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,
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.
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.) 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 be preserved 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.
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.