Figure 4.8: The Endomembrane System Membranes of the nucleus, endoplasmic reticulum (ER), and Golgi apparatus form a network that is connected by vesicles. Parts of the membrane move between these organelles. Membrane synthesized in the smooth ER becomes sequentially part of the rough ER, then the Golgi apparatus, then vesicles formed from the Golgi apparatus. These vesicles may eventually fuse with, and become part of, the cell membrane.