Endocytosis is a process where cells absorb material (molecules such as proteins) from the outside by engulfing it with their cell membrane. The cell is the structural and functional unit of all known living Organisms It is the smallest unit of an organism that is classified as living and is often called In Chemistry, a molecule is defined as a sufficiently stable electrically neutral group of at least two Atoms in a definite arrangement held together by The cell membrane (also called the plasma membrane, plasmalemma, or "phospholipid bilayer" is a Selectively permeable Lipid bilayer It is used by all cells of the body because most substances important to them are large polar molecules, and thus cannot pass through the hydrophobic plasma membrane. In Chemistry, hydrophobicity (from the combining form of water in Attic Greek hydro- and for fear phobos) refers to the physical property of The cell membrane (also called the plasma membrane, plasmalemma, or "phospholipid bilayer" is a Selectively permeable Lipid bilayer The function of endocytosis is the opposite of exocytosis. Exocytosis (ek-soh-sy-TOH-sis Greek: Έξω - external and κύτος - cell is the durable process by which a cell directs secretory vesicles out of the Cell
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Types
The absorption of material from the outside environment of the cell is commonly divided into two processes: phagocytosis and pinocytosis. Phagocytosis is the cellular process of engulfing solid particles by the Cell membrane to form an internal Phagosome, or "food vacuole In Cellular biology, pinocytosis ("cell-drinking" "bulk-phase pinocytosis" "non-specific non-adsorptive pinocytosis" "fluid endocytosis"
- Phagocytosis (literally, cell-eating) is the process by which cells ingest large objects, such as cells which have undergone apoptosis, bacteria, or viruses. Phagocytosis is the cellular process of engulfing solid particles by the Cell membrane to form an internal Phagosome, or "food vacuole The Bacteria ( singular: bacterium) are a large group of unicellular Microorganisms Typically a few Micrometres in length bacteria have A virus (from the Latin virus meaning Toxin or Poison) is a sub-microscopic infectious agent that is unable The membrane folds around the object, and the object is sealed off into a large vacuole known as a phagosome. In general vacuole functions include Removing unwanted structural debris Isolating materials that might be harmful or a threat to the cell Containing In cell biology a phagosome is a Vacuole formed around a particle absorbed by Phagocytosis.
- Pinocytosis (literally, cell-drinking). In Cellular biology, pinocytosis ("cell-drinking" "bulk-phase pinocytosis" "non-specific non-adsorptive pinocytosis" "fluid endocytosis" This process is concerned with the uptake of solutes and single molecules such as proteins. Solubility is the characteristic Physical property referring to the ability of a given substance the Solute, to dissolve in a Solvent. Proteins are large Organic compounds made of Amino acids arranged in a linear chain and joined together by Peptide bonds between the Carboxyl
- Receptor-mediated endocytosis is a more specific active event where the cytoplasm membrane folds inward to form coated pits. Receptor-mediated endocytosis (RME also called clathrin-dependent endocytosis, is a process by which cells internalize molecules ( Endocytosis) by the inward budding These inward budding vesicles bud to form cytoplasmic vesicles.
Endocytosis pathways
There are three types of endocytosis: namely, macropinocytosis, caveolar endocytosis, and clathrin-mediated endocytosis. Clathrin is a Protein that is the major constituent of the 'coat' of the clathrin- Coated pits and coated Vesicles formed during Endocytosis
- Macropinocytosis is the invagination of the cell membrane to form a pocket, which then pinches off into the cell to form a vesicle filled with extracellular fluid (and molecules within it). The cell membrane (also called the plasma membrane, plasmalemma, or "phospholipid bilayer" is a Selectively permeable Lipid bilayer The filling of the pocket occurs in a non-specific manner. The vesicle then travels into the cytosol and fuses with other vesicles such as endosomes and lysosomes. The cytosol or intracellular fluid (or cytoplasmic matrix) is the liquid found inside cells. In Biology, an endosome is a membrane-bound compartment inside cells roughly 300-400 nm in diameter when fully mature Lysosomes are Organelles that contain Digestive enzymes (acid Hydrolases.
- Caveolae consists of the protein caveolin-1 with a bilayer enriched in cholesterol and glycolipids. Cholesterol is a Lipid found in the Cell membranes and transported in the Blood plasma of all Animals It is an essential component of mammalian Glycolipids are Carbohydrate -attached Lipids. Their role is to provide Energy and also serve as markers for cellular recognition Caveolae are flask-shape pits in the membrane that resemble the shape of a cave (hence the name caveolae). Uptake of extracellular molecules are also believed to be specifically mediated via receptors in caveolae. In Chemistry, a molecule is defined as a sufficiently stable electrically neutral group of at least two Atoms in a definite arrangement held together by
- Clathrin-mediated endocytosis is the specific uptake of large extracellular molecules such as proteins, membrane localized receptors and ion-channels. In Biochemistry, a receptor is a Protein molecule embedded in either the Plasma membrane or Cytoplasm of a cell to which a mobile signaling An ion is an Atom or Molecule which has lost or gained one or more Valence electrons giving it a positive or negative electrical charge These receptors are associated with the cytosolic protein clathrin, which initiates the formation of a vesicle by forming a crystalline coat on the inner surface of the cell's membrane. jono is a gimp
Clathrin-mediated endocytosis
The major route for endocytosis in most cells, and the best-understood, is that mediated by the molecule clathrin. Clathrin is a Protein that is the major constituent of the 'coat' of the clathrin- Coated pits and coated Vesicles formed during Endocytosis This large protein assists in the formation of a coated pit on the inner surface of the plasma membrane of the cell. The cell membrane (also called the plasma membrane, plasmalemma, or "phospholipid bilayer" is a Selectively permeable Lipid bilayer This pit then buds into the cell to form a coated vesicle in the cytoplasm of the cell. In so doing, it brings into the cell not only a small area of the surface of the cell but also a small volume of fluid from outside the cell.
Vesicles selectively concentrate and exclude certain proteins during formation and are not representative of the membrane as a whole. AP2 adaptors are multisubunit complexes that perform this function at the plasma membrane. The best-understood receptors that are found concentrated in coated vesicles of mammalian cells are the LDL receptor (which removes LDL from the blood circulation), the transferrin receptor (which brings ferric ions bound by transferrin into the cell) and certain hormone receptors (such as that for EGF). The LDL Receptor is a mosaic protein that mediates the Endocytosis of cholesterol-rich LDL. Low-density lipoprotein ( LDL) is a type of Lipoprotein that transports Cholesterol and Triglycerides from the Liver to peripheral Transferrin is a Blood plasma Protein for Iron Ion delivery Transferrin is a Glycoprotein, which binds iron very tightly but reversibly Epidermal growth factor or EGF is a Growth factor that plays an important role in the regulation of Cell growth, Proliferation, and
At any one moment, about 25% of the plasma membrane of a fibroblast is made up of coated pits. As a coated pit has a life of about a minute before it buds into the cell, a fibroblast takes up its surface by this route about once every 50 minutes. Coated vesicles formed from the plasma membrane have a diameter of about 100nm and a lifetime measured in a few seconds. Once the coat has been shed, the remaining vesicle fuses with endosomes and proceeds down the endocytic pathway. In Biology, an endosome is a membrane-bound compartment inside cells roughly 300-400 nm in diameter when fully mature The actual budding-in process, whereby a pit is converted to a vesicle, is carried out by clathrin assisted by a set of cytoplasmic proteins, which includes dynamin and adaptors such as adaptin. Dynamin is a GTPase responsible for Endocytosis in the eukaryotic cell Adaptin is a protein mediating the formation of Clathrin -coated pits through interaction with membrane-bound receptors There are several types of adaptin each related
Coated pits and vesicles were first seen in thin sections of tissue in the electron microscope by Thomas Roth and Keith Porter in 1964. The importance of them for the clearance of LDL from blood was discovered by R. G Anderson, Michael S. Brown and Joseph L. Goldstein in 1976. Michael Stuart Brown (b April 13, 1941 in Brooklyn New York) is an American Geneticist and Nobel Laureate. Joseph L Goldstein (b April 18, 1940) from Kingstree South Carolina is a Nobel Prize winning Biochemist and Geneticist Coated vesicles were first purified by Barbara Pearse, who discovered the clathrin coat molecule, also in 1976. Barbara Pearse (born March 24, 1948 in Wraysbury, England) is a British biological scientist and Fellow of the Royal Society.
See also
External links
- Endocytosis at biologyreference.com
- Endocytosis - researching endocytic mechanisms at endocytosis.org
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