lizozomii

Lizozomii are membrane-bound organelles present in most eukaryotic cells. They contain a broad set of acid hydrolases that break down proteins, nucleic acids, lipids, and carbohydrates inside an acidic lumen. Through processes such as endocytosis, autophagy, and phagocytosis, they recycle cellular components and help defend against invading material.

Structure and biogenesis: Lysosomes are bounded by a single membrane and maintain an internal pH around 4.5 to 5.0. Most hydrolytic enzymes (proteases, lipases, nucleases, glycosidases) are synthesized in the endoplasmic reticulum, tagged with mannose-6-phosphate in the Golgi, and delivered to lysosomes via M6P receptors. Primary lysosomes contain inactive enzymes; fusion with endosomes or autophagosomes converts them into secondary lysosomes where digestion occurs. Residual bodies are lysosomes that contain indigestible material.

Functions: Lysosomes participate in endocytosis, phagocytosis, and autophagy, enabling cellular turnover and remodeling. They degrade damaged organelles, unwanted macromolecules, and invasive pathogens, and release basic nutrients back into the cytoplasm. They also contribute to antigen processing and other cellular quality-control processes.

Clinical significance: Defects in lysosomal enzymes or transporters cause lysosomal storage disorders, characterized by substrate accumulation. Examples include Pompe disease (acid alpha-glucosidase deficiency), Gaucher disease (glucocerebrosidase deficiency), Tay-Sachs disease (beta-hexosaminidase A deficiency), and Niemann-Pick disease (sphingomyelinase deficiency). Treatments such as enzyme replacement therapy exist for some conditions; research continues in gene therapy and small-molecule approaches.

History: The organelles were identified and named for their degradative function by Christian de Duve in the 1950s. The discovery earned him (along with others) a Nobel Prize in Physiology or Medicine in 1974.