What is endocytosis in a cell? examples, types, and steps

Endocytosis is an example of a bulk transport mechanism used by some cell types. The endocytosis process and steps involve the transportation of molecules into the cell by engulfing it with the cell membrane.

Cells need to eat and drink but have a cell membrane that protects them. Some substances such as some molecules without charge, water, and small molecules cross the cell membrane through the process of diffusion.

However, some molecules that are still needed by the cell are too large to pass through the cell membrane. It is in such instances that endocytosis and exocytosis come to play.

Endocytosis and exocytosis are means of transport used by cells to move substances into and out of the cell’s cytosol respectively without the substance passing through the cell membrane. These two processes ensure there is a balance of nutrients and waste for the regular functioning of the cell.

Endocytosis involves the active transport of molecules into the cell by engulfing it with its membrane whereas, exocytosis pushes molecules out of an animal cell or plant cell. In living systems, the need for homeostasis causes an equal flow of molecules in and out of the cell.

Hence, the number of molecules coming into the cell via endocytosis equals the number of molecules exiting the cell through exocytosis. In other words, the two main categories of vesicle transport are endocytosis and exocytosis.

Endocytosis definition in biology

In biology, we can define endocytosis as a cellular process whereby cells take in materials from the external environment of the cell by engulfing and fusing the material with its cell membrane. “Endocytosis” as a term was coined by Christain De Duve in 1963.

The process of endocytosis requires energy and is used by the cell to procure the essential nutrients and elements from the external environment that are needed for the cell’s growth and reproduction. Not only does endocytosis moves materials it is also used to ingest pathogens in order to neutralize them.

There are basically 3 types of endocytosis commonly known which include phagocytosis, pinocytosis, and receptor-mediated endocytosis. However, there are four types of endocytosis recently observed, which include “caveolae” as one of the endocytosis types. This will be discussed further in the article.

The process of endocytosis

Endocytosis transports materials from the external environment of the cell into the cell via a vacuole. During the process of endocytosis, the plasma membrane (cell membrane) folds itself around the material to be engulfed.

This results in the formation of a vesicular structure that eventually pinches off from the membrane inside the cell. Therefore, the endocytosis process in cells involves steps that include invagination, vesicle formation, and the detachment of the vesicle.

Endocytosis steps

1. Invagination
2. Vesicle formation
3. Detachment

Invagination

This is the first of the endocytosis steps which involves the cellular membrane folding and creating a cavity wherein the material to be engulfed is entrapped.

Vesicle formation

The next step is the cellular membrane folding back to form a vesicle with uniform vesicular membrane thickness. This vesicle formed contains the material to be engulfed.

Detachment

After the formation of the vesicle, the vesicle detaches from the cellular membrane. The vesicle then proceeds for further processing by the cell.

Early endosomes, late endosomes, and lysosomes are the necessary components in the endocytic pathway. The role of the early endosomes is to receive vesicles on the cell surface as well as sort the received molecules. They sort them into other components in the pathway via trans vesicular compartments such as multivesicular bodies or endosomal carrier vesicles.

The late endosomes then receive the molecules from the early endosomes and begin the degradation of the molecules. Late endosomes can also receive molecules from phagosomes or the trans-Golgi network. Then, they deliver the molecules to lysosomes.

Lysosomes contain enzymes that break down proteins, carbohydrates, fats, and other cellular waste products into smaller, simpler molecules. These molecules are then delivered back to the cytoplasm for the cell to use as building materials.

There are four different pathways or types of endocytosis which include pinocytosis, phagocytosis receptor-mediated endocytosis, and caveolae.

Types of endocytosis

1. Phagocytosis
2. Pinocytosis
3. Receptor-mediated endocytosis or clathrin mediated endocytosis
4. Caveolae

Listed above are the different types of endocytosis which will be discussed below.

Phagocytosis

Phagocytosis also described as cell eating is one of the endocytosis types whereby large cells ingest large foreign material or particles. A cell that undertakes this type of endocytosis is referred to as a phagocyte. The phagocytosis process was discovered by Canadian physician William Osler in 1876 and later studied by Élie Metchnikoff in 1882.

Phagocytosis can be defined in biology as the cellular uptake process by which a cell engulfs or ingests large materials with the modulation of the cell membrane. Some white blood cells exhibit phagocytoses such as neutrophils, macrophages, eosinophils, monocytes, and dendritic cells. The process of phagocytosis involves 5 basic steps of endocytosis.

The five-step process of phagocytosis

1. Detection
2. Attachment
3. Ingestion
4. Fusion

Detection

It is the first step of phagocytosis. The phagocyte, in this step, recognizes the antigen or foreign substance and starts the movement towards the target substance or particle.

Attachment

In this process, the phagocytic cell attaches itself to the target substance. Thereafter, a pseudopodia formation is initiated as a result of this attachment. The pseudopodia are the extended plasma membrane that surrounds the target substance or particle.

Ingestion

During ingestion, the membrane of pseudopodia fuses to form a vesicle that encloses the target substance. This vesicle that encloses the target substances is known as a phagosome.

Fusion

At this stage, the phagosome fuses with the lysosome and forms a phagolysosome. Lysosomes are organelles that contain different hydrolytic enzymes that digest the materials or substances enclosed in the vesicle. The digested substance or material is then eliminated from the cell via the process of exocytosis.

Pinocytosis

The process of pinocytosis or cell drinking involves the ingestion or engulfing of small particles or fluid by cells. Pinocytosis can be defined in biology as the process of cellular ingestion or uptake of small-sized particles or fluid by a cell with the modulation of the cell membrane.

The formation of pseudopodia is what differentiates the process of phagocytosis from pinocytosis. In pinocytosis, there is no formation of pseudopodia, rather, only vesicles are formed. The process of pinocytosis follows the basic steps of the endocytosis process whereby a vesicle forms around the fluid or small particles to be engulfed. The vesicles formed are then internalized in the cell and may fuse with the cellular lysosome. Eventually, the fluid or small particles are digested by the hydrolytic enzymes of the lysosomes.

Types of pinocytosis

1. Micropinocytosis
2. Macropinocytosis

There are two types of pinocytosis:

Micropinocytosis

In this type of pinocytosis, small-sized vesicles (0.1µm in diameter) are formed. These vesicles bud off from the cell. The majority of the body cells perform micropinocytosis.

Macropinocytosis

In this type of pinocytosis, large-sized vesicles (0.5 to 5 µm in diameter) are formed. However, unlike in micropinocytosis, these vesicles formed are not formed by the process of budding off from the cell. Rather, in macropinocytosis, the cell membrane ruffles or extends into the extracellular space to engulf the material to be ingested and then roll back into the intracellular space to form vesicles. This type of pinocytosis is initiated by the actin-driven extension of cell membrane ruffles. White blood cells are known to exhibit macropinocytosis.

Receptor mediated endocytosis or clathrin mediated endocytosis

This is one of the endocytosis types that are best studied. The receptor mediated endocytosis definition is a process that is facilitated through a coat protein called clathrin. This is why receptor-mediated endocytosis is also known as clathrin-mediated endocytosis. In this type of endocytosis, the molecules that are to be engulfed fuse with specific receptors on the plasma membrane.

These specific receptors are found in the protein clathrin-rich regions (known as clathrin-coated pits) of the plasma membrane. After the receptor binding of the molecule occurs, the clathrin pits carry out the process of internalization which forms clathrin-coated vesicles. The clathrin coat from the internalized vesicles is then removed as the clathrin-coated pits fuse with the endosome.

Endosomes are membrane-bound intracellular organelles in the cells of eukaryotic organisms. Its main function is to carry out the sorting of materials that have been internalized and also deliver the materials to the lysosomes. The vesicles are then digested as soon as clathrin is removed. This type of endocytosis is responsible for the uptake of certain materials like hormones, metabolites, proteins, and certain types of viruses. The recycling of iron-bound transferrin, the chief endocytic process in plant cells, and the uptake of cholesterol-bound low-density lipoproteins are classical receptor-mediated endocytosis examples.

The clathrin mediated endocytosis is a more efficient endocytic process that is selective compared to pinocytosis. Therefore, the difference between pinocytosis and receptor-mediated endocytosis is that pinocytosis tends to be nonselective in the molecules it ingests into the cell, whereas receptor-mediated endocytosis tends to be more selective. Moreso, in pinocytosis, the endocytosis of non-specific molecules is carried out by the cell whereas, in receptor-mediated endocytosis, its the uptake of specific molecules that occurs.

Caveolae

This caveolin mediated endocytosis is a process that is a clathrin-independent, receptor-mediated- cholesterol, and dynarrin-dependent pathway. In this endocytic process, the invagination of the cell membrane causes the formation of bulb-shaped cavities inside the cells which are referred to as caveolae. The word caveolae means “little caves” and the size of these cavities is usually between 50-80nm.

There are two proteins, that facilitate the formation of caveolae which are caveolins and cavins.

1. Caveolins: These are an integral protein of 21 kDa of the membrane. Three caveolins are involved in the formation and stability of the caveolae which include caveolin-1, caveolin-2, and caveolin-3. Caveolin-3 is a specific protein seen only in muscle cells.
2. Cavins: These are the peripheral proteins of the membrane. There are four types of cavins which include cavin-1, cavin-2, cavin-3, and cavin-4. The cavin-4 is found specifically in muscle cells.

The three caveolin work in conjunction with the four types of cavins to form caveolae. Caveolae are most widely found on endothelial cells but are completely absent in neurons. The caveolae were first reported in the 1950s by Palade and Yamada.

Endocytosis function

1. Endocytosis functions in cells to procure the nutrients for cellular growth and repair.
2. Through the endocytosis process, toxins or unwanted pathogens are seized and eventually neutralized in the cells. Hence, it controls and neutralizes the entry of pathogens.
3. In cells, the processes of endocytosis function in receptor signaling and the uptake of nutrients.
4. The process of endocytosis helps to remodel the plasma membrane.
5. It plays a role in the neurotransmission of the essential signals and modulates the response of cells towards various cell signals.

Endocytosis examples

1. Phagocytosis in immune cells
2. Transportation and endocytosis of the hydrophobic molecules
3. Endocytosis example in pathogenic intracellular parasites
4. Uptake of nutrients, hormones, and enzymes by cells

Phagocytosis in immune cells

An example of endocytosis is phagocytosis in immune cells. In order to neutralize and eliminate pathogenic material from the body, immune cells such as dendritic cells, macrophages, and neutrophils exhibit phagocytosis. A typical example of phagocytosis is the white blood cell engulfing a bacterium.

Macrophages are the largest immune cells that are specialized in the detection of antigens. The substances that the body recognizes as foreign or undesirable substance is known as the antigen, which can be fungi, bacteria, viruses, pollen grains, etc. After the macrophages detect the antigen, they attach to it, leading to ingestion and fusion that results in neutralizing the antigenic material. This neutralized material is eventually eliminated from the cell via the process of exocytosis.

A phagocytic cell, for the process of adherence, sometimes requires certain protein components from the blood known as Opsonin. These specific protein components from the blood form a layer over the antigen to be engulfed, a process known as opsonization. After the completion of opsonization, the phagocytic cell then engulfs the antigen via phagocytosis.

Transportation and endocytosis of the hydrophobic molecules

Among the examples of endocytosis is the transportation and entry of hydrophobic molecules into the cell. The endocytosis of hydrophobic molecules such as steroids, retinol, etc. is done by the molecules binding with soluble proteins. For instance, vitamin D3 is bound to the vitamin D-binding protein, testosterone and estrogens are bound to the sex hormone-binding globulin, vitamin A (retinol) is bound to the retinol-binding protein, and cortisol is bound to the corticosteroid-binding globulin. They are all endocytosed through receptor-mediated endocytosis.

Endocytosis example in pathogenic intracellular parasites

As the body uses endocytosis to neutralize pathogens, some certain pathogenic intracellular parasites make use of endocytosis too to gain access and establish themselves in their host cell. These parasites specifically use receptor-mediated endocytosis. For example, enveloped viruses may enter their host cells by engulfment within coated vesicles (endocytosis). Some pathogens such as Epstein-Barr Virus (EBV), Streptococcus pneumonia, Listeria monocytogenes, and Influenza virus make use of receptor-mediated endocytosis. They trick the immune system and establish themselves in the host cells.

Uptake of nutrients, hormones, and enzymes by cells

The uptake of nutrients, hormones, and enzymes by cells via pinocytosis is one of the common endocytosis examples. For instance, body cells take up hormones and enzymes via pinocytosis. Another example of pinocytosis is the uptake of nutrients by human egg cells from the surrounding environment. Moreso, the microvilli cells of the intestines take up nutrients via pinocytosis.

Endocytosis diagram

Endocytosis vs Exocytosis

Endocytosis and exocytosis are examples of bulk transportation mechanisms in cells. Exocytosis and endocytosis are responsible for the bulk transportation of cellular material. The entry and exit of a material is the factor used to compare and contrast endocytosis and exocytosis.

In exocytosis, cells release or eliminate materials into extracellular space. During, the process of exocytosis, the vesicle fuses with the plasma membrane, and the cellular content is transferred into the extracellular space. The exocytosis process function in cellular communication, the removal of the cellular undesired /waste material, and enables cellular growth and repair. Examples of exocytosis include the release of neurotransmitters into the synaptic cleft, glucagon transportation from the pancreas to the liver, etc.

In endocytosis, on the other hand, cells ingest materials into the cell. The material or substance that is to be brought into the cell is surrounded by an area of the cell membrane which then buds off inside the cell to create a vesicle containing the ingested substance.

Phagocytosis vs endocytosis

The difference between endocytosis vs phagocytosis is that phagocytosis is a type of endocytosis. Endocytosis involves the movement of solid materials or fluids into the cell via the formation of cell membrane vesicle whereas phagocytosis involves the movement of solid materials into the cell via the formation of phagosomes.

Pinocytosis vs endocytosis

When comparing pinocytosis vs endocytosis, it is important to note that pinocytosis is a type of endocytosis. Pinocytosis or cell drinking involves the ingestion or engulfing of small particles or fluid by cells whereas endocytosis is a cellular process whereby cells ingest materials into the cell.

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