Endocytosis: Definition, Types, & Examples

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Endocytosis Definition

Endocytosis is the process by which a cell absorbs materials from the environment by engulfing and fusing them with its plasma membrane. Etymology: Greek éndon, which means “inside,” Plus “kutos,” which means “hollow vessel,” + -“osis,” which denotes a state or situation.

Exocytosis is the opposite of exocytosis. Exocytosis is the opposite biological process, in which the cell expels material from its inside to the external environment.

What is Endocytosis?

Endocytosis is a biological process in which a cell takes in any substance from its surroundings and internalises it (liquid or solid). In 1963, De Duve coined the phrase endocytosis. It is either a process of ingesting pathogens in order to neutralise them or a process by which a cell obtains the nutrients and essential elements required for its growth and multiplication from the external environment.

The cell membrane folds around the material to be engulfed during endocytosis, resulting in the formation of a vesicular structure that pinches away from the membrane inside the cell. It’s crucial to know whether endocytosis is a passive or active process.

The process of endocytosis is active. This means that ATP molecules play a role in the process. Macrophages engulf pathogenic material (such as viruses or bacteria) and, with the help of lysosomes, neutralise it. The material that has been endocytosed is digested by lysosomal hydrolytic enzymes. Endocytosis is a technique used by cells to:

1. Obtain the nutrients required for cell development and repair.

2. Remove the toxin or unwanted pathogens from the cells and neutralise them.

3. Remove any cells that are no longer functioning.

Endocytosis, Endocytosis Definition, What is Endocytosis,

The mechanisms of endocytosis and exocytosis are responsible for the bulk movement of cellular material. The process by which cells remove or release material into the extracellular space is known as exocytosis.

Exocytosis is a process in which cellular material is transported into the extracellular space by fusing a vesicle with the plasma membrane. Exocytosis is necessary for the evacuation of unwanted / waste material from cells, for cellular communication, and for cellular development and repair.

This is the most common way for macrophages to get rid of trash or infectious material. Exocytosis involves the transfer of glucagon from the pancreas to the liver, as well as the release of neurotransmitters into the synaptic cleft.

Endocytosis Pathway Overview

1. Invagination: The cellular membrane folds to form a cavity in which the substance to be ingested is confined.

2. Vesicle Formation: The cellular membrane folds back to create a vesicle with a consistent thickness of vesicular membrane. This vesicle contains the material to be ingested.

3. Once the vesicle is created, it separates from the cellular membrane and is transported to the cell for further processing.

Types of Endocytosis

Phagocytosis, pinocytosis, receptor-mediated endocytosis, and caveolae are the four kinds of endocytosis.

i. Phagocytosis

Phagocytosis, often known as cell devouring, is an endocytosis process in which big cells consume massive foreign particles or materials. A phagocyte is the type of cell that performs endocytosis. Élie Metchnikoff developed the phagocytosis process in 1882.

Phagocytosis is a cellular absorption process in which a cell ingests or engulfs large-scale materials by modulating the plasma membrane. William Osler, a Canadian physician, was the first to discover phagocytosis in 1867.

White blood cells that show phagocytosis include macrophages, monocytes, neutrophils, eosinophils, and dendritic cells. Phagocytosis is a five-step procedure that includes the following steps:

1. Detection: This is the initial stage of phagocytosis, in which the phagocyte detects the foreign material or antigen and begins moving towards it.

2. Attachment: The phagocytic cell adheres to the target. As a result of this connection, pseudopodia begin to develop. The expanded cell membrane that surrounds the target is known as pseudopodia.

3. Ingestion: The membrane of the pseudopodia combines to create a vesicle, which contains the target substance. A phagosome is a vesicle in which the target is contained.

4. Fusion: A phagolysosome is formed when the phagosome and the lysosome merge. Different hydrolytic enzymes are found in lysosomes. The contents in the vesicle are digested by these hydrolytic enzymes.

5. Exocytosis is the process by which the digested substance is removed from the cell.

Phagocytosis Examples

Phagocytosis is a process through which immune cells such as macrophages, dendritic cells, and neutrophils neutralise and eliminate harmful substances from the body. The typical illustration of phagocytosis is a white blood cell devouring a bacteria. The macrophages are the biggest immune cells, specialising in recognising antigens, adhering to them, and then ingesting and fusing, neutralising the antigenic material and finally removing it by exocytosis.

Macrophages are the most immunogenic cells in the body and contain pseudopodia. An antigen is a material that the human body identifies as alien and unwelcome. Viruses, bacteria, fungus, pollen grains, and other microorganisms are examples.

A phagocytic cell may require certain protein components from the blood to build a coating around the antigen to be swallowed during the adhesion process. Opsonin is the name for these proteins, and opsonization is the process of creating a coating over the antigen. Following opsonization, the phagocytic cell engages in phagocytosis, which engulfs the antigen.

ii. Pinocytosis

The process by which the cell engulfs or ingests the tiny particles of the fluid is known as pinocytosis or cell drinking (or endocytosis of fluid). The process of cellular absorption or ingestion of fluid or particles of tiny size by a cell with the modification of the plasma membrane is defined as pinocytosis in biology.

When it comes to the production of pseudopodia, pinocytosis varies from phagocytosis. The development of pseudopodia is lacking in pinocytosis. Rather, vesicles are the only ones that develop. Pinocytosis is similar to endocytosis in that it involves the formation of a vesicle surrounding the tiny particles or fluid to be absorbed.

Internalized in the cell, the vesicles may merge with the cellular lysosome. The contents are eventually digested by the lysosomes’ hydrolytic enzymes.

Types of Pinocytosis

There are two kinds of pinocytosis:

1. Micropinocytosis: is characterised by the formation of tiny vesicles (0.1m in diameter). Micropinocytosis is a condition in which vesicles bud out from the cell and most body cells display micropinocytosis.

2. Macropinocytosis: the formation of enormous vesicles (0.5 to 5 m in diameter). Vesicles, on the other hand, are not generated by the cell splitting off. The plasma membrane ruffles or stretches into the extracellular area to engulf the substance to be swallowed, then rolls back into the intracellular space to form vesicles in micropinocytosis. White blood cells are well-known for exhibiting this mechanism.

Pinocytosis Examples

Pinocytosis manifests itself in the following ways:

1. Hormone and enzyme absorption by body cells.

2. Human egg cells absorb nutrients from their surroundings.

3. Nutrient uptake by microvilli cells in the intestines

Receptor-mediated Endocytosis

Clathrin-mediated endocytosis is another name for this kind of endocytosis. The endocytosis process is aided by a coat protein known as clathrin, as the name suggests. This is one of the most thoroughly researched endocytic mechanisms. The chemicals to be ingested bind to particular receptors on the cell membrane, which are located in the plasma membrane’s protein clathrin-rich areas.

Clathrin-coated pits are the name for these areas. Clathrin pits go through a process of internalisation once the molecule binds to the receptor, resulting in clathrin-coated vesicles. The clathrin coat from the internalised vesicles is subsequently removed when these clathrin-coated pits merge with the endosome.

We must first define what an endosome is. Endosomes are eukaryotic cells’ membrane-bound intracellular cell organelles. Endosomes are responsible for sorting the material that has been absorbed and delivering it to the lysosomes.

Clathrin-mediated Endocytosis

1. The chemical to be ingested binds to particular receptors on the plasma membrane, and the resulting complex drifts to a clathrin-coated pit area.

2. The clathrin-coated pit collects these molecule-receptor complexes, which finally invaginates and forms a vesicle.

3. A clathrin-coated vesicle containing the molecule-receptor eventually merges with the cellular endosome, causing the clathrin coating and receptor molecule to be removed. The receptor molecules are usually recycled back to the plasma membrane.

4. The vesicle next merges with lysosomes, where enzymes breakdown the vesicular content and transfer the necessary components back to the cytoplasm once the clathrin covering is gone.

Pinocytosis is a less efficient and selective endocytic mechanism than receptor-mediated endocytosis. Uptake of cholesterol-bound low-density lipoproteins, recycling of iron-bound transferrin, and the primary endocytic process in plant cells are all instances of receptor-mediated endocytosis.

Low-density lipoprotein receptors that are faulty or missing cause hypercholesterolemia. As a result, cholesterol is not excreted from the body and accumulates in the blood vessels, causing atherosclerosis.

Caveolae

Caveolae are bulb-shaped cavities that develop inside cells as a result of invagination of the plasma membrane, which is a clathrin-independent, receptor-mediated-cholesterol, and dynarrin-dependent pathway endocytic event.

The term “caveolae” literally means “small caves.” These cavities are 50-80nm in size. Caveolins and cavins, two proteins that aid caveolae development, are involved. Caveolins are membrane-integral proteins with a molecular weight of 21 kDa, whereas cavins are membrane-peripheral proteins.

Caveolin-1, caveolin-2, and caveolin-3 are three caveolins involved in the development and stability of caveolae. Caveolin-3 is a unique protein that can only be found in muscle cells. To create caveolae, these three caveolins collaborate with four types of cavins (cavin-1, -2, -3, and -4; cavin-4 is only present in muscle cells). Caveolae are totally absent in neurons, whereas endothelial cells have the greatest caveolae. Caveolae were initially discovered in the 1950s by Palade and Yamada.

Endocytosis Functions

Endocytosis is essentially a cell’s bulk transfer system.

1. Signaling via receptors

2. Nutrient absorption

3. The plasma membrane is being remodelled

4. Preventing pathogen entrance and neutralising pathogens

5. Essential signals’ neurotransmission

6. Modulating the cell’s responsiveness to different cell signals

Endocytosis Examples

By interacting with soluble proteins, hydrophobic compounds such as steroids, retinol, and others are transported and endocytosed. The retinol-binding group is attached vitamin A (retinol), vitamin D3 binds vitamin D, cortisol binds corticosteroid-binding globulin, and testosterone and estrogens bind to the sex hormone-binding globulin.

Similarly, receptor-mediated endocytosis facilitates the evacuation of hydrophobic and insoluble cholesterol from the body by forming a soluble complex with low-density lipoproteins. As a result of the faulty or absent low-density lipoprotein receptors, cholesterol builds up, resulting in hypercholesterolemia and atherosclerosis, both of which can lead to a heart attack.

Endocytosis, on the other hand, is used by the body to neutralise infections. Endocytosis, especially receptor-mediated endocytosis, is used by some harmful intracellular parasites to obtain access and establish themselves intracellularly in the host cell.

Pathogens such as the Epstein-Barr Virus (EBV), influenza virus, Listeria monocytogenes, and Streptococcus pneumonia use receptor-mediated endocytosis to fool the immune system and establish themselves in the host cells.

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