Lysosomes

Lysosomes can be thought of as the dismantling and restructuring units of the cell. They are membrane-bound vesicles containing hydrolytic enzymes -- a full set of hydrolases including amylases, proteases and lipases -- that most eukaryotic cells use to digest (hydrolyse) macromolecules. These enzymes work best at an acidic pH, and they remain inactive until the lysosome comes into contact with another particular organelle or vesicle and fuses with it to form a hybrid structure; once the enzymes' work is done, the lysosome is reformed and reused. Lysosomes arise from the region of the Golgi complex associated with the endoplasmic reticulum.

Forms of lysosomes

Because of this cycle of fusion, digestion and reformation, lysosomes exist in several structural forms and carry out a range of functions. Primary lysosomes are simply membrane-bound vesicles in which the enzymes are still in their inactive state. Secondary lysosomes, also called hybrid lysosomes, form when a lysosome fuses with an endocytic vesicle carrying material that needs to be digested; such a vesicle, called a heterophagic vesicle, is larger than a primary lysosome. A residual body is the vesicle left holding undigested remains once the heterophagic vesicle has released the products of digestion into the cytosol. Lysosomes that instead digest a cell's own material -- such as a damaged organelle -- are called autophagic vesicles, or informally 'suicide bags'; an autophagic vesicle forms when a lysosome fuses with an old, membrane-bound organelle or organic molecule that is being recycled. A human liver cell, for example, recycles about half of its macromolecules every week through this route.

Intracellular and extracellular digestion

Lysosomes carry out both intracellular and extracellular digestion. Intracellular digestion happens through autophagic vesicles or through secondary lysosomes that contain foreign material brought into the cell by processes such as phagocytosis -- for example, the food vacuole of an Amoeba, or the macrophages in human blood that engulf and destroy harmful microbes. Extracellular digestion happens when lysosomal enzymes are released outside the cell -- for example, the acrosome, a cap-like structure on the head of a human sperm, is a modified lysosome containing enzymes such as hyaluronidase, and these enzymes dissolve the protective layers around the ovum to bring about fertilisation. During metamorphosis in many organisms, lysosomal enzymes similarly help the body reuse the tissue of organs that are no longer needed, and they also play a part in destroying malignant cells, for instance through the action of T-lymphocytes. …