Lysosomes are membrane-bound organelles found in nearly all animal cells. They are spherical vesicles that contain hydrolytic enzymes that can break down many types of biomolecules, including proteins, carbohydrates, and lipids. Lysosomes are responsible for cellular homeostasis and turnover, as well as for the removal of damaged or excess cell parts. In addition, lysosomes play a role in innate immunity by providing a first line of defense against invading pathogens.
Lysosomes were first described in 1955 by Christian de Duve, who named them after their ability to break down (“lyse”) large molecules. Since then, much has been learned about their structure and function. For example, it is now known that lysosomes vary in size and number depending on the cell type and stage of development. Additionally, while most lysosomal enzymes are produced by the liver, some are synthesized by other organs, such as the pancreas.
The interior of a lysosome is acidic due to the presence of proton pumps that maintain a pH gradient across the organelle’s membrane. This low pH environment allows lysosomal enzymes to work optimally. Hydrolytic enzymes within lysosomes can break down all major classes of biomolecules: proteins , carbohydrates , nucleic acids , and lipids . These include complex molecules such as glycosaminoglycans (GAGs), which are long chains of sugar units that make up proteoglycans—a class of macromolecules involved in many important biological processes . The ability of lysosomal enzymes to degrade GAGs makes them especially important for degrading extracellular matrix components during tissue remodeling .
In addition to playing a role in digestion and nutrient absorption , lysosomes also participate in autophagy —the process by which cells recycle their own damaged or excess parts . Autophagy occurs when double-membraned vesicles called autophagosomes fuse with lysosomes, delivering their contents (including damaged organelles) to be degraded . This process helps keep cells healthy and functioning properly. Additionally, autophagy has been shown to play an important role in cancer prevention . When cells become damaged or stressed beyond repair , they undergo programmed cell death , or apoptosis—a process mediated in part by autophagy . By removing damaged cells before they have a chance to divide uncontrollably , autophagy helps prevent cancer from developing .