The lysosome is the cell’s digestive system. Found in nearly all eukaryotic cells, lysosomes are responsible for digesting macromolecules and other structures that enter the cell by endocytosis. It is a single membrane organelle that contains hydrolytic enzymes. Most cells contain a few hundred lysosomes. They are highest in number in cells that fight disease, such as leukocytes (white blood cells). They come in a diverse array of shapes and sizes, which makes them difficult to identify by sight. However, they can all be identified as the same type of organelle when stained with specific antibodies. They were initially discovered by the biochemical fractionation of cell extracts. They were not clearly seen until the invention of the electron microscope in 1931.
Enzyme proteins are produced in the rough endoplasmic reticulum and packaged into vesicles. The Golgi complex then takes them and recombines them before breaking them off into specific lysosomes which it releases into the cytoplasm. The lysosomes then float within the cytoplasm until needed. They can also be formed from late endosomes that have entered the cell via endocytosis.
Proteins are transported across the single membrane of the lysosome and into the cytosol. The interior of the lysosome is composed of over 40 different hydrolytic enzymes. All of these enzymes are acid hydrolases that function best in an acidic environment. The types of hydrolases include nucleases, proteases, glycosidases, lipases, sulfatases, phosphases and phospholipases.
The lumen (interior) of the lysosome is kept at a pH of 5. This acidic environment is ideal for hydrolytic enzyme function. The cytosol has a pH of 7.2, which means that even if a lysosome leaks out some of the enzymes, they are unlikely to damage the cell. The acidity of the lysosome is maintained by a hydrogen pump in the lysosomal membrane. The membrane uses energy from the hydrolysis of adenosine triphosphate (ATP) to pump hydrogen into the lumen.
Digestion of macromolecules by the lysosome occurs via one of three processes: phagocytosis, endocytosis or autophagy. Phagocytosis occurs when the lysosome ingests bacteria or other foreign material within the cell. Receptor proteins are recycled on the cell surface during endocytosis. Autophagy means that the lysosome digests internal cellular structures such as organelles, proteins, and microbes that are no longer required by the cell.
Lysosomal storage diseases occur when a genetic defect caused by a mutation in the structural gene that codes for an individual lysosomal hydrolase. One or more of the hydrolases can be affected and the consequences are often very severe. Undigested substrates accumulate within the lysosome. Commonly affected systems are the nervous system and brain function. Lysosomal storage diseases include Hurler’s Disease, Tay-Sachs disease, and inclusion-cell disease. Many of these diseases result in mental developmental delays and premature death.
The lysosome fills an important function not only to cellular function, but also to the organism as a whole. The acidic interior of the lysosome allows its hydrolytic enzymes to digest materials within the cell. When the system fails, due to a genetic mutation of the genes responsible for one or more of the enzymes, the consequences can be severe.
