A lysosome has approximately 50 different hydrolytic enzymes. These enzymes are produced by the endoplasmic reticulum and targeted lysosomes. Lysosome enzymes can digest every type of biological macromolecule. All the enzymes of the lysosome share significant properties – their optical activity at an acid pH and thus are acid hydrolases. The pH optimum of these enzymes is the low pH of the lysosomal compartment, which is 4.6. The internal proton concentration is high due to the presence of proton pumps, that are in the organelle’s membrane and requires ATP to function. These acidic enzymes do not affect the lysosomal membrane because the membrane’s highly glycosylated integral proteins have carbohydrate chains that form a protective lining.
Under the microscope, lysosomes do not appear distinctive or uniform. It is very difficult to identify lysosomes based on morphology. For example, the phagocytic Kupffer cell of the liver have lysosomes of variable sizes.
This bag of destructive enzymes suggests various possible functions. But the key role of the breakdown of materials in the cell is brought from the extracellular environment. For example, single-celled organisms ingest food, which is then enzymatically digested in the lysosomes. This results in the transport of nutrients from the lysosome to the cytosol.
In mammals, the phagocytic cells, such as macrophages and neutrophils, ingest potentially dangerous microorganisms. Such microorganisms are inactivated by the low pH of lysosomes and enzymatically digested. In addition, lysosomes also play an important role in organelle turnover, destruction and replacement of cell’s own organelles. This process is called autophagy. For example, if mitochondria is being destroyed and replaced, then it will be surrounded by a double membrane, called as the autophagosome. The outer membrane then fuses with the membrane of lysosome to form an autophagolysosome. It is calculated that a mitochondria undergoes autophagy every 10 minutes. Also, research has found that when the cell is nutrient deficit, an increased autophagy is observed. This is because the cellular energy is maintained by cannibalizing its own organelles. Autophagy also helps the cell from abnormal protein aggregation.
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