Table of Contents
Show
The human body is packed with more mysteries than we will ever understand, but we do understand the crucial processes that allow each of our cells, organs, tissues and muscles to function. The microscopic movement of material between cellular factories and the parts of the body where complex molecules are needed is an incredibly important process for human beings. This movement of molecules consists of numerous different processes, including endocytosis, phagocytosis and pinocytosis. Recommended Video for you: What is DNA and How Does it Work? What is Endocytosis?Have you ever watched bubbles floating on top of water? When they collide, the smaller bubbles are often consumed or swallowed up the larger ones. On an even smaller scale, at the cellular level, the cells in our body can do a similar thing – taking up molecules from the extracellular fluid. This process is called endocytosis, and is currently happening in millions of your cells at this very instant! While it may seem strange to think of a cell needing to consume something, there are many reasons that a cell requires processes like endocytosis, such as absorbing essential nutrients from the extracellular fluid, consuming entire microorganisms, or transporting dead or dying cells to where they can be eliminated. The actual process of endocytosis is simple and straightforward, although there are a number of different forms. Basically, once a molecule or other substance is encountered by the cell, the cell’s plasma membrane folds inwards surrounding the material, forming a pocket. Eventually, that pocket of membrane closes, and a vesicle is formed, which can separate from the membrane in the inside of the cell and then move to the organelle where it can be used. Endocytosis is the opposite process to exocytosis, which is the means by which a cell can export molecules, whether they are proteins, neurotransmitters, waste products or any other cellular material. We’ve covered all the details of exocytosis in this article.
Three Forms of Endocytosis. (Photo Credit: OpenStax/Wikimedia Commons) There are two main types of endocytosis, phagocytosis and pinocytosis, which are defined by the types of materials being taken up by the cell. Furthermore, there are two less common forms of endocytosis – caveolae and receptor-mediated endocytosis – which we will also briefly discuss. PhagocytosisWhen certain types of cells (phagocytes) consume an entire other cell or macromolecule, this form of endocytosis is called phagocytosis. This process can occur in various cells, including amoeba and white blood cells. For example, when a white blood cell is sent to the site of inflammation or infection, it can then consume unwanted bacteria, viruses, foreign particles or even pieces of dust. At that point, the unwanted material can be broken down into simple, usable molecules, or discarded from the body as waste. Depending on the size of the material being consumed, the speed of this process may vary. Since the molecules being consumed are typically quite large, the vesicles that must be formed require a moderate amount of energy and resources, in the form of ATP. Similar to the process of exocytosis, this is a form of active transport, as the molecules cannot passively be consumed through the cell membrane. It is important to remember that only certain specialized cells are able to perform phagocytosis, whereas pinocytosis, explained below, is something that all cells must perform. PinocytosisThe second main form of endocytosis is pinocytosis, by which a cell takes in a pocket of extracellular fluid and micro molecules at random. This process is also informally called “cell drinking”, which is an accurate description of this fluid uptake. Similar to phagocytosis, the plasma membrane of the cell extends to form a pocket or a bud, and then closes the pocket. This forms a vesicle within the cell, which is then pinched off and can be moved to other areas of the cell in the cytosol. This form of endocytosis is also important because it can transport extracellular liquid through the cell without it interacting or affecting the cytoplasm. As mentioned, this cell drinking occurs in every type of cell. Receptor-Mediated Endocytosis – Clathrin-Mediated and Caveolae-Mediated EndocytosisThe last two types of endocytosis are slightly more specialized, and require different cellular technology to occur. In clathrin-mediated endocytosis, larger macromolecules are taken into a cell through concentrated areas of clathrin-coated vesicles. Clathrin is a coat protein on the pits with a bristle-like action. These pits on a plasma membrane facilitate the easier movement of large molecules, and are found on almost all cell types. Certain molecules have different receptors, and the cathrin-coated pits are able to provide the necessary “keys” for the membrane “lock”. Caveolae is similar to receptor-mediated endocytosis, but the vesicles are not coated in clathrin, but another transport membrane protein called caveolin. They are small bottle-shaped vesicles waiting below the cellular membrane and serve a similar function as the clathrin-coated pits. Caveolae are extremely abundant in certain cell types, often occupying more than 20% of the surface area of the plasma membrane.
(Photo Credit: OpenStax/Wikimedia Commons) A Final WordThese processes are essential to our survival every day, and yet we have no conscious control over them. They are yet another example of how impressive and intricate our bodies are, and how even the simplest processes at the smallest level of life can have measurable impacts on health. As is so often seen and commented on in science, the microcosm reflects the macrocosm. Just as we consume food from our surroundings, so too do our cells, and while we don’t capture our hamburgers with a plasma membrane, there are certainly parallels if you pay attention! Suggested ReadingWas this article helpful?
 endocytosis n., plural: endocytoses [ˌɛndəʊsaɪˈtəʊsɪs] Definition: a cellular process wherein materials from the outside are taken into the cell, e.g., by engulfing What is endocytosis in biology? Endocytosis is a cellular process by which a cell internalizes any material (liquid as well as solid) from the external environment. The term endocytosis was coined in 1963 by De Duve. What is endocytosis used for? It is either the process by which a cell procures the nutrients and essential elements, required for its growth and multiplication, from the external environment or, a process of ingesting pathogens in order to neutralize them. In the process of endocytosis, the cell membrane folds itself around the material to be engulfed resulting in the formation of a vesicular structure that eventually pinches off from the membrane inside the cell. It is important to understand whether endocytosis is an active or passive process. Does the endocytosis process require energy? Endocytosis is an active process. That means ATP molecules are involved in this process. Macrophages engulf the pathogenic material (e.g. viruses or bacteria) and eventually neutralize them with the help of lysosomes. The endocytosed material is digested with the help of hydrolytic enzymes of the lysosomes. Endocytosis is employed by the cells in order to:
Biology definition: An opposite cellular phenomenon wherein the cell expels out the material from its interior to the external environment is known as exocytosis. So, what are exocytosis and endocytosis? Endocytosis and exocytosis are the processes responsible for the bulk transportation of the cellular material. Exocytosis is the process by which cells eliminate or release the material into extracellular space. In the process of exocytosis, during the fusion of a vesicle with the plasma membrane cellular content is transferred into the extracellular space. The exocytosis process is required for the removal of the cellular undesired /waste material, for cellular communication, for enabling cellular growth and repair. This is the usual mechanism employed by macrophages to eliminate waste or pathogenic material. Examples of exocytosis include glucagon transportation from the pancreas to the liver, the release of neurotransmitters into the synaptic cleft, etc. The major difference between the two modes of cellular transport — endocytosis and exocytosis — are enlisted in Table 1.
Endocytosis pathway overview
Types of EndocytosisWhat are the types of endocytosis? The different types are as follows: (1) phagocytosis, (2) pinocytosis, (3) receptor-mediated endocytosis, and (4) caveolae. PhagocytosisPhagocytosis or cell eating is a type of endocytosis wherein large cells ingest large foreign particles or material. The cell which undertakes this endocytosis is known as a phagocyte. The phagocytosis process was discovered in 1882 by Élie Metchnikoff. What is phagocytosis? Phagocytosis (definition in biology) is the cellular uptake process by which a cell ingests or engulfs the material of large size with the modulation of the plasma membrane. Phagocytosis was first discovered in 1867 by a Canadian physician, William Osler. Macrophages, monocytes, neutrophils, eosinophils, and dendritic cells are some of the white blood cells that exhibit phagocytosis. Phagocytosis is a five-step process:
Examples of phagocytosisImmune cells such as macrophages, dendritic cells, and neutrophils exhibit phagocytosis for neutralizing and eliminating the pathogenic material from the body. A white blood cell engulfing a bacterium is a classic example of phagocytosis. The largest immune cells are the macrophages that specialize in detecting the antigen, followed by attaching to it leading to ingestion and fusion resulting in neutralizing the antigenic material and eventually eliminating it by the process of exocytosis. Macrophages have pseudopodia and are the major immunogenic cells in the body. The antigen is the substance, which the body recognizes as foreign and undesirable to the body. It can be viruses, bacteria, fungi, pollen grains, etc. For the process of adherence, sometimes a phagocytic cell needs certain specific protein components from the blood that forms a layer over the antigen to be engulfed. These proteins are known as Opsonin and this process of forming a layer over the antigen is known as opsonization. Once opsonization is complete, the phagocytic cell then engulfs the antigen by the process of phagocytosis. Watch the video below showing phagocytosis under a microscope (a macrophage phagocytosing a bacterial cell). PinocytosisPinocytosis or cell drinking (or endocytosis of fluid) is the process wherein the cell engulfs or ingests the small particles of the fluid. What is pinocytosis?Pinocytosis definition in biology is the process of cellular uptake or ingestion of fluid or particles of small size by a cell with the modulation of the plasma membrane. The process of pinocytosis differs from phagocytosis in the formation of pseudopodia. In pinocytosis, the formation of pseudopodia is absent. Rather, there is the only formation of vesicles. The process of pinocytosis follows basic steps of the process of endocytosis wherein a vesicle is formed around the small particles or fluid to be engulfed. The vesicles are then internalized in the cell and may fuse with the cellular lysosome. Eventually, the contents are digested by the hydrolytic enzymes of the lysosomes. Types of pinocytosisPinocytosis can be of two types:
Pinocytosis examplesThe following are examples of pinocytosis:
Receptor-mediated endocytosisThis endocytosis is also known as clathrin-mediated endocytosis. As suggested by the name, this endocytosis process is facilitated through a coat protein known as clathrin. This is one of the best-studied endocytic processes. The molecules, that are to be engulfed, fuse with specific receptors on the cell membrane, which are found in the protein clathrin-rich regions of the plasma membrane. These regions are known as clathrin-coated pits. After receptor binding of the molecule is carried out, clathrin pits undergo the process of internalization forming clathrin-coated vesicles. These clathrin-coated pits then fuse with the endosome which results in the removal of the clathrin coat from the internalized vesicles. Here we need to understand what an endosome is. Endosomes are the membrane-bound intracellular cell organelle in the eukaryotic cells. The function of endosomes is to carry out the sorting of the material that has been internalized and its delivery to the lysosomes. Once, clathrin is removed, these vesicles are then digested upon. This process ensures the uptake of specific material like metabolites, hormones, proteins, and certain types of viruses. Thus, the key difference between pinocytosis and receptor-mediated endocytosis is that the pinocytosis endocytosis of the non-specific molecules is carried out by the cell whereas in receptor-mediated endocytosis uptake of specific molecules occurs. Essential steps of Receptor-mediated Endocytosis or Clathrin-Mediated Endocytosis
Receptor-mediated endocytosis is a more efficient and selective endocytic process than pinocytosis. The classical examples of receptor-mediated endocytosis are uptake of cholesterol-bound low-density lipoproteins, recycling of iron-bound transferrin, and the chief endocytic process in plant cells. Hypercholesterolemia is the result of the defective or absent low-density lipoprotein receptors. As a result of this, cholesterol is not eliminated from the body and is deposited in the blood vessels resulting in atherosclerosis. CaveolaeThis is a clathrin-independent, receptor-mediated- cholesterol, and dynarrin- dependent pathway endocytic process wherein invagination of the plasma membrane results in the formation of bulb-shaped cavities inside the cells which are known as caveolae. The literal meaning of the word ‘caveolae’ is ‘little caves’. The size of these cavities is 50-80nm. Two proteins, i.e., caveolins and cavins, facilitate the process of formation of caveolae. Caveolins are an integral protein of 21 kDa of the membrane whereas cavins are the peripheral proteins of the membrane. Three caveolins, i.e., caveolin-1, caveolin-2, and caveolin-3 are involved in the formation and stability of the caveolae. Caveolin-3 is a specific protein found only in muscle cells. These three caveolin work in conjunction with four types of cavins (cavin-1, -2, -3, and -4; cavin-4 is specifically found in muscle cells) to form caveolae. Caveolae are completely absent in neurons while these are most widely found on the endothelial cells. Palade and Yamada first reported caveolae in the 1950s. Function of EndocytosisEndocytosis is basically a bulk transportation mechanism of the cells.
Endocytosis ExamplesTransportation and endocytosis of the hydrophobic molecules like, steroids, retinol, etc. by binding with soluble proteins. It has been found that vitamin A (retinol) is bound to the retinol-binding protein, vitamin D3 is bound to the vitamin D-binding protein, cortisol is bound to the corticosteroid-binding globulin, and testosterone and estrogens bound to the sex hormone-binding globulin are endocytosed via receptor-mediated endocytosis. Similarly, the removal of hydrophobic and insoluble cholesterol from the body is facilitated by its soluble complex with low-density lipoproteins through the process of receptor-mediated endocytosis. Thus, defective or absence of low-density lipoprotein receptors results in the accumulation of cholesterol leading to hypercholesterolemia and atherosclerosis, both of which can eventually lead to a heart attack. Although, the body utilizes the process of endocytosis to neutralize the pathogens. There are certain pathogenic intracellular parasites that utilize endocytosis, specifically receptor-mediated endocytosis, to gain access and establish themselves intracellularly in the host cell. Epstein-Barr Virus (EBV), Influenza virus, Listeria monocytogenes, and Streptococcus pneumonia are some of the pathogens that utilize receptor-mediated endocytosis by tricking the immune system and establish themselves in the host cells. Try to answer the quiz below to check what you have learned so far about endocytosis. References:
©BiologyOnline.com. Content provided and moderated by BiologyOnline Editors. |
What forms of endocytosis do white blood cells use?
zusammenhängende Posts
Urheberrechte © © 2024 frojeostern Inc.
