Membranes in eukaryotic cells are constantly forming, transforming from one type to another, fusing with one another, and breaking down. As we discussed in Chapter 5, fragments of membrane move, in the form of vesicles, from the endoplasmic reticulum (ER) to the Golgi apparatus, and from the Golgi apparatus to the cell membrane (see Figure 5.9). Secondary lysosomes form when primary lysosomes from the Golgi apparatus fuse with phagosomes from the cell membrane (see Figure 5.10).

Because all membranes appear similar under the electron microscope, and because they interconvert readily, we might expect all subcellular membranes to be chemically identical. However, that is not the case: there are major chemical differences among the membranes of even a single eukaryotic cell. Membranes are changed chemically when they form parts of certain organelles. In the Golgi apparatus, for example, the membranes of the cis face closely resemble those of the ER in chemical composition, but those of the trans face are more similar to the cell membrane.