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Endocytosis is a
cellular process The cell (from Latin word 'cellula' meaning "small room") is the basic structural and functional unit of life. Every cell consists of a cytoplasm enclosed within a Cell membrane, membrane, which contains many biomolecules such as proteins and ...
in which substances are brought into the cell. The material to be internalized is surrounded by an area of
cell membrane The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane A biological membrane, biomembrane or cell membrane is a selectively permeable membra ...

cell membrane
, which then buds off inside the cell to form a
vesicle Vesicle may refer to: ; In cellular biology or chemistry * Vesicle (biology and chemistry) In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life and liv ...
containing the ingested material. Endocytosis includes
pinocytosisImage:Pinocytosis.svg, 250px, Pinocytosis In cellular biology, pinocytosis, otherwise known as fluid endocytosis and bulk-phase pinocytosis, is a mode of endocytosis in which small particles suspended in extracellular fluid are brought into the cell ...

pinocytosis
(cell drinking) and
phagocytosis Phagocytosis () is the process by which a cell uses its plasma membrane to engulf a large particle (≥ 0.5 μm), giving rise to an internal compartment called the phagosome. It is one type of endocytosis Endocytosis is a cellular process i ...

phagocytosis
(cell eating). It is a form of active transport.


History

The term was proposed by De Duve in 1963.
Phagocytosis Phagocytosis () is the process by which a cell uses its plasma membrane to engulf a large particle (≥ 0.5 μm), giving rise to an internal compartment called the phagosome. It is one type of endocytosis Endocytosis is a cellular process i ...

Phagocytosis
was discovered by
Élie Metchnikoff Ilya Ilyich Mechnikov (russian: Илья́ Ильи́ч Ме́чников, also written as Élie Metchnikoff; 15 July 1916) was a Russian Imperial zoologist Zoology ()The pronunciation of zoology as is typically regarded as nonstandard, thou ...
in 1882.


Endocytosis pathways

Endocytosis pathways can be subdivided into four categories: namely,
receptor-mediated endocytosis Image:Itrafig2.jpg, 400px, Mechanism of clathrin-dependent endocytosis Receptor-mediated endocytosis (RME), also called clathrin-mediated endocytosis, is a process by which cells absorb metabolites, hormones, proteins – and in some cases viruses ...
(also known as clathrin-mediated endocytosis),
caveolae In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanisms, ...
,
pinocytosisImage:Pinocytosis.svg, 250px, Pinocytosis In cellular biology, pinocytosis, otherwise known as fluid endocytosis and bulk-phase pinocytosis, is a mode of endocytosis in which small particles suspended in extracellular fluid are brought into the cell ...

pinocytosis
, and
phagocytosis Phagocytosis () is the process by which a cell uses its plasma membrane to engulf a large particle (≥ 0.5 μm), giving rise to an internal compartment called the phagosome. It is one type of endocytosis Endocytosis is a cellular process i ...

phagocytosis
. *
Clathrin-mediated endocytosis Image:Itrafig2.jpg, 400px, Mechanism of clathrin-dependent endocytosis Receptor-mediated endocytosis (RME), also called clathrin-mediated endocytosis, is a process by which cells absorb metabolites, hormones, proteins – and in some cases viruses ...
is mediated by the production of small (approx. 100 nm in diameter) vesicles that have a morphologically characteristic coat made up of the cytosolic protein
clathrin Clathrin is a protein Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowdery Kendrew in 1958, f ...

clathrin
. Clathrin-coated vesicles (CCVs) are found in virtually all cells and form domains of the plasma membrane termed clathrin-coated pits. Coated pits can concentrate large extracellular molecules that have different
receptors Receptor may refer to: *Sensory receptor, in physiology, any structure which, on receiving environmental stimuli, produces an informative nerve impulse *Receptor (biochemistry), in biochemistry, a protein molecule that receives and responds to a ne ...

receptors
responsible for the receptor-mediated endocytosis of ligands, e.g.
low density lipoprotein Low-density lipoprotein (LDL) is one of the five major groups of lipoprotein which transport all fat molecules around the body in the extracellular water. These groups, from least dense to most dense, are chylomicrons (aka ULDL by the overall den ...
,
transferrin Transferrins are glycoprotein Glycoproteins are protein Proteins are large biomolecules or macromolecules that are comprised of one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of funct ...

transferrin
,
growth factor A growth factor is a naturally occurring substance capable of stimulating cell proliferation Cell proliferation is the process by which ''a cell grows and divides to produce two daughter cells''. Cell proliferation leads to an exponential gro ...
s,
antibodies An antibody (Ab), also known as an immunoglobulin (Ig), is a large, Y-shaped protein used by the immune system to identify and neutralize foreign objects such as pathogenic bacteria and Viral disease, viruses. The antibody recognizes a unique mo ...

antibodies
and many others. :Study in mammalian cells confirm a reduction in clathrin coat size in an increased tension environment. In addition, it suggests that the two apparently distinct clathrin assembly modes, namely coated pits and coated plaques, observed in experimental investigations might be a consequence of varied tensions in the plasma membrane. *
Caveolae In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanisms, ...
are the most commonly reported non-clathrin-coated plasma membrane buds, which exist on the surface of many, but not all cell types. They consist of the cholesterol-binding protein
caveolin In molecular biology, caveolins are a family of integral membrane protein An integral membrane protein (IMP) is a type of membrane protein Membrane proteins are common proteins that are part of, or interact with, biological membranes. Membra ...
(Vip21) with a bilayer enriched in
cholesterol Cholesterol is any of a class of certain organic compound, organic molecules. A cholesterol is a sterol (or chemical modification, modified steroid), a type of lipid. Cholesterol is biosynthesis, biosynthesized by all animal Cell (biology)#Euk ...

cholesterol
and
glycolipid Glycolipids are lipids In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physio ...

glycolipid
s. Caveolae are small (approx. 50 nm in diameter) flask-shape pits in the membrane that resemble the shape of a cave (hence the name caveolae). They can constitute up to a third of the plasma membrane area of the cells of some tissues, being especially abundant in
smooth muscle Smooth muscle is an involuntary non-striated muscle Striated muscle tissue is a muscle tissue Muscle tissue is a soft tissue that composes muscles in animal bodies, and gives rise to muscles' ability to contract. It is also referred to as myo ...

smooth muscle
, type I
pneumocyte A pulmonary alveolus (plural: alveoli, from Latin ''alveolus'', "little cavity") also known as an air sac or air space is one of millions of hollow, distensible cup-shaped cavities in the lungs where oxygen Molecular diffusion#Biology, is exchang ...
s,
fibroblast A fibroblast is a type of biological cell The cell (from Latin ''cella'', meaning "small room") is the basic structural, functional, and biological unit of all known organisms. Cells are the smallest units of life, and hence are often referre ...

fibroblast
s,
adipocyte Adipocytes, also known as lipocytes and fat cells, are the cells that primarily compose adipose tissue Adipose tissue, body fat, or simply fat is a loose connective tissue Connective tissue is one of the many basic types of animal An ...

adipocyte
s, and
endothelial cell Endothelium is a single layer of squamous Epithelium () is one of the four basic types of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. Wit ...
s. Uptake of extracellular molecules is also believed to be specifically mediated via receptors in caveolae. **
PotocytosisPotocytosis is a type of receptor-mediated endocytosis in which small molecules are transported across the plasma membrane of a Cell (biology), cell. The molecules are transported by caveolae (rather than clathrin-coated vesicles) and are deposited d ...
is a form of receptor-mediated endocytosis that uses caveolae vesicles to bring molecules of various sizes into the cell. Unlike most endocytosis that uses caveolae to deliver contents of vesicles to lysosomes or other organelles, material endocytosed via potocytosis is released into the cytosol. *
PinocytosisImage:Pinocytosis.svg, 250px, Pinocytosis In cellular biology, pinocytosis, otherwise known as fluid endocytosis and bulk-phase pinocytosis, is a mode of endocytosis in which small particles suspended in extracellular fluid are brought into the cell ...

Pinocytosis
, which usually occurs from highly ruffled regions of the plasma membrane, is the invagination of the cell membrane to form a pocket, which then pinches off into the cell to form a vesicle (.5–5 µm in diameter) filled with a large volume of extracellular fluid and molecules within it (equivalent to ~100 CCVs). The filling of the pocket occurs in a non-specific manner. The vesicle then travels into the
cytosol The cytosol, also known as cytoplasmic matrix or groundplasm, is one of the liquids found inside cells Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * Monastic cell, a s ...
and fuses with other vesicles such as
endosomes Endosomes are a collection of intracellular sorting organelles In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life and living organisms, including th ...
and
lysosomes A lysosome () is a membrane-bound organelle In cell biology, an organelle is a specialized subunit, usually within a cell (biology), cell, that has a specific function. The name ''organelle'' comes from the idea that these structures are parts ...

lysosomes
. *
Phagocytosis Phagocytosis () is the process by which a cell uses its plasma membrane to engulf a large particle (≥ 0.5 μm), giving rise to an internal compartment called the phagosome. It is one type of endocytosis Endocytosis is a cellular process i ...

Phagocytosis
is the process by which cells bind and internalize particulate matter larger than around 0.75 µm in diameter, such as small-sized dust particles, cell debris,
microorganism A microorganism, or microbe,, ''mikros'', "small") and ''organism In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes ...
s and
apoptotic Apoptosis (from Ancient Greek wikt:ἀπόπτωσις, ἀπόπτωσις, ''apóptōsis'', "falling off") is a form of programmed cell death that occurs in multicellular organisms. Biochemistry, Biochemical events lead to characteristic cell c ...

apoptotic
cells. These processes involve the uptake of larger membrane areas than
clathrin-mediated endocytosis Image:Itrafig2.jpg, 400px, Mechanism of clathrin-dependent endocytosis Receptor-mediated endocytosis (RME), also called clathrin-mediated endocytosis, is a process by which cells absorb metabolites, hormones, proteins – and in some cases viruses ...
and
caveolae In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanisms, ...
pathway. More recent experiments have suggested that these morphological descriptions of endocytic events may be inadequate, and a more appropriate method of classification may be based upon the clathrin-dependence of particular pathways, with multiple subtypes of clathrin-dependent and clathrin-independent endocytosis. Mechanistic insight into non-phagocytic, clathrin-independent endocytosis has been lacking, but a recent study has shown how
Graf1 Rho GTPase activating protein 26 (ARHGAP26) also known as GTPase Regulator Associated with Focal Adhesion Kinase (GRAF) is a protein Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first t ...

Graf1
regulates a highly prevalent clathrin-independent endocytic pathway known as the CLIC/GEEC pathway.


Principal components of endocytic pathway

The endocytic pathway of mammalian cells consists of distinct membrane compartments, which internalize molecules from the plasma membrane and recycle them back to the surface (as in early endosomes and recycling endosomes), or sort them to degradation (as in late endosomes and lysosomes). The principal components of the endocytic pathway are: *Early
endosome Endosomes are a collection of intracellular sorting organelles In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life and living organisms, including th ...
s are the first compartment of the endocytic pathway. Early endosomes are often located in the periphery of the cell, and receive most types of vesicles coming from the cell surface. They have a characteristic tubulo-vesicular structure (vesicles up to 1 µm in diameter with connected tubules of approx. 50 nm diameter) and a mildly acidic pH. They are principally sorting organelles where many endocytosed ligands dissociate from their
receptors Receptor may refer to: *Sensory receptor, in physiology, any structure which, on receiving environmental stimuli, produces an informative nerve impulse *Receptor (biochemistry), in biochemistry, a protein molecule that receives and responds to a ne ...

receptors
in the acid pH of the compartment, and from which many of the receptors recycle to the cell surface (via tubules). It is also the site of sorting into transcytotic pathway to later compartments (like late endosomes or lysosomes) via transvesicular compartments (like multivesicular bodies (MVB) or endosomal carrier vesicles (ECVs)). *Late endosomes receive endocytosed material en route to
lysosome A lysosome () is a membrane-bound organelle found in many animal Cell (biology), cells. They are spherical Vesicle (biology and chemistry), vesicles that contain Hydrolysis, hydrolytic enzymes that can break down many kinds of biomolecules. A ly ...

lysosome
s, usually from early endosomes in the endocytic pathway, from trans-Golgi network (TGN) in the biosynthetic pathway, and from
phagosome In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, M ...
s in the phagocytic pathway. Late endosomes often contain proteins characteristic of nucleosomes, mitochondria and mRNAs including lysosomal membrane glycoproteins and acid hydrolases. They are acidic (approx. pH 5.5), and are part of the trafficking pathway of
mannose-6-phosphate Mannose-6-phosphate (M6P) is a molecule bound by lectin in the immune system The immune system is a network of biological processes that protects an organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''o ...

mannose-6-phosphate
receptors. Late endosomes are thought to mediate a final set of sorting events prior the delivery of material to lysosomes. *
Lysosome A lysosome () is a membrane-bound organelle found in many animal Cell (biology), cells. They are spherical Vesicle (biology and chemistry), vesicles that contain Hydrolysis, hydrolytic enzymes that can break down many kinds of biomolecules. A ly ...

Lysosome
s are the last compartment of the endocytic pathway. Their chief function is to break down cellular waste products, fats, carbohydrates, proteins, and other macromolecules into simple compounds. These are then returned to the cytoplasm as new cell-building materials. To accomplish this, lysosomes use some 40 different types of hydrolytic enzymes, all of which are manufactured in the endoplasmic reticulum, modified in the
Golgi apparatus The Golgi apparatus (), also known as the Golgi complex, Golgi body, or simply the Golgi, is an organelle In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that stu ...

Golgi apparatus
and function in an acidic environment. The approximate pH of a lysosome is 4.8 and by
electron microscopy An electron microscope is a microscope that uses a beam of accelerated electrons as a source of illumination. As the wavelength of an electron can be up to 100,000 times shorter than that of visible light photons, electron microscopes have a hig ...

electron microscopy
(EM) usually appear as large
vacuoles A vacuole () is a membrane-bound organelle In cell biology, an organelle is a specialized subunit, usually within a cell (biology), cell, that has a specific function. The name ''organelle'' comes from the idea that these structures are parts ...
(1-2 µm in diameter) containing electron dense material. They have a high content of lysosomal membrane proteins and active lysosomal hydrolases, but no mannose-6-phosphate receptor. They are generally regarded as the principal hydrolytic compartment of the cell. It was recently found that an eisosome serves as a portal of endocytosis in yeast.


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 Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowdery Kendrew in 1958, f ...

clathrin
. This large protein assists in the formation of a coated pit on the inner surface of the
plasma membrane The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane A biological membrane, biomembrane or cell membrane is a selectively permeable membra ...
of the cell. 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. Coats function to deform the donor membrane to produce a vesicle, and they also function in the selection of the vesicle cargo. Coat complexes that have been well characterized so far include coat protein-I (COP-I), COP-II, and clathrin. Clathrin coats are involved in two crucial transport steps: (i) receptor-mediated and fluid-phase endocytosis from the plasma membrane to early endosome and (ii) transport from the TGN to endosomes. In endocytosis, the clathrin coat is assembled on the cytoplasmic face of the plasma membrane, forming pits that invaginate to pinch off (scission) and become free CCVs. In cultured cells, the assembly of a CCV takes ~ 1min, and several hundred to a thousand or more can form every minute. The main scaffold component of clathrin coat is the 190-kD protein called clathrin heavy chain (CHC), which is associated with a 25- kD protein called clathrin light chain (CLC), forming three-legged trimers called triskelions. Vesicles selectively concentrate and exclude certain proteins during formation and are not representative of the membrane as a whole.
AP2 adaptorsThe AP2 adaptor complex is a multimeric protein Proteins are large biomolecules or macromolecules that are comprised of one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within orga ...
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 The low-density lipoprotein (LDL) receptor (LDL-R) is a mosaic protein A mosaic protein is a protein that is made up of different protein domains, giving the protein multiple functions. These proteins have quaternary structures, as they are made ...
(which removes
LDL Low-density lipoprotein (LDL) is one of the five major groups of lipoprotein 250px, Structure of a chylomicron. ApoA, ApoB, ApoC, ApoE are apolipoproteins; green particles are phospholipids; T is triacylglycerol; C is cholesterol ester. A lipo ...
from circulating blood), the transferrin receptor (which brings ferric ions bound by
transferrin Transferrins are glycoprotein Glycoproteins are protein Proteins are large biomolecules or macromolecules that are comprised of one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of funct ...

transferrin
into the cell) and certain hormone receptors (such as that for EGF). 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 16 minutes. Coated vesicles formed from the plasma membrane have a diameter of about 36 nm and a lifetime measured in a few seconds. Once the coat has been shed, the remaining vesicle fuses with
endosomes Endosomes are a collection of intracellular sorting organelles In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life and living organisms, including th ...
and proceeds down the endocytic pathway. 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 Dynamin is a GTPase GTPases are a large family of hydrolase Hydrolase is a class of enzyme Enzymes () are proteins that act as biological catalysts (biocatalysts). Catalysts accelerate chemical reactions. The molecules upon which enzymes may ...
and adaptors such as adaptin. Coated pits and vesicles were first seen in thin sections of tissue in the electron microscope by Matt Lions and Parker George. The importance of them for the clearance of LDL from blood was discovered by Richard G. Anderson,
Michael S. Brown Michael Stuart Brown Foreign Member of the Royal Society, ForMemRS (born April 13, 1941) is an American geneticist and Nobel laureate. He was awarded the Nobel Prize in Physiology or Medicine with Joseph L. Goldstein in 1985 for describing the reg ...
and Joseph L. Goldstein in 1977. Coated vesicles were first purified by Barbara Pearse, who discovered the clathrin coat molecule in 1976.


Gallery

SARS-CoV-2 Severe acute respiratory syndrome coronavirus 2 (SARS‑CoV‑2) is the coronavirus Coronaviruses are a group of related that cause diseases in s and s. In humans and birds, they cause s that can range from mild to lethal. Mild illnes ...

SARS-CoV-2
binds to the ACE2 receptor of the
epithelial cell Epithelium is one of the four basic types of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume o ...
."> Endocytosis 3.jpg, Stage 1 Endocytosis 4.jpg, Stage 2 Endocytosis 5.jpg, Stage 3 Endocytosis 6.webm, Endocytosis animation (1) Endocytosis 7.webm, Endocytosis animation (2)


See also

*
Active transport In cellular biology Cell biology (also cellular biology or cytology) is a branch of biology that studies the structure, Physiology, function and behavior of cell (biology), cells. All living organisms are made of cells. A cell is the basic uni ...

Active transport
*
Emperipolesis
Emperipolesis
*
Exocytosis Exocytosis () is a form of active transport In cellular biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, phys ...
*
Phagocytosis Phagocytosis () is the process by which a cell uses its plasma membrane to engulf a large particle (≥ 0.5 μm), giving rise to an internal compartment called the phagosome. It is one type of endocytosis Endocytosis is a cellular process i ...

Phagocytosis
*
PinocytosisImage:Pinocytosis.svg, 250px, Pinocytosis In cellular biology, pinocytosis, otherwise known as fluid endocytosis and bulk-phase pinocytosis, is a mode of endocytosis in which small particles suspended in extracellular fluid are brought into the cell ...

Pinocytosis
* Trans-endocytosis


References


External links


A comprehensive review of endocytosis and endocytic mechanisms by Doherty and McMahon



Endocytosis - researching endocytic mechanisms at endocytosis.orgClathrin-mediated endocytosis
ASCB Image & Video Library
Types of Endocytosis (Animation)
{{Membrane transport Cellular processes Membrane biology Cell anatomy