Figure 13–69 Scale models of a brain presynaptic terminal and a synaptic vesicle. The illustrations show sections through a pre-synaptic terminal (a; enlarged in b) and a synaptic vesicle (c) in which proteins and lipids are drawn to scale based on their known stoichiometry and either known or approximated structures. The relative localization of protein molecules in different regions of the presynaptic terminal was inferred from super-resolution imaging and electron microscopy. The model in (a) contains 300,000 proteins of 60 different kinds that vary in abundance from 150 copies to 20,000 copies. In the model in (c), only 70% of the membrane proteins present in the membrane are shown; a complete model would therefore show a membrane that is even more crowded than this picture suggests (Movie 13.7). each synaptic vesicle membrane contains 7000 phospholipid molecules and 5700 cholesterol molecules. each also contains close to  50 different integral membrane protein molecules, which vary widely in their relative abundance and together contribute about 600 transmembrane α helices. The transmembrane v-snare synaptobrevin is the most abundant protein in the vesicle (~70 copies per vesicle). by contrast, the v-aTpase, which uses aTp hydrolysis to pump h+ into the vesicle lumen, is present in 1–2 copies per vesicle. The h+ gradient provides the energy for neurotransmitter import by an h+/neurotransmitter antiport, which loads each vesicle with 1800 neurotransmitter molecules, such as glutamate, one of which is shown to scale. (a and b, from b.g. wilhelm et al., science 344:1023–1028, 2014. with permission from aaas;  c, adapted from s. Takamori et al., Cell 127:831–846, 2006. with permission from elsevier.) vesicle membrane are either recycled or shuttled to lysosomes for degradation. The amount of secretory vesicle membrane that is temporarily added to the plasma membrane can be enormous: in a pancreatic acinar cell discharging digestive enzymes for delivery to the gut lumen, about 900 μm2 of vesicle membrane is inserted into the apical plasma membrane (whose area is only 30 μm2) when the cell is stimulated to secrete.
Control of membrane traffic thus has a major role in maintaining the composition of the various membranes of the cell. To maintain each membrane-enclosed compartment in the secretory and endocytic pathways at a constant size, the balance between the outward and inward flows of membrane needs to be precisely regulated. For cells to grow, however, the forward flow needs to be greater than the retrograde flow, so that the membrane can increase in area. For cells to maintain a constant size, the forward and retrograde flows must be equal. We still know very little about the mechanisms that coordinate these flows.
some regulated exocytosis events serve to enlarge the plasma Membrane
An important task of regulated exocytosis is to deliver more membrane to enlarge the surface area of a cell’s plasma membrane when such a need arises. A spectacular example is the plasma membrane expansion that occurs during the cellularization process in a fly embryo, which initially is a syncytium—a single cell containing about 6000 nuclei surrounded by a single plasma membrane (see Figure 21–15). Within tens of minutes, the embryo is converted into the same number of cells. This process of cellularization requires a vast amount of new plasma membrane, which is added by a carefully orchestrated fusion of cytoplasmic vesicles, eventually forming the plasma membranes that enclose the separate cells. Similar vesicle fusion events are required to enlarge the plasma membrane when other animal cells or plant cells divide during cytokinesis (discussed in Chapter 17).