3.4—rER, Golgi apparatus and protein transport

Ribosomes on rough endoplasmic reticulum make proteins that are destined for secretion, a cell membrane or another membrane-bound compartment. The RER folds and processes the new polypeptide in its lumen.

Transport vesicles carry the protein from the RER to the Golgi apparatus. The Golgi modifies and sorts it, then sends it in vesicles to its destination: outside the cell, into a lysosome or to another membrane system.

Insulin is a useful model of the pathway: a ribosome makes the polypeptide, the RER and Golgi process and package it, and a vesicle fuses with the cell-surface membrane to release it by exocytosis.

Free cytoplasmic ribosomes generally make proteins used inside the cytoplasm. The RER is not the same as the Golgi: one is the ribosome-studded production/folding site, the other the modification and sorting centre.