Each of your cells holds hundreds of tiny acid-filled recycling bins
Lysosomes are the cell's demolition crews: sealed pouches of acid holding more than 60 digestive enzymes that break proteins, sugars and fats back into reusable parts. The design has a built-in safety catch. Their enzymes only work at a pH around 4.5 to 5, so any that leak into the near-neutral cytosol go quiet.
Almost every animal cell has them, red blood cells being the exception, and they are rare in plants. A typical cell carries several hundred, though numbers can fall below 50 during starvation or heavy autophagy. They range from about 0.1 to 1.2 micrometres across, but tubular ones in immune cells called phagocytes can stretch to 15 micrometres. Shapes vary from spheres to ovals to tubes, depending on the cell type and what is being digested.
Material arrives by several routes. Particles from outside come in through endocytosis; worn-out parts of the cell arrive via autophagy, in which damaged structures are wrapped up and delivered for breakdown. The resulting amino acids, simple sugars and fatty acids are shipped back out through membrane transporters to be rebuilt or burned for energy. Lysosomes can also merge with the outer membrane and release their contents, a process called lysosomal exocytosis.
Keeping the interior acidic takes constant work. Proton pumps called vacuolar ATPases push hydrogen ions inside, while an exchanger named ClC-7 moves chloride the other way to balance charge. The membrane itself is coated in heavily sugared proteins, forming a protective layer so the lysosome does not digest itself.
The Belgian biochemist Christian de Duve, working at the Catholic University of Louvain in the 1950s, found them while tracking acid phosphatase by separating cell components in a centrifuge. An unknown particle rich in the enzyme led him to propose membrane-bound digestive organelles, later confirmed by electron microscopy; he shared the 1974 Nobel Prize in Physiology or Medicine. Since then lysosomes have proved to do far more than clean up. They help repair the cell membrane, regulate metabolism and signalling, and aid immunity: in macrophages they destroy engulfed bacteria and viruses, and fragments of microbes are presented to T cells. Released into the cytoplasm, their enzymes can even trigger a form of programmed cell death.
Source: Lysosome
