Endocytosis Definition

• Endocytosis is a biological process through which a cell takes in materials from its surrounding environment, such as proteins, liquids, electrolytes, microbes, and certain macromolecules. These compounds go through specific procedures that reduce them to smaller components, either for cellular use or for excretion.
• The majority of immune system cells that employ endocytosis processes to remove microbial infections from the body are white blood cells.The pathogens are captured, broken down, and destroyed so that they may be eliminated from the body.
• The invention of cellular components like lysosomes, peroxisomes, endosomes, and even exocytosis biological systems, including the endocytosis process, was made by Christian de Devu, a Belgian cytologist and biologist who won numerous Nobel prizes for his work.

Process of Endocytosis: Summary

1. Invagination is the process by which the cell membrane folds, creating a hollow that is filled with extracellular fluid, dissolved chemicals, food particles, foreign objects, pathogens, and/or other substances.
2. A vesicle is an enclosed membrane or chamber that develops around the trapped molecules during invagination when the cell membrane folds back toward itself in a uniform manner. Some cells create cytoplasmic extension channels.
3. The generated vesicle separates from the cell membrane and is processed by the cell after that.

Types of Endocytosis mechanisms

Three different endocytosis processes exist:

• Phagocytosis
• Pinocytosis
• Endocytosis Mediated by Receptors (Clathrin-Mediated Endocytosis)

1. Phagocytosis

• The process through which a cell’s cell membrane expands toward a particle is often referred to as “cell eating.” engulfs it, and folds the membrane around it to create a phagosome. Cellular enzymes later digest the ingested substance in the phagosome. White blood cells (macrophages, monocytes, neutrophils, eosinophils, and dendritic cells) frequently use phagocytosis to remove pathogens from the body in multicellular creatures. Entamoeba spp. and other protozoans employ phagocytosis to get nutrients;
• Canadian doctor William Osler was the first to describe the phagocytosis mechanism (1876).
• Phagocytosis takes place in 5 steps:
• The phagocytic cells locate the target molecule or antigen and travel in its direction.
• The phagocyte subsequently attaches to the specific molecule or antigen.
• Phagocytes have the capacity to surround the pathogen particle by extending their membrane (pseudopodia) to the specific particle. The pseudopodia enclose the particles and expand toward one another.
• The expanded pseudopodia that have united create a vesicle that contains the particle. A phagosome is a vesicle that contains encapsulated particles. This is the vesicle that the phagocyte breaks down.
• A phagolysosome is created when the phagocyte’s lysosomes and phagosome combine. The vesicle’s contents are broken down or digested by the digestive enzymes found in the lysosomes.
• Exocytosis is then used to evacuate the degraded particles from the phagocytic cell.

An example of phagocytosis

• The process used by immune cells, including neutrophils, dendritic cells, and macrophages, is a classic illustration of phagocytosis. The biggest phagocytic cells in the immune system are called macrophages. They function by finding, affixing, consuming, digesting, and then exocytosing the digested particles from the cell’s cytoplasm. Various antigens, such as germs, fungus, dust particles, dead cells, etc., are present. The majority of phagocytic cells in the immune system are macrophages. A pseudopodial membrane covers them. If an antigen is found, they migrate in its direction, extend their pseudopodia, and swallow it. The cell absorbs the antigen upon engulfment, generating a cell vesicle called a phagosome. When the vesicle enters the macrophage, it comes into contact with the lysosomes, producing a phagolysosome. Here, the lysosomal enzymes break down the particle, which is subsequently expelled from the cell through exocytosis.
• The chemical makeup of the particle has a significant impact on how well it adheres to the phagocyte. Some bacterial antigens attach to the phagocyte directly, whereas others require the formation of a film on the bacterial surface, a process known as opsonization, by a blood protein component called an opsonin (such as complements of antibodies). In order for phagocytosis to occur, the opsonin must initially be recognised by the phagocytes.
• Even with an opsonin, certain bacteria with enclosed cell walls can be challenging to break down. Therefore, after the body reacts to their existence, they must be attached to certain antibodies. The phagocytes can then interact with the bacteria that have been enclosed by an antibody.

2. Pinocytosis

• It is a kind of endocytosis also called “cell drinking” or “fluid endocytosis,” invagination allows extracellular fluids’ minute particles to enter the cell via the cell membrane and creates a tiny vesicle inside the cell containing suspended small molecules or particles. For the purpose of digesting the particles, the pinocytic vesicle and the cell endosome unite.
• The main difference between phagocytosis and pinocytosis is that during phagocytosis, ingested particles are confined inside the cells’ extracellular fluids. Its mechanics are similar to those of the other endocytic processes. The fluid particles were moved into the cell lysosomes by the cell membrane invaginating the vesicle that contained them. Digestive enzymes are released from the lysosomes when the vesicle and lysosomes combine. The vesicle is broken down by the enzymes, discharging its components into the cell’s cytoplasm for utilisation.
• In rare cases, instead of interacting with the lysosomes, the vesicles migrate throughout the cell, merging with the cell membrane to produce an effect similar to that of recycling of the lipids and proteins that make up the membrane.
• Pinocytosis takes place by two mechanisms:
• Micropinocytosisis the process by which tiny, budding vesicles, known as caveolae, grow on the cell membrane of body cells. These vesicles have a diameter of around 0.1 um. They are present in the endothelium of blood vessels.
• MacropinocytosisOn white blood cells, a process known as macropinocytosis results in the formation of larger vesicles that range in size from 0.5 to 5 um. These large vesicles are created by cell membrane ruffles (villi), which are extensions that extend into extracellular fluids and have the capacity to fold back on their own. They squirt some extracellular fluid into the fold, creating a vesicle that draws inside the cell.

An example of pinocytosis:

The small intestine’s ability to take up or absorb nutrients

3. Receptor Mediated Endocytosis (Clathrin Mediated Endocytosis)

• This kind of endocytosis, often referred to as clathrin-mediated endocytosis, includes the recycling and internalisation of receptors that are essential for functions including signal transduction (G-protein and tyrosine kinase receptors), nutrient absorption, and synaptic vesicle reconstruction.
• Phosphatidylinositol-4,5-bisphosphate (PIP2) buildup in the cell membrane is what starts this process. The catalysis of phosphoinositide within the plasma membrane results in PIP2 buildup. Phosphoinositide is converted to PIP2 by the hydrolysis of phosphatases and lipid kinase. Clathrin, a cytosolic protein, binds to the vesicle when adapter proteins (AP proteins) and phosphatidylinositol-4,5-bisphosphate (PIP2) are combined. Thus, Clathrin-coated vesicles are created (CCV).
• To generate the clathrin-coated pits, the Clathrin-coated Vesicles (CCV) must invade and develop. The Clathrin-coated vesicles attach to the cell membrane, attracting a number of proteins, such as Actin-binding proteins and Adapter proteins (AP), all of which are crucial for the vesicle’s development.
• A clathrin-coated pit is created when the CCV, which has many receptors associated with ligands and adaptor proteins, invaginates into the membrane and, with the help of the protein dynamin, matures and scissions from the cell membrane.
• The formation of Clathrin-coated pits Since most cells—if not all—contain clathrin-coated vesicles, these vesicles attract the plasma membrane’s adaptor proteins, which build up on the lipid layer of the plasma membrane as soon as a signal is detected.
• In addition to the actin-binding proteins, the adaptor proteins incorporate Clathrin from clathrin-coated vesicles into the lipids of the cell membrane. As a consequence of the concave shape that is lifted from the plasma membrane as an outcome of the negative charge of the lipid layer getting attracted to the positive charge of the clathrin, pits are formed all over the plasma membrane.
• Clathrin on the pits functions as a sensor for endocytosis-initiating signals, and clathrin-coated vesicles are returned to the cell membrane. As long as there are signalling receptors and ligands that activate them, the formation of clathrin-coated pits and clathrin-coated vesicles will occur in a continuous cycle.
• The following stages make up the receptor-mediated endocytosis process:
• The receptors on the cell membrane are attached to the particles (ligands) that need to be generated; these receptors group together to create coated pits. The dynamin proteins subsequently aid in the invagination of the pits, resulting in the formation of a vesicle that pinches off within the cell membrane. The adaptor proteins and clathrin are then lost from the vesicles.
• The early endosome and the uncoated vesicle subsequently combine to produce the late endosome, also known as the sorting vesicle. The late endosome separates the vesicle’s components, such as the receptors and ligands, before recycling them into the cell membrane.
• Lysosomes contain digestive enzymes which hydrolyze the components of the vesicles when they are in contact with the discharged particles. The digested components are subsequently made available for usage by the cell.
• The greatest use of this receptor-mediated endocytosis (clathrin-coated endocytosis) mechanism is the intracellular transport of macromolecules.

Examples of Clathrin-Mediated Endocytosis

• Two well-known instances of clathrin-mediated endocytosis include
• iron-bound transferrin recycling
• Receptor-mediated endocytosis refers to the absorption of cholesterol, which is linked to low-density lipoprotein (LDL), a combination of phospholipid, protein, and cholesterol.

References and Sources

• Color Atlas of Cytology, Histology and Microscopic Anatomy  by Kuehnel
• http://www.brooklyn.cuny.edu/bc/ahp/LAD/C5/C5_Endocytosis.html
• https://www.thoughtco.com/what-is-endocytosis-4163670
• https://bio.libretexts.org/Bookshelves/Cell_and_Molecular_Biology/Book%3A_Basic_Cell_and_Molecular_Biology_(Bergtrom)/17%3A_Membrane_Function/17.4%3A_Endocytosis_and_Exocytosis
• https://www.pathwayz.org/Tree/Plain/ENDOCYTOSIS+%26+EXOCYTOSIS
• https://www.britannica.com/science/pinocytosis
• MBINFO Defining Mechabiology:www.mechabio.info
• https://www.biologyonline.com/dictionary/opsonization
• https://www.britannica.com/science/phagocytosis
• https://www.britannica.com/science/pinocytosis