The interior space, or lumen, of each membrane-enclosed compartment along the secretory and endocytic pathways is equivalent to the lumen of most other membrane-enclosed compartments and to the cell exterior, in the sense that proteins can travel in this space without having to cross a membrane as they are passed from one compartment to another by means of numerous membrane-enclosed transport containers. These containers are formed from the donor compartment and are either small, spherical vesicles, larger irregular vesicles, or tubules. We shall use the term transport vesicle to apply to all forms of these containers.
Within a eukaryotic cell, transport vesicles continually bud off from one membrane and fuse with another, carrying membrane components and soluble lumenal molecules, which are referred to as cargo (Figure 13–2). This vesicular traffic flows along highly organized, directional routes, which allow the cell to secrete, eat, and remodel its plasma membrane and organelles. The secretory pathway leads outward from the endoplasmic reticulum (ER) toward the Golgi apparatus and cell surface, with a side route leading to lysosomes, while the endocytic pathway leads inward from the plasma membrane. In each case, retrieval pathways
The Molecular MechanIsMs of MeMbrane TransporT and The MaInTenance of coMparTMenTal dIversITy
TransporT froM The er Through The  golgI apparaTus
TransporT froM The Trans golgI neTwork To lysosoMes
TransporT InTo The cell froM The plasMa MeMbrane: endocyTosIs
TransporT froM The Trans golgI neTwork To The cell exTerIor: exocyTosIs
Figure 13–1 Exocytosis and endocytosis. (a) In exocytosis, a transport vesicle fuses with the plasma membrane. Its content is released into the extracellular space, while the vesicle membrane (red) becomes continuous with the plasma membrane. (b) In endocytosis, a plasma membrane patch (red) is internalized, forming a transport vesicle. Its content derives from the extracellular space.
696 Chapter 13:  Intracellular Membrane Traffic balance the flow of membrane between compartments in the opposite direction, Figure 13–2 Vesicle transport. Transport vesicles bud off from one compartment bringing membrane and selected proteins back to the compartment of origin and fuse with another. as they do so, they (Figure 13–3).
carry material as cargo from the lumen To perform its function, each transport vesicle that buds from a compartment (the space within a membrane-enclosed must be selective. It must take up only the appropriate molecules and must fuse compartment) and membrane of the donor only with the appropriate target membrane. A vesicle carrying cargo from the ER compartment to the lumen and membrane of the target compartment, as shown.
to the Golgi apparatus, for example, must exclude most proteins that are to stay in the ER, and it must fuse only with the Golgi apparatus and not with any other organelle.
We begin this chapter by considering the molecular mechanisms of budding and fusion that underlie all vesicle transport. We then discuss the fundamental problem of how, in the face of this transport, the cell maintains the molecular and