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.

Christian de Duve, a Belgian scientist at the Laboratory of Physiological Chemistry at the Catholic University of Louvain, is credited with discovering lysosomes in the 1950s. De Duve and his team were studying the distribution of hydrolytic enzymes such as acid phosphatase within cells, using cell fractionation methods to isolate subcellular components. De Duve and his team identified an unknown organelle that was rich in acid phosphatase. This led them to propose the existence of lysosomes as membrane bound organelles containing digestive enzymes capable of breaking down a variety of biological molecules.

Using differential centrifugation and enzyme activity assays, the team confirmed the hypothesis and understood that these organelles play a crucial role in intracellular digestion processes, such as phagocytosis and autophagy. The presence of digestive enzymes was further validated using electron microscopy. De Duve's discovery laid the foundation for new research into lysosomal functions and understanding disorders which could lead to undigested materials accumulating in the cell. De Duve was awarded the Nobel Prize in Physiology or Medicine in 1974.

Lysosomes vary in shape and size depending on their state, what they are digesting, and the cell type they are in. Their shape can differ from spherical and ovoid to occasionally tubular. The size of lysosomes ranges from 0.1-1.2 μm, with some tubular ones reaching up to 15 μm in phagocytes. Several hundred lysosomes can be found within a single cell. However, upon nutrient deprivation or induced autophagy, their numbers can drop below 50 in a cell.

Lysosomes contain a variety of enzymes that enable the cell to break down various biomolecules it engulfs, including peptides, nucleic acids, carbohydrates, and lipids. The enzymes responsible for this hydrolysis require an acidic environment for optimal activity, with a pH ranging from ~4.5–5.0. The interior of the lysosome is acidic compared to the slightly basic cytosol (pH 7.2).