Lecture 10
٢ ٢٢٢ cell Biology Lysosome
Lysosomes are large, spherical organelles that contain enzymes (acid hydrolases). They break up food so it is easier to digest. They are found in animal cells, while in plant cells the same roles are performed by the vacuole. They digest excess or worn-out organelles, food particles, and engulfed viruses or bacteria. The membrane around a lysosome allows the digestive enzymes to work at the 4.5 pH they require. Lysosomes fuse with vacuoles and dispense their enzymes into the vacuoles, digesting their contents.
٣ ٢٢٢ cell Biology How they are created?
They are created by the addition of hydrolytic enzymes to early endosomes from the Golgi apparatus. The name lysosome derives from the Greek words lysis, which means dissolution or destruction, and soma, which means body. They are frequently nicknamed "suicide-bags" or "suicide-sacs" by cell biologists due to their role in autolysis. Lysosomes were discovered by the Belgian cytologist Christian de Duve in 1949.
٤ ٢٢٢ cell Biology ٥ ٢٢٢ cell Biology Lysosome structure
The size of lysosomes varies from 0.1–1.2 µm. At pH 4.8, the interior of the lysosomes is acidic compared to the slightly alkaline cytosol (pH 7.2). The lysosome maintains this pH differential by pumping protons (H+ ions) from the cytosol across the membrane via proton pumps and chloride ion channels. The lysosomal membrane protects the cytosol, and therefore the rest of the cell, from the degradative enzymes within the lysosome. The cell is additionally protected from any lysosomal acid hydrolases that leak into the cytosol as these enzymes are pH- sensitive and function less well in the alkaline environment of the cytosol.
٦ ٢٢٢ cell Biology Primary & secondary lysosomes
Lysosomes are formed by the Golgi apparatus. There are primary and secondary lysosomes. The primary are formed on the rough ER (endoplasmic reticulum). The secondary lysosomes are formed on the smooth ER by following the phagocytosis (process of taking solid materials into cells). Phagosomes fuse with lysosomes and work as one digestive vacuole. Lysosomal enzymes are released into this vacuole in order to digest the bacteria or other materials. Small molecules which are the result leave the vacuole through its membrane and are used to make new molecules. The indigestible materials are deposited outside the cell.
٧ ٢٢٢ cell Biology Primary & secondary lysosomes
٨ ٢٢٢ cell Biology Residual bodies
In lysosomal digestion, residual bodies are vesicles containing indigestible materials. Residual bodies are either secreted by the cell via exocytosis (this generally only occures in macrophages), or they become lipofuscin granules that remain in the cytosol indefinitely. Longer-living cells like neurons and muscle cells usually have a higher concentration of lipofuscin than other more rapidly-proliferating cells.
٩ ٢٢٢ cell Biology ١٠ ٢٢٢ cell Biology The Functions of Lysosomes
Lysosomes are the cells' garbage disposal system. They are used for the digestion of macromolecules from phagocytosis (ingestion of other dying cells or larger extracellular material, like foreign invading microbes) endocytosis (where receptor proteins are recycled from the cell surface) And autophagy (wherein old or unneeded organelles or proteins, or microbes that have invaded the cytoplasm are delivered to the lysosome). Autophagy may also lead to autophagic cell death, a form of programmed self-destruction, or autolysis, of the cell, which means that the cell is digesting itself.
١١ ٢٢٢ cell Biology Enzymes
Some important enzymes found within lysosomes include: Lipase, which digests lipids Amylase, which digest carbohydrates (e.g., sugars) Proteases, which digest proteins Nucleases, which digest nucleic acids phosphoric acid monoesters. Lysosomal enzymes are synthesized in the cytosol and the endoplasmic reticulum, where they receive a mannose-6-phosphate tag that targets them for the lysosome
١٢ ٢٢٢ cell Biology Autophagy
When cells are faced with an inadequate supply of nutrients in their extracellular fluid (ECF), they may begin to cannibalize some of their internal organelles (e.g. mitochondria) for re-use of their components. This phenomenon, called autophagy, involves: formation of a double membrane within the cell which envelops the materials to be degraded into a vesicle called an autophagosome. The autophagosome then fuses with a lysosome forming an autolysosome whose
١٣ ٢٢٢ cell Biology ١٤ ٢٢٢ cell Biology Autophagy
hydrolytic enzymes degrade the materials Intracellular materials, such as old organelles, are brought into a lysosome by a process called autophagy. For example, when a mitochondrion comes to the end of its ten-day life, it is engulfed by membrane derived from the endoplasmic reticulum . The newly enclosed mitochondrion then fuses with a lysosome, resulting in its degradation by the acid hydrolases.
١٥ ٢٢٢ cell Biology Genetic disorders
A group of genetic disorders caused by defective lysosomal enzymes demonstrates the importance of lysosomes. Called lysosomal storage diseases, these disorders are characterized by the harmful accumulation of undigested substances. The accumulated materials impair or kill the affected cells, resulting in skeletal or muscular defects, mental retardation, or even death.
١٦ ٢٢٢ cell Biology