Lysosome : Structure, Functions, Types, Characteristics

Lysosome : Structure, Functions, Types, Characteristics


What are lysosomes?

Lysosomes are organelles formed by the Golgi complex. They contain hydrolytic and paleolithic enzymes that are responsible for digesting external or internal substances, which is why they are responsible for cellular digestion Its structure consists of a rounded shape, characterized as a bag of enzymes, which, if released, could destroy the cell, so its membrane is protected from these enzymes. Lysosomes are characteristic of animal cells, and their size, shape, and content vary depending on their location and function.

Cell Lysosomes. Relatively large organelles, formed by the rough endoplasmic reticulum (RER) and then packaged by the Golgi complex, which contain hydrolytic and proteolytic enzymes that serve to digest the materials of external (heterophagy) or internal (autophagy) origin that reach them. That is to say, they take care of cellular digestion. They have a very simple structure, "similar" to vacuoles, surrounded only by a membrane, they contain a large number of digestive enzymes that break down all the molecules that are useless for lysosome of the cell.


History of the discovery of lysosomes

The term lysosome was introduced by de Duve et al (1955). First discovered this organelle in rat liver cells, then in 1963 they also suggested that lysosomes are sacs containing hydrolysis enzymes that are active in acidic medium.

But long before that in the late 1800s, E. Metchnifoff and Paul Erlich had observed granules in leukocytes that were thought to have something to do with intracellular digestion. C. Duve et al from Belgium examined biochemically while Ak Novikoff et al from the USA examined morphological and cytochemical aspects. Since 1950 with an electron microscope has been able to observe the structure of small structures / micro bodies.

Characteristics of Lysosomes

• Lysosomes are spherical vesicles, 0.1 to 1 µm in diameter. They contain around 50 enzymes, generally hydrolytic, in acidic solution; enzymes need this acidic solution for optimal function. Lysosomes keep these enzymes separate from the rest of the cell, and thus prevent them from chemically reacting with elements and organelles of the cell.

• Cell Lysosomes use their enzymes to recycle the different organelles of the cell, encompassing them, digesting them and releasing their components in the cytosol. This process is called autophagy, and the cell digests its own structures that are not necessary. The material is encompassed by vesicles that come from the endoplasmic reticulum and from the Golgi apparatus, forming an autophagosome. Upon joining the primary lysosome, it forms an autophagolysosome and follows the same process as in the previous case.

• In endocytosis the materials are collected from the cell exterior and encompassed by endocytosis through the plasma membrane, which forms a phagosome. The lysosome binds to the phagosome, forming a phagolysosome, and pours its content into it, degrading the substances in the phagosome. Once the usable molecules are hydrolyzed, they pass into the cell to enter metabolic pathways, and what is not necessary for the cell is discarded outside the cell by exocytosis.

• Lysosomes also dump their enzymes out of the cell (exocytosis) to further breakdown other materials. In view of their functions, their presence is elevated in white blood cells, because they have the function of degrading invading bodies.

• Lysosomes are spherical or oval organelles that are located in the cell cytoplasm.

• In electron microscopy they are easy to locate because it is the darkest organelle (the most stained) of all the cytoplasm in the cell, while the mitochondria have a more grayish staining. The image on the right shows a partial image of the cell, the lysosomes have been indicated with red arrows.

• Lysosomes consist of a membrane that contains a cavity or lumen, it is a closed sac.

• The content of lysosomes in a single cell is highly variable. Basically, the content of a lysosome can appear homogeneous or heterogeneous.

• When they are formed, lysosomes are loaded with enzymes of hydrolytic function; This type of lysosome, known as the primary lysosome, may be the one with a homogeneous appearance inside). From this, the lysosome is charged to catabolize most types of biochemical molecules that are in the cell.


Lysosome Structure & Functions

Structure of Lysosomes

The structure of lysosomes is not very complex. They appear in the form of spherical corpuscles, with variable dimensions that can be between 100 and 150 nanometers (nm) in diameter. Although small, these corpuscles can represent 5% of the total volume of the cell, a percentage which is modifiable depending on the rate of digestion that the cell is carrying, this is the amount of substances that it is “dismantling”.

The most remarkable part, after the enzymes that we have already seen previously, is the lysosomal membrane. It is a simple membrane, which is intended to prevent the enzymes found within the lysosome from dispersing throughout the cytoplasm. Since enzymes are substances that induce processes in which molecules are “destroyed”, it is advisable to keep them safe, since, otherwise, the cell is destroyed, inducing its autolysis.

In the event that the enzymes have been synthesized improperly, it can have serious consequences for the cell and, consequently, for the whole organism. This is because, in this case, the residual products that arise from the metabolic reactions that take place inside the lysosomes would be stored in the cell, which could damage it.

An example of disease due to problems with lysosomes is glycogenosis type II, where the enzyme β-glucosidase is absent, which causes large amounts of glycogen to accumulate in the organs, making it something fatal for the body.


Types of lysosomes 

They are divided into two varieties, primary and secondary lysosomes.

Primary lysosomes
They are also known as vesicles with hydrolases. These are derived from endomembranes. Each of them are newly formed vesicles that detach from the Golgi apparatus and contain hydrolase enzymes. These, in turn, are synthesized in the endoplasmic reticulum and transported via the gallbladder to the Golgi apparatus. Then, they undergo a transformation through a chemical process where a carbohydrate joins with another molecule.

Secondary lysosomes
They are those primary lysosomes that unite with other particles, either external or internal. And although they contain hydrolases, they also include substances that are in digestion. They are divided into two types according to the layer that hydrolyzes:

Autophageal vacuoles:
It is the union of a primary lysosome with different remains of unnecessary or injured cells, known as autophagolysosoma.
Heterophageal vacuoles: Union of the primary lysosome with extracts external to the cell, these are called phagolysosomes.
After absorption, what remains of the lysosome is a residual body. These contain non-assimilable wastes, which accumulate in the cytosol as the cell ages.

Functions of lysosomes


Functions of Lysosome
The enzymes that the lysosomes contain are capable of degrading lipids, polysaccharides and proteins, which are not going to be used by the cell, so the function of the lysosomes is mainly waste degradation.

Products that are no longer useful for the cell are transported to the lysosomes to produce their degradation to simple molecules, then return them to the cytoplasm and be recycled by the cell.

The primary function of lysosomes is to prevent necessary and fundamental cell structures from being degraded.

Lysosome enzymes have the ability to digest bacteria and substances that enter the cell by phagositosis, or endositosis as needed.

Thanks to the lysosomes, the cell's organelles are in constant replacement and renewal, since these are responsible for degrading them, a process called autophagy.

The most important enzymes that can be found in lysosomes are lipase, which is in charge of digesting lipids, glucosidases to digest carbohydrates, proteases, to digest proteins, and nucleases to digest nucleic acids..

Endocytosis

Endocytosis is the entry of macromolecules from outside the cell into the cell through the mechanism of endocytosis, which then these materials will be carried to small and irregular vesicles, called the initial endosomes. Some of the material is sorted and some are reused (discharged into the cytoplasm), which is not carried to the endosome further. In the advanced endosome, the material first meets the hydrolytic enzyme. In the initial endosome, the pH is about 6. There is a decrease in pH (5) in the endosome, causing maturation and forming lysosomes.

Autophagocytosis

The autofagi process is used for the removal and degradation of the cells themselves, such as organelles that are no longer functioning. Initially, part of the rough endoplasmic reticulum envelops organelles and forms autofagosomes. After that, autofagosomes fuse with the hydrolytic enzymes of trans Golgi and develop into lysosomes (or advanced endosomes). This process is useful in liver cells, the transformation of tadpoles into frogs, and human embryos.

Phagocytosis

Phagocytosis is the process of introducing large particles and microorganisms such as bacteria and viruses into cells. First, the membrane will wrap particles or microorganisms and form phagosomes. Then, the phagosome will fuse with the hydrolytic enzymes of the trans Golgi and develop into lysosomes (advanced endosomes).


Process of Formation of Lysosome

The origin and formation of lysosomes has been studied very intensively. From the various findings, there are two opinions regarding the origin and formation of lysosomes, namely:

Evidence has been found that hydrolytic proteins are formed by ribosomes found in the endoplasmic reticulum. From the rough endoplasmic reticulum, the protein is then transplanted to the surface of the Golgi body formation for further processing. After that, the hydrolytic proteins are packaged and wrapped in the form of vesicles and subsequently released as primary lysosomes.

Hydrolytic proteins are formed in ribosomes found in the rough endoplasmic reticulum, and then they are released in the form of vesicles into the GERL (Golgi associated Endoplasmic Reticulum giving rise to Lysosome) area which is adjacent to the mature surface area of ​​the Golgi body. From GERL, then released vesicles called primary lysosomes.

The first lysosomes are formed by cells and have not been involved in cell digestive activity, called primary glycosomes. Secondary silososomes are lysosomes, which are the result of repeated fusion between primary lysosomes with various membrane bound substrates (Albertet al., 1983). Lysosomes are cytoplasmic organelles similar in size to small mitochondria, involved in intracellular digestion (Marks, et al 2000: 132).

Lysosome biogenesis includes the synthesis of membranes and their enzymes. The lysosome membrane originates from the trans golgi mesh membrane. Enzyme synthesis starts from REK, then is transported to AG (sequentially from CIS> Media> Trans) from AG carried into transport vesicles to undolisosomes and finally to lysosomes (Istianti, et al, 1999: 50). Lysosomal enzymes are synthesized with secretion proteins and other proteins in the endoplasmic reticulum and transported in vesicles to the golgi complex.

In this endoplasmic reticulum, manose residues in enzymes prepared to join the phosphorylated lysosomes. This residue is then bound to the manose-6-phosphate receptor on a trans-golgi woven membrane, which is then transported in small vesicles and subsequently developed into lysosomes.


Frequently Asked Questions

What substances can be found in lysosomes?
Answer : Within the lysosomes we can find different types of enzymes, which will be specialized in digesting different types of substances. In order for the enzymes to be activated, the lysosomes must have a medium with an acidic pH, between 4.6 and 5.0. Among the main ones we have three:
  1. Lipases: digest lipids or fats.
  2. Glucosidases: break down and digest carbohydrates.
  3. Proteases: digest protein.
  4. Nucleases: they are in charge of nucleic acids.

Each lysosome can contain about 40 hydrolytic enzymes, that is, enzymes that catalyze a chemical reaction between a molecule of water (H2O) and one or more molecules of other substances.


What is autophagy?
Answer : One mechanism that manages to capture cellular proteins is called autophagy "eat itself." This event helps maintain cellular homeostasis, degrading cellular structures that are no longer needed, and contributes to the recycling of organelles.


Why are lysosomes acidic?
Answer : The acidic nature of the fluid within the lysosome serves two purposes. First, acids help digest and dismember complex molecules from redundant cell fragments. Second, the digestive enzymes and other chemicals that break the lysosome's targets are designed to work in an acidic environment. This feature protects the cell. In the event of a lysosome rupture or leak, the acidic fluid is rapidly neutralized, and lysosomal enzymes and other digestion chemicals will no longer be effective and will not attack healthy cell structures.


What is Role of lysosomes  in Disease
Answer : Lysosomes play a role in both fighting disease and causing the disease. When foreign pathogens, like bacteria, enter a cell, lysosomes can help neutralize them by digesting them. In this way they help with the immune response of an organism.

When lysosomes don't work properly, they can cause disorders called lysosomal storage diseases. If one of the many lysosomal enzymes has an incorrect chemical formula due to a genetic mutation, the substance that is supposed to digest the enzyme accumulates. Such disorders may also be related to cancer and cardiovascular disease. Lysosomes are normally an essential part of the cell waste disposal system, but when one or more enzymes are missing, rapid diagnosis is needed to replace the enzymes and reduce waste accumulation.


What is exocytosis in lysosomes?
Answer: It is a reverse process to endocytosis, it is used for the cell to pour substances outside. Through exocytosis, the content of intracellular vesicles is released to the outside by fusing with the plasma membrane.

In addition, it has a representative process which is called cellular secretion. First the constitutive secretion, which is carried out by eukaryotic cells continuously. Also, regulated secretion works only in specialized cells for secretion. It only occurs when the cell is stimulated by extracellular signal.

Now that you know what lysosomes are and their function, it is possible to conclude that they play a vital role in the animal cell.