3.17: Lysosomes are the cell’s garbage disposals.

Garbage. What does a cell do with all the garbage it generates? Mitochondria wear out after about 10 days of intensive activity, for starters. And white blood cells constantly track down and consume bacterial invaders, which they then have to dispose of. Similarly, the thousands of ongoing reactions of cellular metabolism produce many waste macromolecules that cells must digest and recycle. Many eukaryotic cells deal with this garbage by maintaining hundreds of versatile floating “garbage disposals” called lysosomes (FIGURE 3-33).

Lysosomes are round, membrane-enclosed, acid-filled vesicles that dispose of garbage. They are filled with about 50 different digestive enzymes and a super-acidic fluid, a corrosive broth so powerful that if a lysosome were to burst, it would almost immediately kill the cell by rapidly digesting all of its component parts. The selection of enzymes in the lysosome represents a broad spectrum of chemicals designed for dismantling macromolecules that are no longer needed by the cell or are generated as by-products of cellular metabolism.

Some of the enzymes break down lipids, others carbohydrates, others proteins, and still others nucleic acids. Consequently, when a cell consumes a particle of food or even an invading bacterium via phagocytosis, the cell directs the material to lysosomes for dismantling. And, ever the efficient system, the cell releases most of the component parts of molecules that are digested, such as the amino acids from proteins, back into its cytoplasm, where they can be reused by the cell as raw materials.

Some immune system cells tend to have particularly large numbers of lysosomes, most likely because of their great need for disposing of the by-products of disease-causing bacteria.

With 50 different enzymes necessary for lysosomes to carry out their metabolic salvaging act, malfunctions sometimes occur. In a common genetic disorder called Tay-Sachs disease, an individual inherits an inability to produce a critical lipid-digesting enzyme. Even though the lysosomes cannot digest certain lipids, the cells continue to send lipids to the lysosomes, where they accumulate, undigested. The lysosome swells until it bursts and digests the whole cell or until it chokes the cell to death. This process occurs in large numbers of cells within the first few years of life, and eventually leads to the child’s death.

Although the matter has long been debated among biologists, it does seem that plant cells also contain lysosomes, compartments with similar digestive broths that carry out the same digestive processes as in animals.