Lysosomes

Lysosomes are small membrane-bound organelles in animal cells that contain digestive enzymes. They break down worn-out cell parts, food particles and invading microbes, and they act as the recycling center of the cell.

A lysosome holds around fifty kinds of acid hydrolase, enzymes that cut up proteins, nucleic acids, lipids and sugars. These work best at the acidic pH of about 4.5 to 5, which is maintained by a proton pump in the lysosomal membrane. The enzymes are made in the endoplasmic reticulum, tagged in the Golgi apparatus and delivered in vesicles, so lysosomes are part of the endomembrane system. Christian de Duve discovered them in the 1950s and shared the 1974 Nobel Prize.

Material taken in by endocytosis or by phagocytosis, the engulfing used by macrophages, is delivered to lysosomes to be digested. In autophagy, the cell wraps its own damaged organelles in a membrane and sends them for breakdown, a process studied by Yoshinori Ohsumi (Nobel Prize 2016). Lysosomes also take part in cell death when they leak, as in some forms of apoptosis.

More than fifty lysosomal storage diseases are known, in which a missing enzyme lets its substrate build up. Examples are Tay-Sachs disease, Gaucher disease and Pompe disease. Some are treated with enzyme replacement therapy, and others are targets for gene therapy. Each case is a loss of one of the many enzymes that the cell relies on.

• Endomembrane System — the network they belong to
• Endocytosis — how material reaches them
• Organelle — the wider class