ENDOCYTOSIS: TYPES AND THEIR CHARACTERISTICS, FUNCTIONS, EXAMPLES - BIOLOGY - 2024

The endocytosis includes processes that allow the effective entry of various materials to the cell the cell membrane structure is a fairly strict control, both input and output, a variety of extracellular and cytoplasmic material including materials. Along with other processes such as simple diffusion and osmosis, the cell integrates the material necessary for proper cell function.

As the endocytosis process occurs, molecules of large molecular size, particles and even mixtures in solution enter. This occurs from invaginations or sacs that originate in the membrane and enter the cytoplasm in the form of vesicles where they will be processed by the cellular digestive machinery.

Source: Mariana Ruiz Villarreal derivative work: Gregor_0492

The process of endocytosis (entry of material into the cell), as well as exocytosis (process of material leaving the cell), are exclusive in eukaryotic organisms.

The eukaryotic cell has great energy requirements, since it is larger (on average 1000 times larger) than any prokaryotic organism. It is for this reason that the eukaryotic cell needs mechanisms that allow the entry of materials so that a great variety of biosynthetic reactions occur in it.

Types and their characteristics

Through the process of endocytosis, the cell maintains an effective exchange with the external environment.

During this cellular mechanism, quite diverse materials can enter the cell; thus, the endocytosis process can vary depending on the nature of the material that is engulfed by the cell and if there are mediators in the process.

Those processes in which the cell from the plasma membrane encompasses large particles are called phagocytosis. Similarly, the cell can also encompass molecules and other dissolved substances, calling this type of endocytosis "pinocytosis."

In addition to these processes, the material that enters the cell can be previously selected in specialized regions of the plasma membrane. In this case, endocytosis is mediated by receptors and the material that enters the cell is coupled to these receptors to be transferred to the interior of the cell in special vesicles.

All eukaryotic cells absorb liquids and solutes through pinocytosis, however, only a few specialized cells perform the phagocytosis process, as we will see later.

-Phagocytosis

Phagocytosis

Phagocytosis is a specialized form of endocytosis. In this opportunity, large particles or molecules, which include waste substances, microorganisms, and other cells, are ingested through invaginations of the cell membrane. Due to the nature of this process it is suggested as the cellular action of "eating".

How does phagocytosis occur?

Particles that are recognized to be "consumed" bind to (specialized) receptors that recognize them on the cell surface. These receptors mainly recognize N-acetylglucosamide residues, mannose, in addition to many other proteins, which trigger the extension of pseudopods that surround the particle and engulf it.

The movement of these pseudopodia is determined mainly by the action of actin and myosin filaments on the cell surface.

Once captured in the cell membrane, they enter the cytosol in the form of large vesicles called phagosomes. These will bind to a lysosome (a cellular organelle that contains a wide variety of digestive enzymes) to form a vacuole for processing, breaking down and degrading material called a phagolysosome.

Phagolysosomes can be quite large and heterogeneous since their size and shape are determined by the amount of material that is being digested.

Within this digestive vacuole, the enzymatic activity generates a large quantity of essential products that will be available to be used as an energy source by the cell.

-Pinocytosis

Nutrition of Protozoa. Pinocytosis. Image by: Jacek FH (derived from Mariana Ruiz Villarreal). Taken and edited from

In contrast to the process explained above, pinocytosis is a process in which small particles are continuously ingested, which in most cases are in soluble form. Here, the cell engulfs small amounts of material with the formation of membrane vesicles that are released into the cytoplasm.

The process of pinocytosis is basically considered the cellular action of “drinking”, since most of the material that enters the cell is liquid.

How does pinocytosis occur?

Pinocytosis can occur in two ways; in a “fluid or simple” way or in an “absorbing” way.

Both types of pinocytosis vary depending on how substances in solution or small particles are internalized. In fluid pinocytosis, substances in solution enter the cell as a function of a concentration gradient with the extracellular medium, and it in turn depends on the speed with which pinocytic vesicles form in the cell membrane.

Absorptive pinocytosis is a more efficient process, the entry rate of solutes into the cytoplasm is 100 to 1000 times higher than when it is performed by fluid pinocytosis, constituting a special process of receptor-mediated endocytosis.

-Receptor-mediated endocytosis

Receptor-mediated endocytosis is a specialized pinocytosis process and the best studied of the cellular endocytosis processes. At this point, the substances that enter the cytosol enter the cytosol in a chosen way through the participation of specific receptors that are found in greater concentration in small sectors of the plasma membrane.

The molecules often associate in advance with receptors found in convolutions of the cell surface called "clathrin-coated depressions." These depressions contain in some cases more than 20 receptors, each one specific for a particular macromolecule.

The vesicles formed in these specialized regions of the membrane, will be coated by the clathrin protein, and will include once the vesicle is released in the cytoplasm the membrane receptors (various types of them), and will also internalize small amounts of extracellular fluid.

In contrast, in fluid pinocytosis the material that enters the cell is not selected and the vesicles formed in the cell membrane do not present any clathrin coating, but more frequently by proteins such as caveolin. This process is also called clathrin-independent endocytosis.

There are also some larger vacuoles that enter material in solution into the cell in a process known as "macropinocytosis." During this process there is no material selectivity.

Features

Endocytosis has a wide variety of functions within the cell, however these vary if it is unicellular or multicellular organisms or the type of requirements that the cell has at a certain time.

Phagocytosis functions

The process can be considered a primary feeding process or a defense and waste disposal method. In protozoa and lower metazoan organisms (eg amoebae), phagocytosis is a mechanism for capturing food particles, whether they are waste substances, bacteria or other protozoa.

These organisms detect the material to be ingested through membrane receptors and encompass it with membrane projections, forming a large vesicle that will be processed inside the organism.

On the other hand, in most organisms, phagocytosis fulfills functions other than cellular nutrition. In this case, phagocytosis is used by specialized cells called "professional" phagocytes, which will eliminate both waste substances and invading agents from the body as a defense mechanism.

Functions of pinocytosis

The function of pinocytosis is basically to include material in solution into the cell. The absorbed solutes and metabolites are destined for cellular metabolism and are also used in the synthesis of several proteins of great interest in the functioning of the organism.

On the other hand, the incoming material can be selected to provide first-hand energy for cellular metabolism.

Examples

Endocytosis occurs at various scales within eukaryotic organisms. Here we will mention some outstanding examples:

Phagocytosis

In mammals as well as other vertebrates, there are several classes of cells that are part of the blood tissue called white blood cells together. These cells act like professional phagocytes, which means that they are specialized cells in gobbling up material.

Macrophages, lymphocytes and neutrophils (leukocytes), are responsible for eliminating and ingesting infectious microorganisms from the body.

Phagocytes in the blood generally work best when they can trap the pathogen on a surface, such as the wall of a blood vessel or a fibrin clot.

These cells participate in specific and nonspecific immune functions, there are even phagocytes specialized in presenting antigens to trigger the immune response

In addition to this, macrophages "mainly" are responsible for engulfing and eliminating approximately 10 ¹¹ red blood cells from the blood, as well as other old cells and waste substances, to maintain a process of continuous cell renewal. Together with lymphocytes they operate in the destruction of most pathogens in the body.

Pinocytosis

The pinocytosis process is usually quite effective in incorporating extracellular material. In absorptive pinocytosis, receptors located on clathrin-coated membrane fossa vesicles can recognize growth factors, various hormones, carrier proteins, as well as lipoproteins and other proteins.

A classic example of this process is the capture of cholesterol from receptors on the membrane. Cholesterol is transported to the bloodstream in the form of lipoproteins, the most commonly mobilized being LDC or low-density lipoproteins.

However, a great variety of metabolites such as vitamin B12 and even iron are also captured in the process, materials that the cell cannot internalize through active transport processes. Both determining metabolites in the synthesis of hemoglobin, a protein specialized in the transport of oxygen in the blood.

On the other hand, material is also integrated into the cell efficiently, through fluid pinocytosis. In endothelial cells of blood vessels, the vesicles transport a large amount of solutes and fluids from the bloodstream to the intracellular space.

Endocytosis, "a large-scale process"

Endocytosis is a very common process in eukaryotic cells, in which material is integrated both in solution and in the form of macromolecules and even whole cells and microorganisms.

In the case of receptor-mediated endocytosis, clathrin-coated depressions occupy about 2% of the total surface of the cell membrane. Each of these depressions has a half-life of two minutes, which causes the entire cell membrane to be internalized in a period of between 1 and 2 hours.

This means that 3 to 5% of the membrane is internalized every minute on average, which gives us an idea of ​​the magnitude of the process and the continuous renewal that the cell membrane undergoes.

Macrophages present in blood tissue, for example, "engulf" up to 35% of their cytoplasmic volume in about one hour, 3% of the plasma membrane every minute, and 100% in about half an hour.

Disadvantage of endocytosis

Although it is an essential process for cellular nutrition, the absorption of waste substances and the capture of external microorganisms, during processes such as receptor-mediated endocytosis many viruses and pathogens enter the cell. Influenza and HIV follow this route as a direct way to enter the cell.

What happens after endocytosis?

The vesicles released into the cytoplasm and the material encompassed by them are processed by the lysosomes. In lysosomes there is a powerful enzymatic battery where the substances present in the vesicles are degraded to usable products by cellular metabolism.

However, in the degradation process, various components of the plasma membrane are recovered. The specific receptors of the depressions coated with clathrin and other materials such as various membrane proteins, are sent to the Golgi apparatus or to the cell surface to be reintegrated into it in recycling vesicles.

This recycling process is very convenient and occurs at the same speed with which the vesicles are formed, since the cell membrane only synthesizes 5% of its surface every hour.

References

1. Alcamo, IE (1996) Cliffs Quick Review Microbiology. Wiley Publishing, Inc., New York, New York.
2. Alberts, B., Bray, D., Hopkin, K., Johnson, A., Lewis, J., Raff, M., Roberts, K. & Walter, P. (2004). Essential cell biology. New York: Garland Science. 2nd Edition
3. Madigan, MT, Martinko, JM & Parker, J. (2004). Brook: Biology of Microorganisms. Pearson Education.
4. Cooper, GM, Hausman, RE & Wright, N. (2010). The cell. (pp. 397-402). Marban.
5. Hickman, C. P, Roberts, LS, Keen, SL, Larson, A., I´Anson, H. & Eisenhour, DJ (2008). Integrated Principles of zoology. New York: McGraw-Hill. 14 ^th Edition.
6. Jiménez García, L. J & H. Merchand Larios. (2003). Cellular and molecular biology. Mexico. Editorial Pearson Education.
7. Kühnel, W. (2005). Color Atlas of Cytology and Histology (11th ed.) Madrid, Spain: Editorial Médica Panamericana.
8. Smythe, E. & Warren, G. (1991). The mechanism of receptor-mediated endocytosis. Eur. J. Biochem. 202: 689-699.