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

TRANSPORT FROM THE TRANS GOLGI NETWORK TO lysosoMES
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nucleus
vacuole
cell wall
plasma
membrane
strands of
cytoplasm
amino acids, and other metabolites across the plasma membrane and the vacuolar
membrane. The turgor pressure regulates the activities of distinct transporters
in each membrane to control these fluxes.
Humans often harvest substances stored in plant vacuoles—from rubber to
opium to the flavoring of garlic. Many stored products have a metabolic function.
Proteins, for example, can MBoC6 be preserved m13.40/13.41 for years in the vacuoles of the storage
cells of many seeds, such as those of peas and beans. When the seeds germinate,
these proteins are hydrolyzed, and the resulting amino acids provide a food supply
for the developing embryo. Anthocyanin pigments stored in vacuoles color
the petals of many flowers so as to attract pollinating insects, while noxious molecules
released from vacuoles when a plant is eaten or damaged provide a defense
against predators.
Figure 13–41 The role of the vacuole
in controlling the size of plant cells. A
plant cell can achieve a large increase in
volume without increasing the volume of
the cytosol. Localized weakening of the cell
wall orients a turgor-driven cell enlargement
that accompanies the uptake of water
into an expanding vacuole. The cytosol
is eventually confined to a thin peripheral
layer, which is connected to the nuclear
region by strands of cytosol stabilized by
bundles of actin filaments (not shown).
Multiple Pathways Deliver Materials to Lysosomes
Lysosomes are meeting places where several streams of intracellular traffic converge.
A route that leads outward from the ER via the Golgi apparatus delivers
most of the lysosome’s digestive enzymes, while at least four paths from different
sources feed substances into lysosomes for digestion.
The best studied of these degradation paths is the one followed by macromolecules
taken up from extracellular fluid by endocytosis. A similar pathway found
in phagocytic cells, such as macrophages and neutrophils in vertebrates, is dedicated
to the engulfment, or phagocytosis, of large particles and microorganisms
to form phagosomes. A third pathway called macropinocytosis specializes in the
nonspecific uptake of fluids, membrane, and particles attached to the plasma
membrane. We will return to discuss these pathways later in the chapter. A fourth
pathway called autophagy originates in the cytoplasm of the cell itself and is used
to digest cytosol and worn-out organelles, as we discuss next. The four paths to
degradation in lysosomes are illustrated in Figure 13–42.
EXTRACELLULAR FLUID
CYTOSOL
bacterium
phagosome
phagocytosis
plasma
membrane
early endosome
endocytosis
LATE
ENDOSOME
LYSOSOME
mitochondrion
macropinocytosis
autophagy
autophagosome
Figure 13–42 Four pathways to
degradation in lysosomes. Materials in
each pathway are derived from a different
source. Note that the autophagosome has
a double membrane. In all cases, the final
step is the fusion with lysosomes.