Essential Cell Biology 5th edition

524 CHAPTER 15 Intracellular Compartments and Protein Transport
Figure 15−32 Specialized phagocytic
cells can ingest other cells. (A) Electron
micrograph of a phagocytic white blood
cell (a neutrophil) ingesting a bacterium,
which is in the process of dividing.
(B) Scanning electron micrograph showing
a macrophage engulfing a pair of red
blood cells. The lines point to the edges of
the pseudopods that the phagocytic cells
are extending like collars to envelop their
targets. (A, courtesy of Dorothy F. Bainton;
B, courtesy of Jean Paul Revel.)
dividing
bacterium
plasma
membrane
pseudopods
edges of extending pseudopods
(A)
phagocytic
white blood cell
1 µm
(B)
5 µm
of microorganisms. Binding of antibody-coated bacteria to these receptors
induces the phagocytic cell to extend sheetlike projections of the
plasma membrane, called pseudopods, that engulf the bacterium (Figure
15−32A). These pseudopods fuse at their tips to form a phagosome,
which then fuses with a lysosome, where the microbe is destroyed. Some
pathogenic bacteria have evolved tricks for subverting the system: for
example, Mycobacterium tuberculosis, ECB5 E15.32/15.32
the agent responsible for tuberculosis,
can inhibit the membrane fusion that unites the phagosome with
a lysosome. Instead of being destroyed, the engulfed organism survives
and multiplies within the macrophage. Although the mechanism is not
completely understood, identifying the proteins involved will provide
therapeutic targets for drugs that could restore the macrophages’ ability
to eliminate the infection.
Phagocytic cells also play an important part in scavenging dead and
damaged cells and cell debris. Macrophages, for example, ingest more
than 10 11 worn-out red blood cells in the human body each day (Figure
15−32B).
Fluid and Macromolecules Are Taken Up by Pinocytosis
Eukaryotic cells continually ingest bits of their plasma membrane, along
with small amounts of extracellular fluid, in the process of pinocytosis.
The rate at which plasma membrane is internalized in pinocytic vesicles
varies from cell type to cell type, but it is usually surprisingly large.
A macrophage, for example, swallows 25% of its own volume of fluid
each hour. This means that it removes 3% of its plasma membrane each
minute, or 100% in about half an hour. Pinocytosis occurs more slowly in
fibroblasts, but even more rapidly in some phagocytic amoebae. Because
a cell’s total surface area and volume remain unchanged during this process,
as much membrane is being added to the cell surface by exocytosis
as is being removed by endocytosis (see Figure 15–19). It is not known
how eukaryotic cells maintain this remarkable balance.
Pinocytosis is carried out mainly by the clathrin-coated pits and vesicles
that we discussed earlier (see Figures 15–20 and 15–21). After they
pinch off from the plasma membrane, clathrin-coated vesicles rapidly