The heterogeneous morphology of lysosomes contrasts with the relatively uniform structures of many other cell organelles. The diversity reflects the wide variety of digestive functions that acid hydrolases mediate, including the breakdown of intraand extracellular debris, the destruction of phagocytosed microorganisms, and the production of nutrients for the cell. Their morphological diversity, however, also reflects the way lysosomes form. Late endosomes containing material received from both the plasma membrane by endocytosis and newly synthesized lysosomal hydrolases fuse with preexisting lysosomes to form structures that are sometimes referred to as endolysosomes, which then fuse with one another (Figure 13–39). When the majority of the endocytosed material within an endolysosome has been digested so that only resistant or slowly digestible residues remain, these organelles become “classical” lysosomes. These are relatively dense, round, and small, but they can enter the cycle again by fusing with late endosomes or endolysosomes. Thus, there is no real distinction between endolysosomes and lysosomes: they are the same except that they are in different stages of a maturation cycle. For this reason, lysosomes are sometimes viewed as a heterogeneous collection of distinct organelles, the common feature of which is a high content of
Figure 13–38 Histochemical visualization of lysosomes. These electron micrographs show two sections of a cell stained to reveal the location of acid phosphatase, a marker enzyme for lysosomes. The larger membrane-enclosed organelles, containing dense precipitates of lead phosphate, are lysosomes. Their diverse morphology reflects variations in the amount and nature of the material they are digesting. The precipitates are produced when tissue fixed with glutaraldehyde (to fix the enzyme in place) is incubated with a phosphatase substrate in the presence of lead ions. red arrows in the top panel indicate two small vesicles thought to be carrying acid hydrolases from the golgi apparatus. (courtesy of daniel s. friend.) 0.2–0.5 µm pH~7.2 pH~5.0 ACID HYDROLASES: nucleases proteases glycosidases lipases phosphatases sulfatases phospholipases CYTOSOL + H+ pump ATPADPPi H+ Figure 13–37 Lysosomes. The acid hydrolases are hydrolytic enzymes that are active under acidic conditions. an h+ aTpase in the membrane pumps h+ into the lysosome, maintaining its lumen at an acidic ph.
digestion of contents 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, 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.