Lysosome

A lysosome (/ˈlaɪsəˌsoʊm/) is a membrane-bound organelle that is found in all mammalian cells, with the exception of red blood cells (erythrocytes). There are normally hundreds of lysosomes in the cytosol, where they function as the cell’s degradation center. Their primary responsibility is catabolic degradation of proteins, polysaccharides and lipids into their respective building-block molecules: amino acids, monosaccharides, and free fatty acids. The breakdown is done by various enzymes, for example proteases, glycosidases and lipases.

With an acidic lumen limited by a single-bilayer lipid membrane, the lysosome holds an environment isolated from the rest of the cell. The lower pH creates optimal conditions for the over 60 different hydrolases inside.

Lysosomes receive extracellular particles through endocytosis, and intracellular components through autophagy. They can also fuse with the plasma membrane and secrete their contents, a process called lysosomal exocytosis. After degradation lysosomal products are transported out of the lysosome through specific membrane proteins or via vesicular membrane trafficking to be recycled or to be utilized for energy.

Aside from cellular clearance and secretion, lysosomes mediate biological processes like plasma membrane repair, cell homeostasis, energy metabolism, cell signaling, and the immune response.