Once proteins are synthesized by ribosomes and the rER, they are transported to the Golgi apparatus where further processing takes place. This represents the end of the protein synthesis chain. The Golgi apparatus performs two main functions in relation to protein processing and packaging. First, it attaches sugars to proteins to form glycoproteins in a process called glycosylation. It receives the newly formed polypeptides from the rough ER. Due to resident glycosyltransferase enzymes, the Golgi can attach sugars to the proteins making glycoproteins. Second, the Golgi then condenses the proteins, packaging them into membrane bound secretion granules.
The Golgi apparatus has a distinct cup-shaped structure with cisternae, or flattened membranes, arranged in parallel sheets. Proteins enter the Golgi apparatus on the convex, or forming face, side and exit on the concave, or maturing face, side.