Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

748 Chapter 13: Intracellular Membrane Traffic
WOUND
(A) CYTOKINESIS (B) PHAGOCYTOSIS (C) PLASMA MEMBRANE (D) CELLULARIZATION
REPAIR
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 μm 2 of vesicle membrane
is inserted into the apical plasma membrane (whose area is only 30 μm 2 )
when the cell is stimulated to secrete.
Control of membrane traffic thus has a major MBoC6 role in m13.70/13.73
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.
Figure 13–70 Four examples of regulated
exocytosis leading to plasma membrane
enlargement. The vesicles fusing with the
plasma membrane during cytokinesis (A)
and phagocytosis (B) are thought to be
derived from endosomes, whereas those
involved in wound repair (C) may be derived
from plasma membranes. The vast amount
of new plasma membrane inserted during
cellularization in a fly embryo occurs by the
fusion of cytoplasmic vesicles (D).
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).
Many animal cells, especially those subjected to mechanical stresses, frequently
experience small ruptures in their plasma membrane. In a remarkable
process thought to involve both homotypic vesicle–vesicle fusion and exocytosis,
a temporary cell-surface patch is quickly fashioned from locally available internal-membrane
sources, such as lysosomes. In addition to providing an emergency
barrier against leaks, the patch reduces membrane tension over the wounded
area, allowing the bilayer to flow back together to restore continuity and seal the
puncture. This membrane repair process, the fusion and exocytosis of vesicles is
triggered by the sudden increase of Ca 2+ , which is abundant in the extracellular
space and rushes into the cell as soon as the plasma membrane is punctured.
Figure 13–70 shows four examples in which regulated exocytosis leads to plasma
membrane expansion.
Polarized Cells Direct Proteins from the Trans Golgi Network to the
Appropriate Domain of the Plasma Membrane
Most cells in tissues are polarized, with two or more molecularly and functionally
distinct plasma membrane domains. This raises the general problem of how the