Lysosome Definition, Structure, Functions, Types

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Lysosome Definition

Lysosomes are membranous, spherical sacs that are made up of enzymes. They are acidic hydrolase enzymes that are capable of digesting cellular macromolecules. The membrane of the lysosome helps keep its internal compartments acidic, and also separates the digestion enzymes and the cells’ other components. Lysosome enzymes are created by the proteins of the endoplasmic reticulum , which is then enclosed in vesicles created by the Golgi system. Lysosomes are created by budding out of in the Golgi complex.

The word “lysosome” comes out of Greek word Lysis (meaning dissolve or destroy) in addition to Soma (meaning the body). Animal cells could contain many Lysosomes (several hundred) yeast and plant cells usually have one massive the lysosome (vacuole).

Alongside macromolecules, Lysosomes are also used to degrade cells after they cease to exist. They can be found in nearly all animal cells (except red blood cell) they are especially present in organs or tissues involved in high enzymatic reaction. They include organs/tissues like kidneys, livers as well as macrophages, pancreas and several other. The cells of these organs and tissues have a high concentration of Lysosomes.

Types of Lysosomes

There are two main types, these include:

  1. Primary lysosomes
  2. Secondary lysosomes

Primary lysosomes

They originate from Golgi apparatus and appear as tiny vesicles. Although primary lysosomes are a common sight in the Golgi apparatus however, they also appear in monocytes and granulocytes. These lysosomes are covered by one phospholipid layer, and also contain acid hydrolases.

The pH of the acid present in these vesicles is crucial because it activates or inhibit the enzymes. In the end, most primary granules are able to fuse with phagosomes. This causes the development secondary Lysosomes.

Secondary lysosomes

Secondary lysosomes form when primary lysosomes fusion with the phagosomes/pinosome (they are sometimes referred to as an endosomes). The fusion causes enzymes that were previously inactive to be active and capable of digesting biomolecules such as nucleic acids and lipids, among others.

As compared to primary lysosomes secondary lysosomes are bigger and are capable of releasing their contents (enzymes) beyond the cell in order to break down foreign materials.

Lysosome Enzymes

Most the lysosomal enzymes work in an acidic environment. This is the reason they are classified to as acid hydrolases. They comprise around 45 enzymes, which are classified into six major categories:

  • Nucleases – Nucleases are essential enzymes that can hydrolyze nucleic acids. Nucleases are classified into the deoxyribonuclease (acts by interacting with DNA) and ribonuclease that hydrolyzes the RNA. The hydrolysis process on nucleic acid results in the creation of nitrogen bases, sugars and phosphates.
  • Proteases – Proteases include enzymes such as collagenase or peptidases which work on proteins and convert them into amino acids.
  • Glycosidases – Beta galactosidase work on glycosidic bonds in polysaccharides, converting polysaccharides into monosaccharides. For example galactosidase is an enzyme that is able to convert these bonds from galactose to glucose, and lactose to galactose to glucose and.
  • Phosphatases – Phosphatases are excellent instances of Phosphatases are acid phosphodiesterases. They are essential enzymes that work on organic substances, releasing the phosphate. But, the compound needs to contain a phosphate group.
  • Lipases – Lipases are composed of esterases and phospholipiases, which work on lipids in order to create alcohol and acids.
  • Sulphatases – These are enzymes that bind to organic compounds to release sulphates.

Lysosomes are unable to digest themselves. Most proteins within its membranes contain large quantities of sugar and carbohydrate. Because of group digestive enzymes are not able to break down proteins in the membrane.

Morphology and Structure of Lysosome

Morphology and Structure of Lysosome
Morphology and Structure of Lysosome
  • Lysosomes tend to be small, with sizes ranging between 0.1-0.5 um, but they can grow to 1.2 um.
  • They are simple in their shape; they’re formed by an lipid bilayer which encloses the fluid, which is filled with a range in hydrolytic enzymes.
  • The lipids that form the bilayer are phospholipids which are a class of molecules with hydrophilic phosphate group heads the glycerol molecule and hydrophobic tails of fatty acids. Because of these distinct characteristics, phospholipids naturally form double-layered membranes when they are placed in water-based solutions.
  • The heads of the phosphate group move towards the outside of the layer, whereas the tails of fatty acids move towards the interior of the layer to stay far from the water.
  • Phospholipids form a number of other cells’ membranes including the cell membrane that surrounds the whole cell and it’s nuclear membrane (or nuclear envelope) that covers the nucleus and the Golgi apparatus and the endoplasmic and reticulum.
  • Lysosomes form by budding off from the Golgi system, and the hydrolytic enzymes contained within them are created within the endoplasmic-reticulum.
  • The enzymes are identified by the mannose-6-phosphate molecule, and transferred into the Golgi apparatus through vesicles and then packed into Lysosomes.

Functions of Lysosome

The lysosomes’ lysosome-specific enzymes contain can degrade polysaccharides and lipids as well as proteins. They aren’t likely to be utilized by cells, therefore the role of Lysosomes is mostly waste removal.

The products that are no longer essential to cells are taken to Lysosomes for their degradation into simple molecules. They then back to the cytoplasm, where they are recycled by cells. The main function of lysosomes is to keep essential cell structures from being destroyed.

Lysosome enzymes digest substances and bacteria that enter cells via either phagocytosis or endocytosis when necessary. Through lysosomes cells’ organelles undergo continuous replacement and renewal because they are accountable for their degrading in a process known as autophagy.

The most significant enzymes located in lysosomes is lipase that is responsible of digesting lipids, glucosidases that digestion carbohydrates and proteases in order to break down proteins as well as nucleases, which break down nucleic acids.


Endocytosis refers to the entrance of macromolecules outside of the cell into it through the process of endocytosis and then the materials are carried into small and irregular vesicles. They are called the first endosomes. A portion of the material is separated, and some can be recycled (discharged into the cell cytoplasm) and are not transported to the endosome for any further transport. In the endosome that is advanced, the material is first exposed to an enzyme called hydrolytic. In the beginning endosome, the pH is around 6. The pH decreases to pH (5) within the endosome, leading to maturation and the formation of the lysosomes.


The autophagic process is utilized to eliminate and degrade of cells in themselves including organelles that have stopped functioning. At first, the endoplasmic reticulum’s rough surface encases organelles, and then forms autophagosomes. Autophagosomes subsequently fuse with hydrolytic enzymes from trans-Golgi, and grow into Lysosomes (or advanced endosomes). This process is beneficial in liver cells, and the transformation of frogs from tadpoles or human embryos.


Phagocytosis can be described as the procedure of inducing massive particles and microorganisms like viruses and bacteria into cells. In the beginning, the membrane will contain microorganisms or particles, and create Phagosomes. The phagosomes will join with the hydrolytic enzymes in the trans-Golgi, and grow into the lysosomes (advanced endosomes).

Formation of Lysosome

The development and origin of lysosomes are being thoroughly studied. Based on the numerous research findings there are two theories concerning the formation and origin of lysosomes. The two theories are:

Evidence suggests to show that the proteins hydrolytic are produced by ribosomes in the endoplasmic retina. From the rough endoplasmic retina it is transferred onto on the outside of the Golgi body to further process. The hydrolytic proteins are packed and encased in the form of vesicles. They are then released as primary Lysosomes.

Hydrolytic proteins are produced in ribosomes located within the rough endoplasmic reticulum which, after that they are released as vessels into the GERL (Golgi connected Endoplasmic Reticulum leading to the formation of Lysosome) space that is situated near to the mature surface that is the Golgi body. From GERL they release vesicles known as primary Lysosomes are released.

The first lysosomes were formed by cells but haven’t been implicated in digestive function, referred to as primary glycosomes. Lysosomes that are secondary are lysosomes which result from the frequent fusion of primary lysosomes and various membrane-bound substrates. Lysosomes can be described as cytoplasmic organelles comparable in size to mitochondria of smaller sizes which are which are involved in digestion within the cell.

Lysosome biogenesis involves the production of membranes and enzymes. The lysosome membrane is derived from the trans-Golgi mesh. Enzyme production begins from REK which is then transferred into AG (sequentially through CIS> Media > Trans) from AG and then transported through transport vesicles , which then go to endolysosomes, ultimately to Lysosomes. Lysosomal enzymes are produced by secretion proteins, as well as other proteins from the endoplasmic Reticulum. They are transported via vesicles into in the Golgi complex.

In this endoplasmic and reticulum mannose residues from enzymes are made to connect Lysosomes that are phosphorylated. The residue is then bonded by the mannose-6-phosphate receptor the trans-Golgi-woven membrane which is then transported into small vesicles . They then develop into Lysosomes.

Characteristics of Lysosomes

  • Lysosomes are spherical vesiclesthat range from 0.1 up to one um size. They are home to about 50 enzymes, which are generally hydrolytic in acidic solutions The enzymes require the acidic solution to function at their optimal functioning. Lysosomes protect these enzymes from the cells and therefore keep them from chemically reacting with organelles and elements in the cell.
  • Cell Lysosomes utilize its enzymes for recycling various organelles within the cell. They cover them, degrading them and then releasing their constituents into the cell’s cytosol. This process is referred to as autophagy. In this process, the cells digest their own cells which are not required. The matter is covered by vesicles derived from the endoplasmic Reticulum and the Golgi apparatus, creating an autophagosome. When it joins the primary lysosome, it creates an autophagolysosome. It follows the same procedure as in the previous instance.
  • In the process of endocytosis, the substances are taken from the cell’s exterior and then absorbed by endocytosis across the plasma membrane. It creates an Phosome. The lysosome is bound to the phagosome and forms an phagolysosome and then pours its contents into it, destroying the substances contained in the phagosome. After the usable molecules have been hydrolyzed, the molecules enter the cell to be absorbed into metabolism, and what isn’t needed by the cell’s removal is removed from the cell via exocytosis.
  • Lysosomes also flush their enzymes from their cells (exocytosis) to degrade other substances. Because of their roles they are found within white blood cells due to their function of degrading invaders’ bodies.
  • Lysosomes are oval or spherical organelles found in the cell’s the cytoplasm.
  • In electron microscopy , they are easily identified since it is the most dark organelle (the most stained) of cells’ cytoplasm and mitochondria exhibit darker staining that is grayish. The image to right shows a portion of an view of the cell. the lysosomes are marked by red Arrows.
  • Lysosomes comprise the membrane, which has the lumen or cavity. it’s a closed sac.
  • The content of lysosomes within one cell can be very diverse. In essence, the contents of a lysosome could appear homogeneous , or heterogeneous.
  • Once they’re formed lysosomes are stuffed with hydrolytically-active enzymes The type of lysosome is referred to by its name as the principal lysosome may be the one that has an identical look inside). This is because the lysosome can catabolize all kinds of biochemical compounds that exist present in cells.

Lysosomal Storage Diseases

Certain metabolic disorders that are inherited can result in problems with the functioning of the lysosomes. These are referred to as Lysosomal Storage Disorders LSDs. There are about 50 distinct LSDs. Each kind of LSD is uncommon and occurs in less than one in 100,000 births. However as a whole, LSDs occur in 1 of 5,000 to 10,000. LSDs typically occur in people who are deficient in a specific enzyme that can break down large molecules, such as proteins or the lipids. Since the enzyme is not present the larger molecules can’t be broken down and eventually, they build up inside the cell, and eventually kill it.

The majority of LSDs are passed down through an autosomal recessive manner. This means that they can be disguised by a duplicate of an allele that does not have being affected by the change (a dominant allele) and can be due to a mutation on an autosomal genomes, which include all chromosomes, with the exception of the sex chromosomes, X and The Y. Tay-Sachs ‘ disease can be one of the most well-known LSD which is recessively acquired. Because of the lack of function in the enzyme called hexosaminidase glycolipids build up inside the brain, causing problems with normal brain function. The result is that nerve cells fall apart, and mental and physical functioning to decrease. The cure isn’t there and the majority of deaths occur by the age of four.

Some LSDs are X linked and occur due to an alteration on the X the chromosome. One of these LSD can be Fabry disease. It is extremely rare and occurs in one of 40,000-20,000 live births. People with Fabry disease are deficient in the enzyme alpha galactosidase A, which causes the glycolipid globotriaosylceramide to build up within the body. It is characterized by burning pains in the extremities and full of body pain and tinnitus nausea, kidney and cardiac complications and papules on the skin that are called angiokeratomas. The mutation which causes Fabry disease is found on the X-chromosome. However, females who have only 1 copy of this affected gene can also exhibit symptoms. Since males have just one the X chromosomes and have a single copy of the gene, their symptoms are likely to be more serious. The life expectancy of those suffering from this condition across the United States is 58.2 for males and 75.4 for females.

I-cell disease

The cause is genetic deficiencies in the N-acetylglucosamine Phosphortransferas. This enzyme is essential to add mannose-6-phosphate to the lysosome-targeted proteins. The result is that lysosomal enzymes are not being targeted properly. In the end, large amounts of them are present in the urine as well as the blood stream.

Mechanism of Lysosome

Mechanism of Lysosome
Mechanism of Lysosome | Image Source:

Since lysosomes are tiny digestive machines, they get to work whenever the cell ingests certain food items. When the substance is inside the cell, lysosomes are attached to release enzymes. They breakdown complex molecules which may include complex sugars as well as proteins. But what happens if food is limited and cells are in need of food? The lysosomes get to work even when there isn’t food available for the cell. If the signal is released the lysosomes actually digest cells organelles to obtain the necessary nutrients.

Acidic nature of Lysosomes

The acidic nature Lysosome’s fluid serves two functions.

The first is that the acids assist by digesting and dismembering complex molecules of cells that are redundant. The digestive enzymes and other substances that break down the lysosome targets have been designed to function in an acidic atmosphere. This protects cells.

If the lysosome breaks or begins to leak in acidic fluid, it is quickly neutralized and the enzymes in the lysosomal system and other digestion chemicals cease to work and will not harm healthy cell structures.

Maintaining an acidic pH within the lysosome therefore is essential to its function as well as for cell protection. The lysosome does this by performing chemical reactions that involve proton pumps located on its surface as well as inside the membrane, to move hydrogen ions or protons through the membrane, and then to the inside.

The hydrogen ions keep an acidity in the internal fluid.

Observation of lysosome under Microscope

Lysosomes can be too small to observe using an optical microscope. This is why electron microscopes are employed to examine the lysosomes. It is nevertheless possible to see the in-cell lysosome (vacuole) within the plant cell. Here is the procedure to examine the vacuole of a plant:


  • An onion
  • Glycerin
  • Safraning solution
  • A pair of forceps
  • A dropper
  • Microscope glass slides
  • A light compound microscope
  • Distilled water
  • Microscope cover slip
  • Two watch glasses


  1. Drop a few drops pure water into a glass watch
  2. Utilizing a pair of forceps, cut off the membrane of the skin of an onion. Place it in the glass with the help of a small amount of water.
  3. Include a few drops safranin into the other watch glass empty.
  4. Choose the onion’s membrane with the forceps, then place it in the glass with safranin . Allow it to rest for around 30 seconds.
  5. Take the membrane back and place it back into the watch glass using the distilled water.
  6. Place a drop of glycerin into the center of a microscope slide
  7. Put the membrane of onion onto the slide of glass (in the form of glycerin) and cover it with an cover slip
  8. Slide the slide under the microscope and watch


In addition to seeing many irregular cells, and the cell nucleus Students will be able to be able to clearly see a huge vacuole that is located in the center inside the cell.



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