For animation of how materials pass from ER to and through Golgi click here. It is clear that newly made protein and lipid passes through the Golgi from the cis face to the trans face, and that vesicles from one stack can fuse with another in vitro . Vesicles carry newly made proteins to next part of Golgi for additional modifications. Each part of the Golgi holds on to its characteristic enzymes, and proteins pass through. Older texts generally assume this model is correct.
Vesicles carry enzymes to modify and sort proteins.
The enzyme composition of each individual sac is constantly changing as it ages and passes from cis to trans face. Recent evidence favors this model. See Science article for an extended discussion. O glycosylation of glycoproteins. Assemble sugars of proteoglycans 4.