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Lysosomes are organelles that contain digestive enzymes. They digest excess or worn-out organelles, food particles and engulfed viruses or bacteria. Lysosomes are like the stomach of the cell.
Lysosomes contain a variety of enzymes, enabling the cell to break down various biomolecules it engulfs, including peptides, nucleic acids, carbohydrates, and lipids. The enzymes responsible for this hydrolysis require an acidic environment for optimal activity.
In addition to being able to break down polymers, lysosomes are capable of fusing with other organelles and digesting large structures or cellular debris, through cooperation with phagosomes. They are able to conduct autophagy, clearing out damaged structures. Similarly, they are able to break-down virus particles or bacteria in phagocytosis of macrophages.
The size of lysosomes varies from 0.1 m to 1.2 m. With a pH ranging from 4.5 to 5.0, the interior of the lysosomes is acidic compared to the slightly basic cytosol (pH 7.2). The lysosomal membrane protects the cytosol, and therefore the rest of the cell, from the degradative enzymes within the lysosome. The cell is additionally protected from any lysosomal acid hydrolases that drain into the cytosol, as these enzymes are pH-sensitive and do not function well or at all in the alkaline environment of the cytosol. This ensures that cytosolic molecules and organelles are not destroyed in case there is leakage of the hydrolytic enzymes from the lysosome.
The lysosome maintains its pH differential by pumping in protons (H+ ions) from the cytosol across the membrane via proton pumps and chloride ion channels. Vacuolar H+-ATPases are responsible for transport of protons, while the counter transport of chloride ions is performed by ClC-7 Cl/H+ antiporter. In this way, a steady acidic environment is maintained.
Its versatile capacity for degradation by the import of enzymes with specificity for different substrates, cathepsins are the major class of hydrolytic enzymes, while lysosomal alpha-glucosidase (GAA) is responsible for carbohydrates, and ACP2 is necessary to release phosphate groups of phospholipids.