Some cells make proteins to send out: digestive enzymes from the pancreas, mucus from the lining of the nose, antibodies from white blood cells. Those proteins follow one route through the cell, a connected set of membranes called the endomembrane system.
Key idea
A protein made for export follows a path: ribosomes on the rough ER build it into the ER, a vesicle carries it to the Golgi apparatus, the Golgi modifies and sorts it, and another vesicle carries it to the cell membrane, where it is released.
Why it works
The instructions start in the nucleus as DNA; a copy (mRNA) leaves through pores in the nuclear envelope. The ribosomes do the building. Ribosomes floating free in the cytoplasm make proteins that stay in the cytoplasm. Ribosomes stuck to the rough ER (that is what makes it "rough") feed their protein straight into the ER, which folds it. The Golgi works like a post office: it adds tags such as sugar chains and sends each protein to the right place. The smooth ER, with no ribosomes, makes lipids instead. Lysosomes are made by the same path: they are vesicles of digestive enzymes from the Golgi.
Worked example: Follow the label
In the early 1960s, George Palade's lab gave pancreas cells radioactive amino acids for a few minutes, then followed where the label went. In what order would it appear?
- Amino acids are built into proteins at the ribosomes on the rough ER, so the label shows up there first.
- Next it moves in vesicles to the Golgi.
- Then it appears in secretory vesicles near the cell membrane, and finally outside the cell.
Answer: Rough ER, Golgi, secretory vesicles, outside the cell
Common mistake
Sending proteins from the nucleus straight to the Golgi, or saying the Golgi makes proteins. The nucleus holds the instructions; ribosomes make the proteins; the ER and Golgi fold, tag and ship them. Check your path: it always passes through the ER before the Golgi, and vesicles carry the protein between each stop.