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.


                      Contributed By: John B. Ferguson, Sc.B., M.Phil., Ph.D.
                      Professor of Biology, Division of Natural Sciences and
                      Mathematics, Bard College. Director, Distinguished
                      Scientist Lecture Series, Bard Center.
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