Lysosome

Lysosome, membrane-bound sac found in nucleated cells that contains digestive enzymes that break down complex molecules in the body. Lysosomes are numerous in disease-fighting cells, such as white blood cells, that destroy harmful invaders or cell debris.

Lysosomes vary greatly in size, typically ranging from 0.05 to 0.5 micrometers in diameter. Each lysosome is surrounded by a membrane that protects the cell from the lysosome’s digestive enzymes—if the lysosome breaks open, the enzymes would destroy the cell. Proteins embeded in the lysosome membrane protect the activity of the enzymes by maintaining the proper internal acidity. Membrane proteins also transport digested products out of the lysosome.

Lysosome enzymes are manufactured in the rough endoplasmic reticulum and processed in the Golgi apparatus. They are delivered by sacs known as transport vesicles to fuse with three types of membrane-bound structures: endosomes, phagosomes, and autophagosomes. Endosomes form when the cell membrane surrounds nutritional molecules like polysaccharides, complex lipids, nucleic acids, or proteins. In a process called endocytosis, these molecules are broken down for reuse. Phagosomes form when the cell membrane engulfs large objects, like debris from sites of injury or inflammation or disease-causing bacteria, in a process called phagocytosis. Autophagosomes form when the endoplasmic reticulum wraps around spent cell structures, such as mitochondria, that are destined for recycling. In all cases the digestive enzymes supplied by the lysosomes digest the membrane-bound objects into simple compounds that are delivered to the cytoplasm as new cell-building materials.

Lysosome enzyme disorders can cause disease. Infants born with Tay-Sachs disease lack an enzyme that breaks down a complex lipid called ganglioside. When this lipid accumulates in the body, it damages the central nervous system, causes mental retardation, and results in death by age five. The inflammation and pain associated with rheumatoid arthritis and gout are related to the escape of lysosome enzymes.

Some scientists classify plant vacuoles as a type of lysosome. These membrane-bound structures are much larger than other lysosomes, measuring up to 20 micrometers in diameter. Vacuoles maintain water pressure within plant cells, called turgor, preventing wilting. Vacuoles may also provide long-term storage of polysaccharides, lipids, proteins, pigments, and harmful materials such as rubber or opium that may deter predators.