The lysosome is a single-membrane-bound vesicular organelle used for the digestion of large molecules. They are present in almost all animal cells, often in numbers of hundreds. Lysosomes contain various hydrolases that break down proteins, nucleic acids, polysaccharides, lipids, and other substances. Proton pumps on the membrane continuously transport protons in the lysosome to maintain the pH of the internal solution around 5. The membrane also contains various transport proteins to move hydrolyzed products outside.
Functions of Lysosomes
Lysosomes are involved in digesting invading organisms such as bacteria, viruses, and aging cells. These substances are recognized by immune cells and engulfed through a process called phagocytosis. They are enclosed in phagosomes that transport them to the lysosomes for fusion. The substances enter the lysosomes to be broken down into smaller molecules which are then released to the cytoplasm for reuse. To eliminate undigested materials, lysosomes fuse with the cell membrane and excrete the residues outside the cell.
Lysosomes are also involved in the removal of aging organelles and abnormal proteins within cells through a process called autophagy. These substances are encapsulated by vesicles and fused with lysosomes to be degraded.
They also digest large molecules to provide nourishment for organisms. Lipoproteins form spherical complexes with phospholipids, cholesterol and fatty acids in the blood. They are phagocytosed by cells and fused with lysosomes. Hydrolytic enzymes break these complexes down again into small molecules again, which are used as raw materials for metabolism. Small peptide directly absorbed by the human body from food are engulfed by cells and broken down into amino acids within lysosomes. During periods of starvation, lysosomes also break down large biomolecules of cells to provide the necessary energy and nutrients. For some single-celled eukaryotes, lysosomes serve as their digestive system.
Lysosomes play a vital role in programmed cell death. In early development, human embryos grow tails whose cells are degraded by lysosomal enzymes in a later process.