Each living cell has a set of structures that enable it to demonstrate all the properties of a living organism. In order to function properly, the cell must receive enough nutrients, break down them and release energy, which then will be used to ensure vital processes.
At the first stage of complex energy management processes are cell lysosomes that are laced up along the edges of flattened dictosome tanks (Golgi complex).
How are lysosomes arranged?
Lysosomes are spherical single-membrane bodies with a diameter of 0.2 to 2 ฮผm, in which a complex of hydrolytic enzymes is enclosed. They are able to break down any natural polymer or substance of complex structure that enters the cell as a nutrient substrate or a foreign agent:
- proteins and polypeptides;
- polysaccharides (starch, dextrins, glycogen);
- nucleic acids;
- lipids.
This efficiency is provided by about 40 different types of enzymes contained both in the lysosome matrix and on the inside of the membrane in an adhered state.
Chemistry of Lysosomes
The membrane surrounding the lysosome protects the organelles and other components of the cell from digestion by a complex of enzymes. But in the vesicle itself all the enzymes are of protein origin, why aren't they cleaved by proteases?
The fact is that inside the lysosomes, the enzymes are in a glycosylated state. This carbohydrate โshellโ makes them poorly recognized for proteolytic enzymes.
The reaction of the medium inside the lysosome is weakly acidic (pH 4.5โ5), in contrast to the almost neutral reaction of the hyaloplasm. It creates favorable conditions for the action of enzymes and is provided by the work of the H + -ATPase, which pumps protons into the organelle.
Lysosome conversion process
Morphologically, two main types of lysosomes are distinguished in the cell - primary and secondary.
Primary lysosomes are small vesicles, smooth-walled or fringed, separated from the tanks of the Golgi complex. They contain a set of hydrolytic enzymes previously formed on the membranes of granular (rough) EPR. Before absorption of the nutrient substrate, the lysosomes are inactive.
To start the work of enzymes, food particles or liquids must get into the lysosome. This happens in two ways:
- By autophagy, when a food particle is absorbed by the lysosome from the surrounding cytoplasm. At the same time, the organelle membrane is invaginated at the point of contact with the particle and forms an endocytotic vesicle, and then laced into the lysosome.
- By heterophagy, when the lysosome merges with endocytotic vesicles, which appear in the cytoplasm of the cell as a result of absorption of solid particles or liquids from the outside.
Secondary lysosomes are vesicles containing both enzymes and a substrate for digestion. They are characterized by pronounced hydrolytic activity and are formed as a result of absorption of the substrate by the primary lysosome.
Despite the fact that the functions of the lysosome are reduced to the digestion (splitting) of solid organic particles and dissolved substances, the universality of the process is ensured by the ability of secondary lysosomes:
- merge with primary lysosomes, which introduce a new portion of enzymes;
- merge with new food particles or endocytotic vesicles, supporting a continuous process of splitting;
- merge with other secondary lysosomes, forming a large structure that can absorb other organelles of the cell;
- absorb pinocytotic vesicles, turning into a multivesicular body.
The structure of the lysosome does not fundamentally change. Usually it only increases in size.
Other types of lysosomes
Sometimes the cleavage of substances that have fallen into the lysosome is not complete. Undigested particles are not removed from the organelle, but accumulate in it. After the supply of hydrolytic enzymes is depleted, the contents are compacted and processed, the structure of the lysosome becomes more complex, layered. Pigment substances may also be deposited. The lysosome is converted to a residual body.
Subsequently, the residual bodies remain in the cell or are removed from it by exocytosis.
Autophagosomes can be found in protist cells. By their nature, they belong to secondary lysosomes. The remains of large components of the cell and cytoplasmic structures are found inside these organelles. They are formed during cell damage, aging of cellular organelles and serve to utilize the components of the cell, releasing monomers.
Cell lysosome functions
Lysosomes, first of all, provide the cell with the necessary building material, depolymerizing the substances that enter it.
The breakdown of carbohydrates is an important link in the energy metabolism of cells, supplying a substrate for conversion in mitochondria.
Lysosomes are also a defense link in the bodyโs immune system:
- After phagocytosis of bacteria by leukocytes, lysosomes pour their contents into the cavity of the phagocytic vesicle and destroy the harmful microorganism.
- Proteolytic enzymes are released during apoptosis - programmed cell death.
- Dispose of damaged and "aged" cell organelles.
In combination with cell proliferation, the participation of lysosomes in the process of utilization of various structures ensures the renewal of the body.