Lysosomes are the cell’s degradation compartments and are primarily involved in the digestion of proteins, DNA, RNA, polysaccharides, and complex lipids into their respective monomers.
A well-known role of the lysosomes is to degrade and recycle the waste materials generated in their respective cells.
Diagram of lysosomes
- Lysosomes are formed by the budding-off of the Golgi apparatus and the collection of hydrolytic enzymes that are synthesized in the endoplasmic reticulum.
- The endoplasmic reticulum (ER) enzymes are initially tagged by mannose-6-phosphate and then they are transported to the Golgi apparatus via vesicles, where they are packed into the lysosomes.
- Structurally, lysosomes are tiny organelles, ranging from 0.1 to 1.2 µm. They have a simple structure covered by a membrane that is made up of a phospholipid bilayer.
- Inside lysosomes, various acid hydrolase enzymes are found.
- The lipids, especially the phospholipids, of all membrane systems contain a hydrophilic phosphate group and a glycerol molecule as heads and hydrophobic fatty acids as tails.
- Due to this amphipathic nature, phospholipids typically form membrane bilayers when they are kept in a solution containing water molecules. In which, the phosphate group of the lipids (hydrophilic) turns toward the hydrophilic environment (outside of the bilayer) and interacts with water molecules, while the hydrophobic fatty acid tails turn toward the inside of the bilayer (hydrophobic environment) to keep themselves away from water.
- In this way, phospholipids form cellular membranes, including cell membranes and membranes of cellular organelles such as a nucleus, mitochondria, chloroplast, lysosome, Golgi apparatus, and the endoplasmic reticulum.
Lysosomes are polymorphic, which means functionally they appear in different forms.
- Primary lysosomes
- Residual bodies
1. Primary lysosomes: These types of lysosomes are formed by the fusion of Golgi vesicles with late endosomes.
2. Phagolysosomes: These types of lysosomes are also called digestive vacuoles or heterophagosomes (phagosome + lysosome = phagolysosome).
3. Autophagosomes: These types of phagosomes will be formed during cellular degradation, a process called autophagy. It involves in renovation and turnover of cellular components that have previously been degraded by autophagy, eg., mitochondria, ER, peroxisomes, GC (matriphagy). Autophagy also occurs in the liver cells during starvation to provide nutrition for the remaining cellular constituents.
4. Residual bodies: These are also known as telolysosomes, which contain undigested materials.
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