Lysosomes Are Acidic Organelles That Contain a Battery of Degradative Enzymes

Lysosomes provide an excellent example of the ability of in-tracellular membranes to form closed compartments in which the composition of the lumen (the aqueous interior of the compartment) differs substantially from that of the surrounding cytosol. Found exclusively in animal cells,

Plasma membrane controls movement of molecules in and out of the cell and functions in cell-cell signaling and cell adhesion.

Mitochondria, which are surrounded by a double membrane, generate ATP by oxidation of glucose and fatty acids.

Lysosomes, which have an acidic lumen, degrade material internalized by the cell and worn-out cellular membranes and organelles.

Nuclear envelope, a double membrane, encloses the contents of the nucleus; the outer nuclear membrane is continuous with the rough ER.

Nucleolus is a nuclear subcompartment where most of the cell's rRNA is synthesized.

Nucleus is filled with chromatin composed of DNA and proteins; in dividing cells is site of mRNA and tRNA synthesis.

Smooth endoplasmic reticulum (ER) synthesizes lipids and detoxifies certain hydrophobic compounds.

Rough endoplasmic reticulum (ER) functions in the synthesis, processing, and sorting of secreted proteins, lysosomal proteins, and certain membrane.

9| Golgi complex processes and sorts secreted proteins, lysosomal proteins, and membrane proteins synthesized on the rough ER.

10 Secretory vesicles store secreted proteins and fuse with the plasma membrane to release their contents.

1l| Peroxisomes detoxify various molecules and also break down fatty acids to produce acetyl groups for biosynthesis.

12| Cytoskeletal fibers form networks and bundles that support cellular membranes, help organize organelles, and participate in cell movement.

13 Microvilli increase surface area for absorption of nutrients from surrounding medium.

14 Cell wall, composed largely of cellulose, helps maintain the cell's shape and provides protection against mechanical stress.

15 Vacuole stores water, ions, and nutrients, degrades macromolecules, and functions in cell elongation during growth.

16 Chloroplasts, which carry out photosynthesis, are surrounded by a double membrane and contain a network of internal membrane-bounded sacs.

▲ FIGURE 5-19 Schematic overview of a "typical" animal cell and plant cell and their major substructures. Not every cell will contain all the organelles, granules, and fibrous structures shown here, and other substructures can be present In some. Cells also differ considerably In shape and In the prominence of various organelles and substructures.

▲ FIGURE 5-20 Cellular structures that participate in delivering materials to lysosomes. (a) Schematic overview of three pathways by which materials are moved to lysosomes. Soluble macromolecules are taken into the cell by invagination of coated pits in the plasma membrane and delivered to lysosomes through the endocytic pathway (1). Whole cells and other large, insoluble particles move from the cell surface to lysosomes through the phagocytic pathway (2|). Worn-out organelles and bulk cytoplasm are delivered to lysosomes through the autophagic pathway ( 3). Within the acidic lumen of lysosomes, hydrolytic enzymes degrade proteins, nucleic acids, and other large molecules. (b) An electron micrograph of a section of a cultured mammalian cell that had taken up small gold particles coated with the egg protein ovalbumin. Gold-labeled ovalbumin (black spots) is found in early endosomes (EE) and late endosomes (LE), but very little is present in autophagosomes (AV). (c) Electron micrograph of a section of a rat liver cell showing a secondary lysosome containing fragments of a mitochondrion (M) and a peroxisome (P). [Part (b) from T E. Tjelle et al., 1996, J. Cell Sci. 109:2905. Part (c) courtesy of D. Friend.]

lysosomes are responsible for degrading certain components that have become obsolete for the cell or organism. The process by which an aged organelle is degraded in a lysosome is called autophagy ("eating oneself"). Materials taken into a cell by endocytosis or phagocytosis also may be degraded in lysosomes (see Figure 5-20a). In phagocytosis, large, insoluble particles (e.g., bacteria) are enveloped by the plasma membrane and internalized.

Lysosomes contain a group of enzymes that degrade polymers into their monomeric subunits. For example, nucleases degrade RNA and DNA into their mononucleotide building blocks; proteases degrade a variety of proteins and peptides; phosphatases remove phosphate groups from mononucleotides, phospholipids, and other compounds; still other enzymes degrade complex polysaccharides and glycol-ipids into smaller units. All the lysosomal enzymes work most efficiently at acid pH values and collectively are termed acid hydrolases. Two types of transport proteins in the lyso-somal membrane work together to pump H+ and Cl_ ions (HCl) from the cytosol across the membrane, thereby acidifying the lumen (see Figure 7-10b). The acid pH helps to denature proteins, making them accessible to the action of the lysosomal hydrolases, which themselves are resistant to acid denaturation. Lysosomal enzymes are poorly active at the neutral pH of cells and most extracellular fluids. Thus, if a lysosome releases its enzymes into the cytosol, where the pH is between 7.0 and 7.3, they cause little degradation of cy-tosolic components. Cytosolic and nuclear proteins generally are not degraded in lysosomes but rather in proteasomes, large multiprotein complexes in the cytosol (see Figure 3-13).

Lysosomes vary in size and shape, and several hundred may be present in a typical animal cell. In effect, they function as sites where various materials to be degraded collect. Primary lysosomes are roughly spherical and do not contain obvious particulate or membrane debris. .Secondary lyso-somes, which are larger and irregularly shaped, appear to result from the fusion of primary lysosomes with other membrane-bounded organelles and vesicles. They contain particles or membranes in the process of being digested (Figure 5-20c).

HTay-Sachs disease is caused by a defect in one enzyme catalyzing a step in the lysosomal breakdown of gangliosides. The resulting accumulation of these glycolipids, especially in nerve cells, has devastating consequences. The symptoms of this inherited disease are usually evident before the age of 1. Affected children commonly become demented and blind by age 2 and die before their third birthday. Nerve cells from such children are greatly enlarged with swollen lipid-filled lysosomes. I

ports into peroxisomes an enzyme required for the oxidation of these fatty acids. Persons with the severe form of ADL are unaffected until midchildhood, when severe neurological disorders appear, followed by death within a few years. I

U-Z^L-j Plant seeds contain glyoxisomes, small organelles •—jS^ that oxidize stored lipids as a source of carbon and ■Jin,'in energy for growth. They are similar to peroxi-somes and contain many of the same types of enzymes as well as additional ones used to convert fatty acids into glucose precursors. I

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