Lysosomes are organelles in eukaryotic cells that contain digestive enzymes to digest macromolecules. They are built in the Golgi apparatus. At pH 5, the interior of the lysosomes is more acidic that the cytosol. The lysosome membrane stabilizes the low pH by pumping in protons (H+) from the cytosol, and also protects the cytosol, and therefore the rest of the cell, from the digestive enzymes within the lysosome. The digestive enzymes need the acidic environment of the lysosome to function correctly. All these enzymes are produced in the endoplasmatic reticulum, and transported and processed through the Golgi apparatus. The Golgi apparatus produces lysosomes by budding (budding means unequal division, resulting in one large and one small membrane-enclosed space). The most important enzymes in lysosomes are:
- Lipase, which digests lipids,
- Carbohydrases, which digest carbohydrates (e.g., sugars),
- Proteases, which digest proteins,
- Nucleases, which digest nucleic acids.
The lysosomes are used for the digestion of macromolecules from phagocytosis (ingestion of food), from the cell's own recycling process (where old components are continuously destroyed and replaced by new ones), and from apoptosis, the programmed self-destruction of the cell, which means that the cell is digesting itself. There are a number of illnesses that are caused by the malfunction of the lysosomes or one of their digestive proteins, e.g., Tay-Sachs disease, or Pompe's disease. These are caused by a defective or missing digestive protein, which leads to the accumulation of substrates within the cell, resulting in impaired cell metabolism.