside a cell end up out on the cell surface, or exported entirely — say, a digestive enzyme dumped into your gut or a hormone released into the blood? It follows a fixed delivery route, like a package moving through sorting centers from factory to doorstep. That route is the protein secretory pathway.
The pathway is the ordered journey: a ribosome bound to the rough ER builds the protein and threads it into the ER, where it folds and gets initial sugar tags; transport vesicles carry it to the Golgi apparatus, which finishes modifying, sorts, and re-packages it; final vesicles then travel to the plasma membrane and fuse with it, releasing the cargo outside (exocytosis) or inserting it into the membrane. The short summary is ER to Golgi to vesicle to cell surface. Proteins destined for lysosomes branch off this same pathway at the Golgi.
This pathway is how cells supply the outside world and their own surface: it makes the antibodies, hormones, digestive enzymes, mucus, and signaling molecules that let a body function. It is also why the organelles of the endomembrane system are not separate gadgets but a coordinated production line. Proteins meant to stay in the cytosol skip this route entirely — they are made on free ribosomes and never enter the ER.
An antibody made by a plasma cell is built in the ER, decorated and sorted in the Golgi, packed into a vesicle, and finally squirted out of the cell — a textbook trip through the secretory pathway.
Antibody export traces the whole ER-Golgi-vesicle-surface route end to end.
Not every protein takes this route — only those entering the ER (membrane, lysosomal, and exported proteins). Cytosolic and nuclear proteins are made on free ribosomes and never travel it.