The cell membrane, ER, Golgi complexes, and nuclear membrane together make up a topologically connected set of membranes with a very large surface area.

Connections between these organelles are made and broken as their membranes divide, fuse, and pinch off vesicles. Together they form an integrated whole with an inside and an outside. The inner surface of the cell membrane, the ribosome surface of the ER, the outer surface of the Golgi complex and nuclear envelope, and even the inside of the nucleus through the pores, all are topologically inside, and at times are connected.

In contrast, the outer cell surface, the side of the ER without ribosomes, the inner region of the Golgi complex, and the space between the two layers of the nuclear envelope all are outside, in the same sense of lacking access to the cytoplasm.