See also peroxisome
Structure and Function
Lysosomes are spherical sacs, manufactured by the Golgi apparatus, bounded by a single membrane and having no internal structure. They are commonly 0.1 - 0.5 µm in diameter and contain hydrolytic enzymes which must be kept separate from the rest of the cell so as to prevent damage to it. Lysosomes are responsible for the breakdown of unwanted structures, such as old organelles or even whole cells, as occurs in mammary cells after lactation.
In white blood cells they are used to digest bacteria. Enzymes are sometimes released outside the cell as occurs during the replacement of cartilage with bone during development. The heads of sperm contain a special lysosome, the acrosome, for digesting a path through to the ovum.
Digestion and Breakdown
Lysosomes contain over three dozen different kinds of hydrolytic enzymes including
The pH within the lysosome is about pH 5, substantially less than that of the cytoplasm which is about pH 7.2. All the enzymes in the lysosome work best at an acid pH and this adaptation reduces the risk of their digesting their own cell if they should escape from the lysosome.
Materials within the cell scheduled for digestion are first deposited within lysosomes. These may be:
other organelles such as mitochondria that have ceased functioning properly
food molecules or, in some cases, food particles taken into the cell by endocytosis
foreign particles like bacteria that are engulfed by neutrophils
antigens that are taken up by the antigen-recognising cells of the immune system - either macrophages or B-cells
Programmed Cell Death
At one time, it was thought that lysosomes were responsible for programmed cell death (PCD), the killing of cells scheduled to be removed from a tissue such as the resorption of a tadpole's tail as it metamorphoses into a frog. It has, however, since been discovered that these examples of PCD, otherwise known as apoptosis, take place by an entirely different mechanism.