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

724 Chapter 13: Intracellular Membrane Traffic
hydrolase
intralumenal
vesicle
late endosome
endolysosome
Figure 13–39 A model for lysosome
maturation. Late endosomes fuse with
preexisting lysosomes (bottom arrow) or
preexisting endolysosomes (top arrow).
Endolysosomes eventually mature into
lysosomes as hydrolases complete the
digestion of their contents, which can include
intralumenal vesicles. Lysosomes also fuse
with phagosomes, as we discuss later.
digestion of
contents
lysosome
hydrolytically active, low-pH compartments
hydrolytic enzymes. It is especially hard to apply a narrower definition than this in
plant cells, as we discuss next.
Plant and Fungal Vacuoles Are Remarkably Versatile Lysosomes
Most plant and fungal cells (including yeasts) contain one or several very large,
MBoC6 m13.38/13.39
fluid-filled vesicles called vacuoles. They typically occupy more than 30% of the
cell volume, and as much as 90% in some cell types (Figure 13–40). Vacuoles are
related to animal cell lysosomes and contain a variety of hydrolytic enzymes, but
their functions are remarkably diverse. The plant vacuole can act as a storage
organelle for both nutrients and waste products, as a degradative compartment,
as an economical way of increasing cell size, and as a controller of turgor pressure
(the osmotic pressure that pushes outward on the cell wall and keeps the plant
from wilting) (Figure 13–41). The same cell may have different vacuoles with distinct
functions, such as digestion and storage.
The vacuole is important as a homeostatic device, enabling plant cells to withstand
wide variations in their environment. When the pH in the environment
drops, for example, the flux of H + into the cytosol is balanced, at least in part, by
an increased transport of H + into the vacuole, which tends to keep the pH in the
cytosol constant. Similarly, many plant cells maintain an almost constant turgor
pressure despite large changes in the tonicity of the fluid in their immediate environment.
They do so by changing the osmotic pressure of the cytosol and vacuole—in
part by the controlled breakdown and resynthesis of polymers such as
polyphosphate in the vacuole, and in part by altering the transport rates of sugars,
(A)
(B)
cell wall
vacuole
cytosol
chloroplasts
vacuole
tonoplast
10 µm
Figure 13–40 The plant cell vacuole.
(A) A confocal image of cells from an
Arabidopsis embryo that is expressing an
aquaporin—YFP (yellow fluorescent protein)
fusion protein in its tonoplast, or vacuole
membrane (green); the cell walls have been
false-colored orange. Each cell contains
several large vacuoles. (B) This electron
micrograph of cells in a young tobacco
leaf shows the cytosol as a thin layer,
containing chloroplasts, pressed against
the cell wall by the enormous vacuole.
(A, courtesy of C. Carroll and L. Frigerio,
based on S. Gattolin et al., Mol. Plant
4:180–189, 2011. With permission from
Oxford University Press; B, courtesy of
J. Burgess.)