Lysosome

Lysosomes (de Duve, 1955) are spherical or pleomorphic organelles, usually 0.05–0.5 µm across. They contain acid hydrolases — proteases (cathepsins), nucleases, glycosidases, lipases, phospholipases, phosphatases and sulfatases — that work best at acidic pH. The lumen is kept at pH ≈4.5–5.0 by a V-type H⁺-ATPase; this protects the cell, because enzymes that leak into the neutral cytosol (pH 7.2) are largely inactive. The lysosomal membrane is lined by heavily glycosylated proteins (LAMP-1, LAMP-2) that shield it from self-digestion, and contains transporters that export the products of digestion (amino acids, sugars, nucleosides) to the cytosol for reuse.

Lysosomes form when late endosomes receive M6P-tagged hydrolases from the trans-Golgi network. Material reaches them by three routes: endocytosis (via endosomes), phagocytosis (phagosomes fuse with lysosomes to form phagolysosomes — central to killing microbes by neutrophils and macrophages) and autophagy (autophagosomes fuse to form autolysosomes). Indigestible remnants remain as residual bodies, which accumulate as lipofuscin in long-lived cells. Lysosomes can also exocytose their contents (membrane repair, osteoclast bone resorption) and act as a nutrient-sensing platform for mTORC1.