How do cells dispose of their unwanted parts?
Cells are continually making new molecules, while old ones are broken down and recycled. This is especially important in cell signalling, where old signals need to be stopped in order for new signals to be responded to. The main site for this breakdown is the lysosome, which acts as a cellular stomach.
A typical human cell has about a hundred lysosomes, each containing a collection of potent hydrolytic (digestive) enzymes, which break down substances by hydrolysis, enclosed in a membrane. Old organelles, other cellular waste and, in immune system cells, old red blood cells or bacteria engulfed by the cell are all wrapped in membranes of their own. These then fuse with the lysosome, where they are quickly broken down into small molecules that can then be reused.
In cases where one of the lysosomal enzymes fails, the cell cannot keep up with removing waste. In the rare genetic condition Tay–Sachs disease, for example, an enzyme that mops up a fatty chemical in neurons is defective. This chemical, a ganglioside, then accumulates and eventually destroys cells in the brain and spinal cord, causing a progressive loss of mental and physical function that usually results in death before the age of five.
The right packaging is also crucial for recycling and disposal. Cells constantly pinch off bits of outer membrane, turning a small pit in the membrane into a vesicle, which is brought inside (endocytosis). Larger vesicles import material from outside. The vesicles then fuse with an extensive network of tubes and bags, known as endosomes, which sort incoming material.
New vesicles can also bud off from here, and shift designated contents onward – to other parts of the cell, back to the outer membrane (through exocytosis) or to the lysosomes. Cell surface receptors are recycled as part of this process, too.