Lysosomes are membranous bags of hydrolytic enzymes used for the controlled intracellular

10 Lysosomes are membranous bags of hydrolytic enzymes used for the controlled intracellular

digestion of macromolecules. They contain about 40 types of hydrolytic enzymes, including proteases, nucleases, glycosidases, lipases, phospholipases, phosphatases, and sulfatases. All are acid hydrolases. For optimal activity they require an acid environment, and the lysosome provides this by maintaining a pH of about 5 in its interior. In this way the contents of the cytosol are doubly protected against attack by the cell's own digestive system. The membrane of the lysosome normally keeps the digestive enzymes out of the cytosol, but even if they should leak out, they can do little damage at the cytosolic pH of about 7.2.

Like all other intracellular organelles, the lysosome not only contains a unique collection of enzymes, but also has a unique surrounding membrane. Transport proteins in this membrane allow the final products of the digestion of macromolecules, such as amino acids, sugars, and nucleotides, to be transported to the cytosol, from where they can be either excreted or reutilized by the cell. An H + pump in the lysosomal membrane utilizes the energy of ATP hydrolysis to pump H + into the lysosome, thereby maintaining the lumen at its acidic pH (Figure 13-17). Most of the lysosomal membrane proteins are unusually highly glycosylated, which is thought to help protect them from the lysosomal proteases in the lumen.

As we discuss later, endocytosed materials are initially delivered to organelles called endosomes before being delivered to lysosomes. Endosomes also have H + pumps that keep their lumen at a low pH, although not as low as that of lyso-somes (Figure 13-18). We shall see that these pH

differences are often used to load and unload cargo molecules from their receptors during vesicular transport along the endocytic pathway.

Ly s o s o m e s Are He t e ro g e n e o u s 11

Lysosomes were initially discovered by biochemical fractionations of cell extracts; only later were they seen clearly in the electron microscope. They are extraordinarily diverse in shape and size but can be identified as members of a single family of organelles by histochemistry, using the precipitate produced by the action of an acid hydrolase on its substrate to show which organelles contain the enzyme (Figure 13-19). By this criterion, lysosomes are found in all eucaryotic cells.

The heterogeneity of lysosomal morphology contrasts with the relatively uniform structures of most other cellular organelles. The diversity reflects the wide variety of digestive functions mediated by acid hydrolases, including the breakdown of intra- and extracellular debris, the destruction of phagocytosed microorganisms, and the production of nutrients for the cell. For this reason lysosomes are sometimes viewed as a heterogeneous collection of distinct organelles whose common feature is a high content of hydrolytic enzymes. It is especially hard to apply a narrower definition than this in plant cells, as we see next.

P la n t a n d Fu n g a l Va c u o le s Are Re m a rk a b ly Ve rs a t ile

Ly s o s o m e s 12

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 13-20). Vacuoles are related to lysosomes of animal cells, containing a variety of hydrolytic enzymes, but their functions are remarkably diverse. The plant vacuole can act as a storage organelle for nutrients and for waste products, as a degradative compartment, as an economical way of increasing cell size (Figure 13-21), and as a controller of turgor pressure (the osmotic pressure that pushes outward on the cell wall and keeps the plant from wilting). Different vacuoles with distinct functions (for example, digestion and storage) are often present in the same cell.

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 increased transport of H + into the vacuole so as to keep the pH in the cytosol constant. Similarly, many plant cells maintain an almost constant

turgor pressure in the face of large changes in the tonicity of the fluid in their immediate environment. They do so by changing the osmotic pressure of the cytosol and vacuolein part by the controlled breakdown and resynthesis of polymers such as polyphosphate in the vacuole and in part by altering rates of transport of sugars, amino acids, and other metabolites across the plasma membrane and the vacuolar membrane. The turgor pressure controls these fluxes by regulating the activities of the distinct sets of transporters in each lipid bilayer.

Substances stored in plant vacuoles in different species range from rubber to opium to the flavoring of garlic. Often, the 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, the proteins are hydrolyzed and the mobilized amino acids provide a food supply for the developing embryo. Anthocyanin pigments that are stored in vacuoles color the petals of many flowers to attract pollinating insects, while noxious molecules that are released from vacuoles when a plant is eaten or damaged provide a defense against predators.

Ma t e ria ls Are D e liv e re d t o Ly s o s o m e s b y Mu lt ip le P a t h w a y s