Lysosomes Definition, Structure, Function, Types
- Lysosomes are dense, membrane-bound granular structures that contain hydrolytic enzymes primarily responsible for intracellular and extracellular digestion.
- The word “lysosome” is made up of two words “lysis” which means decomposition and “soma” which means body.
- It is an important cellular organelle responsible for the inter and extracellular decomposition of substances.
- They are most commonly found in animal cells, while only in some lower plant groups (slime molds and saprophytic fungi).
- Lysosomes are produced freely in the cytoplasm. In animals, it is found in almost all cells except red blood cells.
- They are found in more abundant quantities in cells related to enzymatic reactions such as liver cells, pancreatic cells, kidney cells, spleen cells, leukocytes, macrophages, etc.
Structure of Lysosomes
Structure of Lysosomes
- Lysosomes do not have any characteristic shape or structure, that is, they are pleomorphic
- They are mostly globular or granular in appearance.
- It is 0.2-0.5 μm in size and is surrounded by a single compositional single lipoprotein membrane.
- The membrane contains highly glycosylated lysosome-associated membrane proteins (LAMP) and lysosomal integral membrane proteins (LIMP).
- LAMP and LIMP form a layer on the inner surface of the membrane.
- They protect the membrane from attack by the numerous hydrolytic enzymes retained within it.
- The lysosomal membrane has a hydrogen proton pump that is responsible for maintaining the enzyme’s pH conditions. The acidic medium maintained by the proton pump that pumps H + into the lumen ensures the functionality of the lysosomal enzymes.
- Inside the membrane, the organelle contains enzymes in the crystalline form.
For the degradation of extra and intracellular material, lysosomes are filled with enzymes called hydrolases. It contains around 40 varieties of enzymes that are classified into the following main types, namely:
- Proteases, which digest proteins
- Lipases, which digests lipids
- Amylase, which digests carbohydrates
- Nucleases, which digest nucleic acids
- Phosphoric acid monoesters
Collectively, the group of enzymes is called hydrolases that cause cleavage of substrates by adding water molecules. Most lysosomal enzymes work in the acidic environment.
Types of Lysosomes
- Small sac-like structures that enclose enzymes synthesized by the rough endoplasmic reticulum.
- Simply called as storage granules that store enzymes.
- Formed by the fusion of the primary lysosome with phagosomes.
- Contains wrapped material plus enzymes.
- The materials are progressively digested.
Lysosomes fulfill two main functions:
- Intracellular Digestion
- To digest food, the lysosome membrane fuses with the vacuole membrane of food and throws enzymes inside.
- The digested food then diffuses through the vacuole membrane and enters the cell for energy and growth.
- Autolytic Action
- The cellular organelles that need to be mounted are covered by vesicles or vacuoles by the autophagy process to form an autophagosome.
- The autophagosome is destroyed by the action of lysosomal enzymes.
The processes in which lysosomes play crucial roles include:
Collection of exogenous material into the cell by phagocytosis or pinocytosis and digestion of the ingested material after fusion of the newly formed vacuole with a lysosome.
A normal physiological process that deals with the destruction of cells in the body. It is essential to maintain homeostasis, for normal functioning due to protein degradation, replacement of destroyed cell organelles for the formation of new cells.
c. Extracellular Digestion
Primary lysosomes secrete hydrolases to the exterior by exocytosis, resulting in the degradation of extracellular materials.
Eg. Saprophytic fungi
It refers to the death of a complete set of cells by the decomposition of the lysosomal membrane. It occurs during the metamorphosis of amphibians and insects.
The acrosome of the sperm head is a giant lysosome that breaks down and releases enzymes on the surface of the egg. This provides the way for sperm to enter the egg by digesting the membrane of the egg.
Lysosomes digest many complex molecules, such as carbohydrates, lipids, proteins, and nucleic acids, that the cell recycles for other uses. The pH of lysosomes is acidic (around pH 5) because their hydrolytic enzymes work best at this pH rather than at the neutral pH of the rest of the cell. Hydrolytic enzymes specifically break down large molecules through hydrolysis. During the hydrolysis process, a molecule of water is added to a substance, causing it to sink. Like the human body’s digestive system, which breaks down food using enzymes, the lysosome can be thought of as the cell’s “digestive system” because it breaks down enzyme-using molecules.
Lysosomes digest several different types of molecules. They can digest the food molecules that enter the cell into smaller pieces if an endocytic vesicle (a vesicle that introduces particles into the cell) fuses with them. They can also perform autophagy, which is the destruction of malfunctioning organelles. Also, lysosomes have a role in phagocytosis, which is when a cell gobbles down a molecule to break it down; Also known as “eating cells.” For example, white blood cells called phagocytes ingest invasive bacteria to break it down and destroy it, and the bacteria are enclosed by a vesicle with which lysosomes fuse. These lysosomes then break down the bacteria.
Lysosomal Storage Diseases
Some inherited metabolic disorders can cause defects in the smooth functioning of lysosomes. These disorders are called lysosomal storage diseases, or LSD. There are around 50 different LSDs. Each type of LSD is rare, occurring in less than 1 in 100,000 births; however, as a group, LSDs occur at 1 in 5,000-10,000. LSDs generally occur when a person is deficient in an enzyme that breaks down large molecules like protein or lipids. Due to the lack of the enzyme, large molecules cannot break down, and eventually accumulate within the cell and destroy it.
Most LSDs are inherited in an autosomal recessive pattern. This means that it can be masked with a copy of one allele without the mutation (a dominant allele) and is caused by a mutation in one of the autosomal chromosomes, which are all chromosomes except the X and Y sex chromosomes. Tay-Sachs disease is An example of a known LSD that is inherited recessively. Due to insufficient function of the enzyme hexosaminidase A, glycolipids accumulate in the brain and interfere with normal function. This causes nerve cells to break down and physical and mental function to decrease. There is no cure, and death generally occurs at age four.
Some LSDs are X-linked; they occur due to a mutation on the X chromosome. One of those LSDs is Fabry disease. Fabry disease is rare and occurs in 1 in 40,000-120,000 live births. People with Fabry disease are deficient in the enzyme alpha galactosidase A, which causes glycolipid globotriaosylceramide to accumulate in the body. Symptoms include fatigue, burning limbs or pain throughout the body, tinnitus, nausea, heart and kidney complications, and papules on the skin called angiokeratomas. The mutation that causes Fabry disease is found on the X chromosome, but women with only one copy of the mutated gene also show symptoms. Since men only have one X chromosome, their symptoms tend to be more severe.