Because many types of molecules are unable to cross cell membranes on their own, the endomembrane system divides the interior of a cell into two distinct “worlds,” one inside the spaces defined by these membranes and one outside these spaces. A molecule within the interior of the ER can stay in the ER or end up in the interior of the Golgi apparatus or even outside the cell by the budding off and fusing of a vesicle between these organelles. Similarly, a molecule associated with the ER membrane can move to the Golgi membrane or the plasma membrane by vesicle transport. Molecules in the cytosol are in a different physical space, separated by membranes of the endomembrane system. This physical separation allows specific functions to take place within the spaces defined by the membranes and also within the membrane itself.

In spite of forming a continuous and interconnected system, the various compartments have unique properties and maintain distinct identities determined in part by which lipids and proteins are present in their membranes.

Vesicles not only bud off from and fuse with organelles but also with the plasma membrane. When a vesicle fuses with the plasma membrane, the process is called exocytosis. It provides a way for a vesicle to empty its contents to the extracellular space or to deliver proteins embedded in the vesicle membrane to the plasma membrane (Fig. 5.18). The process also works in reverse: A vesicle can bud off from the plasma membrane, enclosing material from outside the cell and bringing it into the cell interior. This process is called endocytosis. Together, exocytosis and endocytosis provide a way to move material into and out of cells without passing through the cell membrane.

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