The Human Body

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.