Lysosomes (Greek words lysis, meaning "to separate", and soma, "body") are the cell's waste disposal system and can digest some compounds. Lysosomes digest macromolecules by: Phagocytosis (ingestion of other dying cells, extracellular material, foreign invading microbes) Endocytosis (recycling of receptor proteins from the cell surface) Autophagy (old or unneeded organelles, proteins, or invaded microbes). They digest bacteria (or other forms of waste) that invade a cell and helping repair damage to the plasma membrane by serving as a membrane patch, sealing the wound.
Phagocytosis, Endocytosis and Exocytosis
Lysosomes contain acid hydrolase enzymes that break down waste materials and cellular debris. Thus described as the stomach of the cell. Lysosomes digest excess or worn-out organelles, food particles, and engulf viruses or bacteria. The membrane around a lysosome allows the digestive enzymes to work at the pH they require. Lysosomes fuse with autophagic vacuoles and dispense their enzymes into the autophagic vacuoles, digesting their contents. They are frequently nicknamed "suicide-bags" or "suicide-sacs" by cell biologists due to their autolysis. A group of genetic inherited disorders called lysosomal storage diseases (LSD) results from the dysfunction of lysosomes.
The size of a lysosome varies from 0.1–1.2 μm ф. The interior of the lysosomes is acidic (pH 4.8) compared to the slightly basic cytosol (pH 7.2). The lysosome maintains this pH differential by pumping protons (H + ions) from the cytosol via proton pumps and chloride ion channels. The lysosomal membrane protects the cytosol, and rest of the cell, from the degradative enzymes within the lysosome. The cell is additionally protected from any lysosomal acid hydrolases that drain into the cytosol, as these enzymes are pH-sensitive and do not function in the alkaline environment of the cytosol. This ensures that cytosolic molecules and organelles are not lysed in case there is heavy leakage of the hydrolytic enzymes from the lysosome
Digestion of food particle and lysis of damaged cell organelles
During endocytosis, a portion of the cell’s plasma membrane pinches off to form a vesicle that will eventually fuse with an organelle within the cell. Without active replenishment, the plasma membrane would continuously decrease in size. It is thought that lysosomes participate in this dynamic membrane exchange system and are formed by a gradual maturation process from endosomes. Lysosomes sustain by transcription of genes in the nucleus. mRNA transcripts exit the nucleus and translated into proteins in the cytosol by ribosomes. The nascent peptide chains are translocated into the RER where they are modified. Upon exiting the endoplasmic reticulum and entering the Golgi apparatus via vesicular transport, a specific lysosomal tag, mannose 6-phosphate, is added to the peptides. The presence of 6-phosphate receptors in the Golgi apparatus, is crucial for proper packaging into vesicles destined for the lysosomal system.
Upon leaving the Golgi apparatus, the lysosomal enzyme-filled vesicle fuses with a late endosome, an acidic organelle with an approximate pH 5.5. This acidic environment causes dissociation of the lysosomal enzymes from the mannose-6-phosphate receptors. The enzymes are packed into vesicles for further transport to establish lysosomes. The late endosome itself can eventually grow into a mature lysosome, as evidenced by the transport of endosomal membrane components from the lysosomes back to the endosomes.
Weak bases with lipophilic properties accumulate in acidic compartments of cell like lysosomes. The plasma and lysosomal membranes are permeable for neutral and uncharged weak bases and non permeable to the charged protonated weak bases which accumulate within lysosomes. Their concentration within lysosomes may reach 100 to 1000 fold higher than extracellular concentrations. This phenomenon is called "lysosomotropism" or "acid trapping“. A significant part of the clinically approved drugs are lipophilic weak bases with lysosomotropic properties.