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95-413: The chain molecules in the fibers are highly oriented along the fiber axis. As a result, a higher proportion of the chemical bond contributes more to fiber strength than in many other synthetic fibers. Aramids have a very high melting point (>500 °C (932 °F)). Common aramid brand names include Kevlar , Nomex , and Twaron . The term aramid is shortened from aromatic polyamide . It

190-422: A "characteristic tight pavement-like appearance". But this is not always the case, such as when the cells are derived from a tumor. In these cases, it is often necessary to use certain biochemical markers to make a positive identification. The intermediate filament proteins in the cytokeratin group are almost exclusively found in epithelial cells, so they are often used for this purpose. Cancers originating from

285-474: A cell and the extracellular matrix , or they build up the paracellular barrier of epithelia and control the paracellular transport . Cell junctions are the contact points between plasma membrane and tissue cells. There are mainly 5 different types of cell junctions: tight junctions , adherens junctions , desmosomes , hemidesmosomes , and gap junctions . Tight junctions are a pair of trans-membrane protein fused on outer plasma membrane. Adherens junctions are

380-450: A chemical bond, where the higher the associated electronegativity then the more it attracts electrons. Electronegativity serves as a simple way to quantitatively estimate the bond energy , which characterizes a bond along the continuous scale from covalent to ionic bonding . A large difference in electronegativity leads to more polar (ionic) character in the bond. Ionic bonding is a type of electrostatic interaction between atoms that have

475-440: A connexion). Epithelial tissues are derived from all of the embryological germ layers : However, pathologists do not consider endothelium and mesothelium (both derived from mesoderm) to be true epithelium. This is because such tissues present very different pathology. For that reason, pathologists label cancers in endothelium and mesothelium sarcomas , whereas true epithelial cancers are called carcinomas . Additionally,

570-494: A covalent bond as an orbital formed by combining the quantum mechanical Schrödinger atomic orbitals which had been hypothesized for electrons in single atoms. The equations for bonding electrons in multi-electron atoms could not be solved to mathematical perfection (i.e., analytically ), but approximations for them still gave many good qualitative predictions and results. Most quantitative calculations in modern quantum chemistry use either valence bond or molecular orbital theory as

665-453: A direction in space, allowing them to be shown as single connecting lines between atoms in drawings, or modeled as sticks between spheres in models. In a polar covalent bond , one or more electrons are unequally shared between two nuclei. Covalent bonds often result in the formation of small collections of better-connected atoms called molecules , which in solids and liquids are bound to other molecules by forces that are often much weaker than

760-447: A high degree of orientation with other fibers such as ultra-high-molecular-weight polyethylene , a characteristic that dominates their properties. Para-aramid Meta-aramid Others Chemical bond A chemical bond is the association of atoms or ions to form molecules , crystals , and other structures. The bond may result from the electrostatic force between oppositely charged ions as in ionic bonds or through

855-477: A large electronegativity difference. There is no precise value that distinguishes ionic from covalent bonding, but an electronegativity difference of over 1.7 is likely to be ionic while a difference of less than 1.7 is likely to be covalent. Ionic bonding leads to separate positive and negative ions . Ionic charges are commonly between −3 e to +3 e . Ionic bonding commonly occurs in metal salts such as sodium chloride (table salt). A typical feature of ionic bonds

950-586: A lone pair that can be shared is described as an electron-pair donor or Lewis base . The electrons are shared roughly equally between the atoms in contrast to ionic bonding. Such bonding is shown by an arrow pointing to the Lewis acid. (In the Figure, solid lines are bonds in the plane of the diagram, wedged bonds point towards the observer, and dashed bonds point away from the observer.) Transition metal complexes are generally bound by coordinate covalent bonds. For example,

1045-400: A model of the chemical bond in 1913. According to his model for a diatomic molecule , the electrons of the atoms of the molecule form a rotating ring whose plane is perpendicular to the axis of the molecule and equidistant from the atomic nuclei. The dynamic equilibrium of the molecular system is achieved through the balance of forces between the forces of attraction of nuclei to the plane of

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1140-513: A net positive charge, and the other to assume a net negative charge. The bond then results from electrostatic attraction between the positive and negatively charged ions . Ionic bonds may be seen as extreme examples of polarization in covalent bonds. Often, such bonds have no particular orientation in space, since they result from equal electrostatic attraction of each ion to all ions around them. Ionic bonds are strong (and thus ionic substances require high temperatures to melt) but also brittle, since

1235-400: A part of the shell of two different atoms and cannot be said to belong to either one exclusively." Also in 1916, Walther Kossel put forward a theory similar to Lewis' only his model assumed complete transfers of electrons between atoms, and was thus a model of ionic bonding . Both Lewis and Kossel structured their bonding models on that of Abegg's rule (1904). Niels Bohr also proposed

1330-442: A permanent dipole in one molecule and an induced dipole in another molecule. Hydrogen bonds of the form A--H•••B occur when A and B are two highly electronegative atoms (usually N, O or F) such that A forms a highly polar covalent bond with H so that H has a partial positive charge, and B has a lone pair of electrons which is attracted to this partial positive charge and forms a hydrogen bond. Hydrogen bonds are responsible for

1425-500: A plaque (protein layer on the inside plasma membrane) which attaches both cells' microfilaments. Desmosomes attach to the microfilaments of cytoskeleton made up of keratin protein. Hemidesmosomes resemble desmosomes on a section. They are made up of the integrin (a transmembrane protein) instead of cadherin. They attach the epithelial cell to the basement membrane. Gap junctions connect the cytoplasm of two cells and are made up of proteins called connexins (six of which come together to make

1520-438: A sheet of polarised cells forming a tube or tubule with cilia projecting into the lumen ." Primary cilia on epithelial cells provide chemosensation, thermoception , and mechanosensation of the extracellular environment by playing "a sensory role mediating specific signalling cues, including soluble factors in the external cell environment, a secretory role in which a soluble protein is released to have an effect downstream of

1615-747: A singular layer of cells as simple epithelium, either simple squamous, simple columnar, or simple cuboidal, or in layers of two or more cells deep as stratified (layered), or compound , either squamous, columnar or cuboidal. In some tissues, a layer of columnar cells may appear to be stratified due to the placement of the nuclei. This sort of tissue is called pseudostratified. All glands are made up of epithelial cells. Functions of epithelial cells include diffusion , filtration, secretion , selective absorption , germination , and transcellular transport . Compound epithelium has protective functions. Epithelial layers contain no blood vessels ( avascular ), so they must receive nourishment via diffusion of substances from

1710-451: A sodium cyanide crystal. When such crystals are melted into liquids, the ionic bonds are broken first because they are non-directional and allow the charged species to move freely. Similarly, when such salts dissolve into water, the ionic bonds are typically broken by the interaction with water but the covalent bonds continue to hold. For example, in solution, the cyanide ions, still bound together as single CN ions, move independently through

1805-402: A starting point, although a third approach, density functional theory , has become increasingly popular in recent years. In 1933, H. H. James and A. S. Coolidge carried out a calculation on the dihydrogen molecule that, unlike all previous calculation which used functions only of the distance of the electron from the atomic nucleus, used functions which also explicitly added the distance between

1900-421: A vacancy which allows the addition of one or more electrons. These newly added electrons potentially occupy a lower energy-state (effectively closer to more nuclear charge) than they experience in a different atom. Thus, one nucleus offers a more tightly bound position to an electron than does another nucleus, with the result that one atom may transfer an electron to the other. This transfer causes one atom to assume

1995-434: Is a common type of bonding in which two or more atoms share valence electrons more or less equally. The simplest and most common type is a single bond in which two atoms share two electrons. Other types include the double bond , the triple bond , one- and three-electron bonds , the three-center two-electron bond and three-center four-electron bond . In non-polar covalent bonds, the electronegativity difference between

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2090-453: Is a covalent bond in which the two shared bonding electrons are from the same one of the atoms involved in the bond. For example, boron trifluoride (BF 3 ) and ammonia (NH 3 ) form an adduct or coordination complex F 3 B←NH 3 with a B–N bond in which a lone pair of electrons on N is shared with an empty atomic orbital on B. BF 3 with an empty orbital is described as an electron pair acceptor or Lewis acid , while NH 3 with

2185-441: Is a covalent bond with a significant ionic character . This means that the two shared electrons are closer to one of the atoms than the other, creating an imbalance of charge. Such bonds occur between two atoms with moderately different electronegativities and give rise to dipole–dipole interactions . The electronegativity difference between the two atoms in these bonds is 0.3 to 1.7. A single bond between two atoms corresponds to

2280-405: Is a product of p -phenylene diamine (PPD) and terephthaloyl dichloride (TDC or TCl) . Production of PPTA relies on a co-solvent with an ionic component ( calcium chloride , CaCl 2 ) to occupy the hydrogen bonds of the amide groups, and an organic component ( N -methyl pyrrolidone , NMP) to dissolve the aromatic polymer . This process was invented by Leo Vollbracht at Akzo. Apart from

2375-449: Is a situation unlike that in covalent crystals, where covalent bonds between specific atoms are still discernible from the shorter distances between them, as measured via such techniques as X-ray diffraction . Ionic crystals may contain a mixture of covalent and ionic species, as for example salts of complex acids such as sodium cyanide , NaCN. X-ray diffraction shows that in NaCN, for example,

2470-509: Is characterized by its excellent resistance to heat, as it neither melts nor ignites in normal levels of oxygen. It is used extensively in the production of protective apparel, air filtration, thermal and electrical insulation, as well as a substitute for asbestos . Meta-aramids are also produced in the Netherlands and Japan by Teijin Aramid under the trade name Teijinconex , and by Toray under

2565-442: Is discussed. Sometimes, even the non-bonding valence shell electrons (with the two-dimensional approximate directions) are marked, e.g. for elemental carbon . C . Some chemists may also mark the respective orbitals, e.g. the hypothetical ethene anion ( \ C=C / ) indicating the possibility of bond formation. Strong chemical bonds are the intramolecular forces that hold atoms together in molecules . A strong chemical bond

2660-413: Is formed from the transfer or sharing of electrons between atomic centers and relies on the electrostatic attraction between the protons in nuclei and the electrons in the orbitals. The types of strong bond differ due to the difference in electronegativity of the constituent elements. Electronegativity is the tendency for an atom of a given chemical element to attract shared electrons when forming

2755-465: Is free (by virtue of its wave nature ) to be associated with a great many atoms at once. The bond results because the metal atoms become somewhat positively charged due to loss of their electrons while the electrons remain attracted to many atoms, without being part of any given atom. Metallic bonding may be seen as an extreme example of delocalization of electrons over a large system of covalent bonds, in which every atom participates. This type of bonding

2850-448: Is no clear line to be drawn between them. However it remains useful and customary to differentiate between different types of bond, which result in different properties of condensed matter . In the simplest view of a covalent bond , one or more electrons (often a pair of electrons) are drawn into the space between the two atomic nuclei. Energy is released by bond formation. This is not as a result of reduction in potential energy, because

2945-574: Is often very strong (resulting in the tensile strength of metals). However, metallic bonding is more collective in nature than other types, and so they allow metal crystals to more easily deform, because they are composed of atoms attracted to each other, but not in any particularly-oriented ways. This results in the malleability of metals. The cloud of electrons in metallic bonding causes the characteristically good electrical and thermal conductivity of metals, and also their shiny lustre that reflects most frequencies of white light. Early speculations about

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3040-472: Is sometimes concerned only with the functional group of the molecule. Thus, the molecular formula of ethanol may be written in conformational form, three-dimensional form, full two-dimensional form (indicating every bond with no three-dimensional directions), compressed two-dimensional form (CH 3 –CH 2 –OH), by separating the functional group from another part of the molecule (C 2 H 5 OH), or by its atomic constituents (C 2 H 6 O), according to what

3135-455: Is that the species form into ionic crystals, in which no ion is specifically paired with any single other ion in a specific directional bond. Rather, each species of ion is surrounded by ions of the opposite charge, and the spacing between it and each of the oppositely charged ions near it is the same for all surrounding atoms of the same type. It is thus no longer possible to associate an ion with any specific other single ionized atom near it. This

3230-443: Is the type of epithelium that forms the glands from the infolding of epithelium and subsequent growth in the underlying connective tissue. They may be specialized columnar or cuboidal tissues consisting of goblet cells , which secrete mucus . There are two major classifications of glands: endocrine glands and exocrine glands : Some epithelial cells are ciliated , especially in respiratory epithelium , and they commonly exist as

3325-433: The atom in which a positively charged center is surrounded by a number of revolving electrons, in the manner of Saturn and its rings. Nagaoka's model made two predictions: Rutherford mentions Nagaoka's model in his 1911 paper in which the atomic nucleus is proposed. At the 1911 Solvay Conference, in the discussion of what could regulate energy differences between atoms, Max Planck stated: "The intermediaries could be

3420-546: The carcinogenic HMPT , still no practical alternative of dissolving the polymer is known. The use of the NMP/CaCl 2 system led to an extended patent dispute between Akzo and DuPont. After production of the polymer, the aramid fiber is produced by spinning the dissolved polymer to a solid fiber from a liquid chemical blend. Polymer solvent for spinning PPTA is generally 100% anhydrous sulfuric acid (H 2 SO 4 ). Besides meta-aramids like Nomex, other variations belong to

3515-488: The epigenome of these cells, which enables a rapid response to immunological challenges. The slide shows at (1) an epithelial cell infected by Chlamydia pneumoniae ; their inclusion bodies shown at (3); an uninfected cell shown at (2) and (4) showing the difference between an infected cell nucleus and an uninfected cell nucleus. Epithelium grown in culture can be identified by examining its morphological characteristics. Epithelial cells tend to cluster together, and have

3610-428: The linear combination of atomic orbitals and ligand field theory . Electrostatics are used to describe bond polarities and the effects they have on chemical substances. A chemical bond is an attraction between atoms. This attraction may be seen as the result of different behaviors of the outermost or valence electrons of atoms. These behaviors merge into each other seamlessly in various circumstances, so that there

3705-481: The melting point ) of a substance. Van der Waals forces are interactions between closed-shell molecules. They include both Coulombic interactions between partial charges in polar molecules, and Pauli repulsions between closed electrons shells. Keesom forces are the forces between the permanent dipoles of two polar molecules. London dispersion forces are the forces between induced dipoles of different molecules. There can also be an interaction between

3800-440: The silicate minerals in many types of rock) then the structures that result may be both strong and tough, at least in the direction oriented correctly with networks of covalent bonds. Also, the melting points of such covalent polymers and networks increase greatly. In a simplified view of an ionic bond , the bonding electron is not shared at all, but transferred. In this type of bond, the outer atomic orbital of one atom has

3895-499: The 1960s, calling it Nomex paper. Yantai Metastar Special Paper introduced an aramid paper in 2007, which is called metastar paper. Both Dupont and Yantai Metastar make meta-aramid and para-aramid paper. During the 1990s, an in vitro test of aramid fibers showed they exhibited "many of the same effects on epithelial cells as did asbestos , including increased radiolabeled nucleotide incorporation into DNA and induction of ODC ( ornithine decarboxylase ) enzyme activity", raising

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3990-461: The Danish physicist Øyvind Burrau . This work showed that the quantum approach to chemical bonds could be fundamentally and quantitatively correct, but the mathematical methods used could not be extended to molecules containing more than one electron. A more practical, albeit less quantitative, approach was put forward in the same year by Walter Heitler and Fritz London . The Heitler–London method forms

4085-480: The FTC definition of an aramid, at least 85% of these linkages must be attached to two aromatic rings. Below 85%, the material is instead classed as nylon. Aramids are divided into two main types according to where the linkages attach to the rings. Numbering the carbon atoms sequentially around a ring, para-aramids have the linkages attached at positions 1 and 4, while meta-aramids have them at positions 1 and 3. That is,

4180-597: The Teijin Aramid Company. In 2011, Yantai Tayho introduced similar fiber which is called Taparan in China (see Production ). Para-aramids are used in many high-tech applications, such as aerospace and military applications, for "bullet-proof" body armor fabric . Both meta-aramid and para-aramid fiber can be used to make aramid paper. Aramid paper is used as electrical insulation materials and construction materials to make honeycomb core. Dupont made aramid paper during

4275-491: The approximations differ, and one approach may be better suited for computations involving a particular system or property than the other. Unlike the spherically symmetrical Coulombic forces in pure ionic bonds, covalent bonds are generally directed and anisotropic . These are often classified based on their symmetry with respect to a molecular plane as sigma bonds and pi bonds . In the general case, atoms form bonds that are intermediate between ionic and covalent, depending on

4370-437: The aramid fiber range. These are mainly of the copolyamide type, best known under the brand name Technora , as developed by Teijin and introduced in 1976. The manufacturing process of Technora reacts PPD and 3,4'-diaminodiphenylether (3,4'-ODA) with terephthaloyl chloride (TCl) . This relatively simple process uses only one amide solvent, and therefore spinning can be done directly after the polymer production. Aramids share

4465-487: The atoms involved can be understood using concepts such as oxidation number , formal charge , and electronegativity . The electron density within a bond is not assigned to individual atoms, but is instead delocalized between atoms. In valence bond theory, bonding is conceptualized as being built up from electron pairs that are localized and shared by two atoms via the overlap of atomic orbitals. The concepts of orbital hybridization and resonance augment this basic notion of

4560-412: The attachment points are diametrically opposite each other in para-aramids, and two atoms apart in meta-aramids. The illustration thus shows a para-aramid. Aromatic polyamides were first introduced in commercial applications in the early 1960s, with a meta -aramid fiber produced by DuPont as HT-1 and then under the trade name Nomex . This fiber, which handles similarly to normal textile apparel fibers,

4655-480: The attraction of the two electrons to the two protons is offset by the electron-electron and proton-proton repulsions. Instead, the release of energy (and hence stability of the bond) arises from the reduction in kinetic energy due to the electrons being in a more spatially distributed (i.e. longer de Broglie wavelength ) orbital compared with each electron being confined closer to its respective nucleus. These bonds exist between two particular identifiable atoms and have

4750-401: The basis of what is now called valence bond theory . In 1929, the linear combination of atomic orbitals molecular orbital method (LCAO) approximation was introduced by Sir John Lennard-Jones , who also suggested methods to derive electronic structures of molecules of F 2 ( fluorine ) and O 2 ( oxygen ) molecules, from basic quantum principles. This molecular orbital theory represented

4845-512: The blood and lymphatic vessels are of a specialised form of epithelium called endothelium . Epithelium lines both the outside ( skin ) and the inside cavities and lumina of bodies. The outermost layer of human skin is composed of dead stratified squamous , keratinized epithelial cells. Tissues that line the inside of the mouth, the esophagus , the vagina , and part of the rectum are composed of nonkeratinized stratified squamous epithelium. Other surfaces that separate body cavities from

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4940-537: The blood vessels in the underlying tissue. The basement membrane acts as a selectively permeable membrane that determines which substances will be able to enter the epithelium. The basal lamina is made up of laminin (glycoproteins) secreted by epithelial cells. The reticular lamina beneath the basal lamina is made up of collagen proteins secreted by connective tissue . Cell junctions are especially abundant in epithelial tissues. They consist of protein complexes and provide contact between neighbouring cells, between

5035-498: The bonded atoms is small, typically 0 to 0.3. Bonds within most organic compounds are described as covalent. The figure shows methane (CH 4 ), in which each hydrogen forms a covalent bond with the carbon. See sigma bonds and pi bonds for LCAO descriptions of such bonding. Molecules that are formed primarily from non-polar covalent bonds are often immiscible in water or other polar solvents , but much more soluble in non-polar solvents such as hexane . A polar covalent bond

5130-399: The bonds between sodium cations (Na ) and the cyanide anions (CN ) are ionic , with no sodium ion associated with any particular cyanide . However, the bonds between the carbon (C) and nitrogen (N) atoms in cyanide are of the covalent type, so that each carbon is strongly bound to just one nitrogen, to which it is physically much closer than it is to other carbons or nitrogens in

5225-412: The cell shapes. However, when taller simple columnar epithelial cells are viewed in cross section showing several nuclei appearing at different heights, they can be confused with stratified epithelia. This kind of epithelium is therefore described as pseudostratified columnar epithelium . Transitional epithelium has cells that can change from squamous to cuboidal, depending on the amount of tension on

5320-575: The cells, the stretch that they experience rapidly activates cell division. Alternatively, when too many cells accumulate, crowding triggers their death by activation epithelial cell extrusion . Here, cells fated for elimination are seamlessly squeezed out by contracting a band of actin and myosin around and below the cell, preventing any gaps from forming that could disrupt their barriers. Failure to do so can result in aggressive tumors and their invasion by aberrant basal cell extrusion. Epithelial tissues have as their primary functions: Glandular tissue

5415-554: The connectivity −(NH−C 6 H 4 −CO) n −. Well-known aramid polymers such as Kevlar , Twaron , Nomex , New Star, and Teijinconex) are prepared from diamine and diacid (or equivalent) precursors. These polymers can be further classified according to the linkages on the aromatic subunits. Nomex, Teijinconex, and New Star contain predominantly the meta-linkage. They are called poly-metaphenylene isophthalamides (MPIAs). By contrast, Kevlar and Twaron both feature para-linkages. They are called p -phenylene terephthalamides (PPTAs). PPTA

5510-440: The corresponding inner surfaces of body cavities , and the inner surfaces of blood vessels . Epithelial tissue is one of the four basic types of animal tissue , along with connective tissue , muscle tissue and nervous tissue . These tissues also lack blood or lymph supply. The tissue is supplied by nerves. There are three principal shapes of epithelial cell: squamous (scaly), columnar, and cuboidal. These can be arranged in

5605-540: The covalent bonds that hold the molecules internally together. Such weak intermolecular bonds give organic molecular substances, such as waxes and oils, their soft bulk character, and their low melting points (in liquids, molecules must cease most structured or oriented contact with each other). When covalent bonds link long chains of atoms in large molecules, however (as in polymers such as nylon ), or when covalent bonds extend in networks through solids that are not composed of discrete molecules (such as diamond or quartz or

5700-469: The density of two non-interacting H atoms. A double bond has two shared pairs of electrons, one in a sigma bond and one in a pi bond with electron density concentrated on two opposite sides of the internuclear axis. A triple bond consists of three shared electron pairs, forming one sigma and two pi bonds. An example is nitrogen. Quadruple and higher bonds are very rare and occur only between certain transition metal atoms. A coordinate covalent bond

5795-485: The difference between the maximum and minimum valencies of an element is often eight. At this point, valency was still an empirical number based only on chemical properties. However the nature of the atom became clearer with Ernest Rutherford 's 1911 discovery that of an atomic nucleus surrounded by electrons in which he quoted Nagaoka rejected Thomson's model on the grounds that opposite charges are impenetrable. In 1904, Nagaoka proposed an alternative planetary model of

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5890-401: The electron pair bond. In molecular orbital theory, bonding is viewed as being delocalized and apportioned in orbitals that extend throughout the molecule and are adapted to its symmetry properties, typically by considering linear combinations of atomic orbitals (LCAO). Valence bond theory is more chemically intuitive by being spatially localized, allowing attention to be focused on the parts of

5985-445: The electrons." These nuclear models suggested that electrons determine chemical behavior. Next came Niels Bohr 's 1913 model of a nuclear atom with electron orbits. In 1916, chemist Gilbert N. Lewis developed the concept of electron-pair bonds , in which two atoms may share one to six electrons, thus forming the single electron bond , a single bond , a double bond , or a triple bond ; in Lewis's own words, "An electron may form

6080-470: The epithelium are classified as carcinomas . In contrast, sarcomas develop in connective tissue . When epithelial cells or tissues are damaged from cystic fibrosis , sweat glands are also damaged, causing a frosty coating of the skin. The word epithelium uses the Greek roots ἐπί ( epi ), "on" or "upon", and θηλή ( thēlē ), "nipple". Epithelium is so called because the name was originally used to describe

6175-488: The epithelium. Stratified or compound epithelium differs from simple epithelium in that it is multilayered. It is therefore found where body linings have to withstand mechanical or chemical insult such that layers can be abraded and lost without exposing subepithelial layers. Cells flatten as the layers become more apical, though in their most basal layers, the cells can be squamous, cuboidal, or columnar. Stratified epithelia (of columnar, cuboidal, or squamous type) can have

6270-452: The filaments that support these mesoderm-derived tissues are very distinct. Outside of the field of pathology, it is generally accepted that the epithelium arises from all three germ layers. Epithelia turn over at some of the fastest rates in the body. For epithelial layers to maintain constant cell numbers essential to their functions, the number of cells that divide must match those that die. They do this mechanically. If there are too few of

6365-447: The fluid flow, and mediation of fluid flow if the cilia are motile . Epithelial cells express many genes that encode immune mediators and proteins involved in cell-cell communication with hematopoietic immune cells. The resulting immune functions of these non-hematopoietic, structural cells contribute to the mammalian immune system ("structural immunity"). Relevant aspects of the epithelial cell response to infections are encoded in

6460-446: The following specializations: Epithelial tissue cells can adopt shapes of varying complexity from polyhedral to scutoidal to punakoidal. They are tightly packed and form a continuous sheet with almost no intercellular spaces. All epithelia is usually separated from underlying tissues by an extracellular fibrous basement membrane. The lining of the mouth, lung alveoli and kidney tubules are all made of epithelial tissue. The lining of

6555-436: The forces between ions are short-range and do not easily bridge cracks and fractures. This type of bond gives rise to the physical characteristics of crystals of classic mineral salts, such as table salt. A less often mentioned type of bonding is metallic bonding . In this type of bonding, each atom in a metal donates one or more electrons to a "sea" of electrons that reside between many metal atoms. In this sea, each electron

6650-562: The heels of the invention of the voltaic pile , Jöns Jakob Berzelius developed a theory of chemical combination stressing the electronegative and electropositive characters of the combining atoms. By the mid 19th century, Edward Frankland , F.A. Kekulé , A.S. Couper, Alexander Butlerov , and Hermann Kolbe , building on the theory of radicals , developed the theory of valency , originally called "combining power", in which compounds were joined owing to an attraction of positive and negative poles. In 1904, Richard Abegg proposed his rule that

6745-402: The high boiling points of water and ammonia with respect to their heavier analogues. In some cases a similar halogen bond can be formed by a halogen atom located between two electronegative atoms on different molecules. At short distances, repulsive forces between atoms also become important. In the (unrealistic) limit of "pure" ionic bonding , electrons are perfectly localized on one of

6840-415: The inner lining of blood vessels and the heart , and is known as vascular endothelium, and lining lymphatic vessels as lymphatic endothelium. Another type, mesothelium , forms the walls of the pericardium , pleurae , and peritoneum . In arthropods, the integument , or external "skin", consists of a single layer of epithelial ectoderm from which arises the cuticle , an outer covering of chitin ,

6935-837: The ion Ag reacts as a Lewis acid with two molecules of the Lewis base NH 3 to form the complex ion Ag(NH 3 ) 2 , which has two Ag←N coordinate covalent bonds. In metallic bonding, bonding electrons are delocalized over a lattice of atoms. By contrast, in ionic compounds, the locations of the binding electrons and their charges are static. The free movement or delocalization of bonding electrons leads to classical metallic properties such as luster (surface light reflectivity ), electrical and thermal conductivity , ductility , and high tensile strength . There are several types of weak bonds that can be formed between two or more molecules which are not covalently bound. Intermolecular forces cause molecules to attract or repel each other. Often, these forces influence physical characteristics (such as

7030-626: The molecule undergoing chemical change. In contrast, molecular orbitals are more "natural" from a quantum mechanical point of view, with orbital energies being physically significant and directly linked to experimental ionization energies from photoelectron spectroscopy . Consequently, valence bond theory and molecular orbital theory are often viewed as competing but complementary frameworks that offer different insights into chemical systems. As approaches for electronic structure theory, both MO and VB methods can give approximations to any desired level of accuracy, at least in principle. However, at lower levels,

7125-403: The nature of the chemical bond , from as early as the 12th century, supposed that certain types of chemical species were joined by a type of chemical affinity . In 1704, Sir Isaac Newton famously outlined his atomic bonding theory, in "Query 31" of his Opticks , whereby atoms attach to each other by some " force ". Specifically, after acknowledging the various popular theories in vogue at

7220-624: The negatively charged electrons surrounding the nucleus and the positively charged protons within a nucleus attract each other. Electrons shared between two nuclei will be attracted to both of them. "Constructive quantum mechanical wavefunction interference " stabilizes the paired nuclei (see Theories of chemical bonding ). Bonded nuclei maintain an optimal distance (the bond distance) balancing attractive and repulsive effects explained quantitatively by quantum theory . The atoms in molecules , crystals , metals and other forms of matter are held together by chemical bonds, which determine

7315-504: The number of their layers and by the shape and function of the cells. The basic cell types are squamous, cuboidal, and columnar, classed by their shape. By layer, epithelium is classed as either simple epithelium, only one cell thick (unilayered), or stratified epithelium having two or more cells in thickness, or multi-layered – as stratified squamous epithelium , stratified cuboidal epithelium , and stratified columnar epithelium , and both types of layering can be made up of any of

7410-431: The outside environment are lined by simple squamous, columnar, or pseudostratified epithelial cells. Other epithelial cells line the insides of the lungs , the gastrointestinal tract , the reproductive and urinary tracts, and make up the exocrine and endocrine glands . The outer surface of the cornea is covered with fast-growing, easily regenerated epithelial cells. A specialised form of epithelium, endothelium , forms

7505-400: The paper." World capacity of para-aramid production was estimated at 41,000 t (40,000 long tons; 45,000 short tons) per year in 2002 and increases each year by 5–10%. In 2007 this means a total production capacity of around 55,000 tonnes per year. Aramids are generally prepared by the reaction between an amine group and a carboxylic acid halide group. Simple AB homopolymers have

7600-411: The possibility of carcinogenic implications. However, in 2009, it was shown that inhaled aramid fibrils are shortened and quickly cleared from the body and pose little risk. A declaration of interest correction was later provided by the author of the study stating that "This review was commissioned and funded by DuPont and Teijin Aramid, but the author alone was responsible for the content and writing of

7695-417: The relative electronegativity of the atoms involved. Bonds of this type are known as polar covalent bonds . Epithelial cell Epithelium or epithelial tissue is a thin, continuous, protective layer of cells with little extracellular matrix . An example is the epidermis , the outermost layer of the skin . Epithelial ( mesothelial ) tissues line the outer surfaces of many internal organs ,

7790-408: The rigidity of which varies as per its chemical composition. The basal surface of epithelial tissue rests on a basement membrane and the free/apical surface faces body fluid or outside. The basement membrane acts as a scaffolding on which epithelium can grow and regenerate after injuries. Epithelial tissue has a nerve supply , but no blood supply and must be nourished by substances diffusing from

7885-457: The ring of electrons and the forces of mutual repulsion of the nuclei. The Bohr model of the chemical bond took into account the Coulomb repulsion – the electrons in the ring are at the maximum distance from each other. In 1927, the first mathematically complete quantum description of a simple chemical bond, i.e. that produced by one electron in the hydrogen molecular ion, H 2 , was derived by

7980-466: The sharing of electrons as in covalent bonds , or some combination of these effects. Chemical bonds are described as having different strengths: there are "strong bonds" or "primary bonds" such as covalent , ionic and metallic bonds, and "weak bonds" or "secondary bonds" such as dipole–dipole interactions , the London dispersion force , and hydrogen bonding . Since opposite electric charges attract,

8075-412: The sharing of one pair of electrons. The Hydrogen (H) atom has one valence electron. Two Hydrogen atoms can then form a molecule, held together by the shared pair of electrons. Each H atom now has the noble gas electron configuration of helium (He). The pair of shared electrons forms a single covalent bond. The electron density of these two bonding electrons in the region between the two atoms increases from

8170-439: The solution, as do sodium ions, as Na . In water, charged ions move apart because each of them are more strongly attracted to a number of water molecules than to each other. The attraction between ions and water molecules in such solutions is due to a type of weak dipole-dipole type chemical bond. In melted ionic compounds, the ions continue to be attracted to each other, but not in any ordered or crystalline way. Covalent bonding

8265-433: The structure and properties of matter. All bonds can be described by quantum theory , but, in practice, simplified rules and other theories allow chemists to predict the strength, directionality, and polarity of bonds. The octet rule and VSEPR theory are examples. More sophisticated theories are valence bond theory , which includes orbital hybridization and resonance , and molecular orbital theory which includes

8360-449: The time, of how atoms were reasoned to attach to each other, i.e. "hooked atoms", "glued together by rest", or "stuck together by conspiring motions", Newton states that he would rather infer from their cohesion, that "particles attract one another by some force , which in immediate contact is exceedingly strong, at small distances performs the chemical operations, and reaches not far from the particles with any sensible effect." In 1819, on

8455-559: The trade name Arawin , in China by Yantai Tayho under the trade name New Star and by SRO Group under the trade name X-Fiper , and a variant of meta-aramid in France by Kermel under the trade name Kermel. Based on earlier research by Monsanto Company and Bayer , para -aramid fiber with much higher tenacity and elastic modulus was also developed in the 1960s and 1970s by DuPont and AkzoNobel , both profiting from their knowledge of rayon , polyester and nylon processing. In 1973, DuPont

8550-411: The two atoms in the bond. Such bonds can be understood by classical physics . The force between the atoms depends on isotropic continuum electrostatic potentials. The magnitude of the force is in simple proportion to the product of the two ionic charges according to Coulomb's law . Covalent bonds are better understood by valence bond (VB) theory or molecular orbital (MO) theory . The properties of

8645-408: The two electrons. With up to 13 adjustable parameters they obtained a result very close to the experimental result for the dissociation energy. Later extensions have used up to 54 parameters and gave excellent agreement with experiments. This calculation convinced the scientific community that quantum theory could give agreement with experiment. However this approach has none of the physical pictures of

8740-486: The underlying connective tissue, through the basement membrane . Cell junctions are especially abundant in epithelial tissues. Simple epithelium is a single layer of cells with every cell in direct contact with the basement membrane that separates it from the underlying connective tissue. In general, it is found where absorption and filtration occur. The thinness of the epithelial barrier facilitates these processes. In general, epithelial tissues are classified by

8835-445: The valence bond and molecular orbital theories and is difficult to extend to larger molecules. Because atoms and molecules are three-dimensional, it is difficult to use a single method to indicate orbitals and bonds. In molecular formulas the chemical bonds (binding orbitals) between atoms are indicated in different ways depending on the type of discussion. Sometimes, some details are neglected. For example, in organic chemistry one

8930-633: Was introduced in 1972, accepted in 1974 by the Federal Trade Commission of the USA as the name of a generic category of fiber distinct from nylon , and adopted by the International Standards Organisation in 1977. Aromatic in the longer name refers to the presence of aromatic rings of six carbon atoms. In aramids these rings are connected via amide linkages each comprising a CO group attached to an NH group. In order to meet

9025-401: Was the first company to introduce a para-aramid fiber, calling it Kevlar ; this remains one of the best-known para-aramids and/or aramids. In 1978, Akzo introduced a similar fiber with roughly the same chemical structure calling it Twaron . Due to earlier patents on the production process, Akzo and DuPont engaged in a patent dispute in the 1980s. Twaron subsequently came under the ownership of

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