LYSOSOME

subcellular organelle that is found in all eucaryotic cells and is responsible for the cell's digestion of macromolecules, old cell parts, and microorganisms. Each lysosome is surrounded by a membrane that maintains an acidic environment within the interior. Lysosomes contain a wide variety of hydrolytic enzymes (acid hydrolases) that break down macromolecules such as nucleic acids, proteins, and polysaccharides. These enzymes are active only in the lysosome's acidic interior; their acid-dependent activity protects the cell from self-degradation in case of lysosomal leakage or rupture, since the pH of the cell is neutral to slightly alkaline. Lysosomes were discovered by the Belgian cytologist Christian Ren de Duve in the 1950s. Lysosomes originate by budding off from the membrane of the trans-Golgi network. Materials to be digested are transported to the lysosomes by three main pathways: endocytosis, autophagocytosis, and phagocytosis. In endocytosis, extracellular macromolecules are taken up into the cell to form membrane-bound vesicles called endosomes that fuse with lysosomes. Autophagocytosis is the process by which old organelles are removed from a cell; they are enveloped by internal membranes that then fuse with lysosomes. Phagocytosis is carried out by specialized cells (e.g., macrophages) that engulf large extracellular particles such as dead cells or foreign invaders (e.g., bacteria) and target them for lysosomal degradation. Many of the products of lysosomal digestion, such as amino acids and nucleotides, are recycled back to the cell for use in the synthesis of new cellular components. Lysosomal storage diseases are genetic disorders in which a genetic mutation affects the activity of one or more of the acid hydrolases. In such diseases, the normal metabolism of specific macromolecules is blocked and the macromolecules accumulate inside the lysosomes, causing severe physiological damage or deformity. Hurler's syndrome, which involves a defect in the metabolism of mucopolysaccharides, is a lysosomal storage disease.