It is a tiny, circular, membrane-bound cell organelle as it is enclosed by a membrane and contains acidic digestive enzymes (hydrolase enzymes). Lysosome was discovered by Christian de Duve in the liver cells in 1949. The term lysosome and the name ‘suicide bags’ to lysosomes is also given by de Duve. They are found in all eukaryotic cells except for some fungi like yeast, Neurospora. They are also absent in some protists like Euglena and meristematic cells and mature mammalian RBCs. It is also known as the scavenger of the cells as it digests the unwanted debris or residues left in the cell.
Number of lysosomes:
The number of lysosomes in a cell varies from cell to cell. For example, the cells that are involved in intracellular digestion activity contain more number of lysosomes such as white blood cells (WBCs) and phagocytes as they ingest and digest harmful foreign particles. Similarly, Osteoclasts have more lysosomes as they digest old bone cells and also helps in replacing the cartilage with bony material.
Structure of Lysosomes:
They don’t have a specific shape or structure. They are pleomorphic cell organelles which means they can change their shape depending on their functions. However, they are generally globular or granular in shape.
The diameter of a lysosome ranges from 0.4 to 8 micrometre and is bounded by a single lipoprotein (made of lipids and proteins) bilayer membrane (similar to the plasma membrane). Besides this, glycosylated lysosomal associated membrane proteins (LAMP) and lysosomal integral membrane proteins (LIMP) are present in the membrane. These proteins form a layer on the inner side of the membrane and protect the membrane from the damaging effect of hydrolytic enzymes present in the matrix of the lysosome.
Size of lysosome:
They are generally spherical in shape and bounded by a membrane. They also have a matrix (the material inside the lysosome). Based on the type or nature of the matrix present, the lysosome can be of four types. So, it is also a polymorphic organelle as it can exist in different forms or types such as;
Types of lysosomes:
i) Primary lysosomes: They have inactive hydrolytic enzymes. They are also known as storage granules as they store chemicals that are required by the cell to degrade materials.
The enzymes found in primary lysosomes are synthesized by the ribosomes located on the rough endoplasmic reticulum and stored in the endoplasmic reticulum (ER). From ER they are transferred to the Golgi body in the form of vesicles where they are covered with a membrane. The primary lysosome contains only one type of enzymes, whereas, the secondary lysosome contains different types of hydrolytic enzymes.
ii) Secondary lysosomes: They contain active hydrolytic enzymes. They can be phagocytic vacuole (Phagosome) that carry out phagocytosis, and they can be present in the cytoplasm in the form of the autophagic vacuole (cyto-lysosome), which performs autophagy or self-destruction.
For example; Digestive vacuole (heterophagosomes or phagosome) are formed when foreign material is engulfed by the cell through phagocytosis or pinocytosis. The body containing the engulfed material is associated with a primary lysosome. The lysosome in the cell fuses with the phagosome to form the phagolysosomes, which is a secondary lysosome. After their fusion, the engulfed material in the phagosome is digested by the hydrolytic enzymes of the lysosome and the undigested waste is released from the cell membrane.
The rate of digestion of the phagosome depends on the nature and amount of the material engulfed by the phagosome. The digestion produces simpler molecules of low molecular weight which passes through the membrane of the lysosome and may be secreted out of the cell and if they are useful they can stay in the cytoplasm to be used by the cell.
iii) Tertiary lysosomes: They are also known as residual bodies. They contain undigested material that is left after digestion and absorption of food particles. So, they are lysosomes containing undigested material left due to incomplete digestion.
iv) Autophagosomes: They are also known as autolysosomes. They carry out the digestion of internal parts of the cell. For example, an old, damaged, or infected mitochondria in a cell will be digested by this type of lysosome. So, it also has digestive enzymes but it uses these enzymes to digest only unwanted internal parts of the cell. Sometimes, when an old cell needs to be digested then autophagosomes rupture inside the cell and enzymes are released and the entire cell is digested. Due to this reason, the lysosomes are also known as suicidal bags of the cell. Similarly, an Autophagic Vacuole engulfs worn-out parts of the cell to digest it. However, it does not destroy or digest the entire cell.
The first three types of lysosomes are also known as heterophagosomes as they are not involved in the digestion of intracellular structures. They digest the outside particles that they engulf like food particles, pathogens, etc.
So, the internal structure and shape of lysosomes may vary (polymorphism) due to the presence of different types of chemicals and due to the association of primary lysosome with different materials engulfed or digested by the cell.
Origin of lysosomes:
Lysosomes originate in different ways based on the tissue in which they are located and their function in a specific cell such as
- It can be a pinocytic vacuole, which is formed by engulfing liquid. So, it is extracellular in origin.
- It may be enclosures that contain leftovers of digested material.
- It may originate from the Golgi complex which is the most common method of its origin.
Chemistry of Lysosome:
A lysosome contains around 40 types of digestive or hydrolytic enzymes that can break down or digest a large number of molecules. It is a body rich in acid hydrolase. Its enzymes work best at acidic pH. To maintain the acidity in the matrix of the lysosome, the hydrogen ions are continuously pumped into the lysosome. So, the average pH of the inside of the lysosome is 5 which makes it more acidic than the cytoplasm, which has a pH of 7.
Besides this, there are certain enzymes that are not found in lysosomes such as oxidative enzymes like dehydrogenase and cytochrome oxidase. When the membrane of the lysosome ruptures the enzymes are released and become active. These enzymes remain inactive as long as the membrane remains intact or does not undergo any change after coming in contact with other cellular structures.
Types of enzymes in lysosomes:
Generally, there are six categories or types of enzymes present in lysosomes as listed below:
- Proteases: They digest or break down proteins.
- Nucleases: They digest nucleic acids such as DNA, RNA. For example, DNase and RNase.
- Glycosidases: They break the glycosidic bond.
- Esterases: They help in the breakdown of ester bonds.
- Phosphatases: They help in the breakdown of phosphate bonds.
- Sulfatases: They help in the breakdown of sulphate bonds.
Some of the major enzymes found in the lysosome are as follows;
- Acid ribonuclease that breaks down the RNA
- Acid deoxyribonuclease, which breaks down the DNA
- Acid phosphate that breaks down the phosphate monoesters
- Phosphoprotein phosphatase break down the phosphoproteins
- Cathepsin A and B break down the polypeptides
- Collagenase break down the collagen
Functions of Lysosomes:
- It performs intracellular digestion in which its membrane fuses with the food vacuole membrane and then it transfers its enzymes inside the vacuole. The digested food moves out of the vacuole membrane and used for growth and energy by the cell.
- It performs autolysis in which it digests the cell organelle or the entire cell. Auto phagosomes ruptured and their enzymes are released in dead cells, aging cells or damaged cells or malfunctioned organelles to digest them.
- It also performs Heterophagy (extracellular digestion) in which it digests the outside material that may be pathogens, food particles, harmful substances, etc. It secretes hydrolases (enzymes) into the extracellular environment through exocytosis. Thus, it can also break down extracellular material and so they are also called digestive bags.
- It also performs autophagy in which it digests the internal food reserves.
- The acrosome of the sperm head also contains lysosomal enzymes. It ruptures and releases enzymes on the egg’s surface to carry out fertilization.
- They also help to destroy carcinogens, cancer-causing substances.
- It also performs regression of the tadpole tail during the development of the tadpole.
- In some cells such as lymphocytes, the breakdown of lysosomes may activate mitosis. So, it also plays a role in cell division.
So, it is mainly responsible for the breakdown of complex substances into simpler substances. We can say that it is a waste disposable system of our body cells as it digests bacteria, viruses, worn-out cell organelle, and more.
