Calcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks as the minerals calcite and aragonite and is the main component of pearls and the shells of marine organisms and eggs. Calcium carbonate is the active ingredient in agricultural lime and is created when calcium ions in hard water react with carbonate ions to create limescale, it is medicinally used as a calcium supplement or as an antacid, but excessive consumption can be hazardous. Calcium carbonate shares the typical properties of other carbonates. Notably it reacts with acids, releasing carbon dioxide:CaCO3 + 2 H+ → Ca2+ + CO2 + H2Oreleases carbon dioxide upon heating, called a thermal decomposition reaction, or calcination, to form calcium oxide called quicklime, with reaction enthalpy 178 kJ/mol:CaCO3 → CaO + CO2Calcium carbonate will react with water, saturated with carbon dioxide to form the soluble calcium bicarbonate. CaCO3 + CO2 + H2O → Ca2This reaction is important in the erosion of carbonate rock, forming caverns, leads to hard water in many regions.
An unusual form of calcium carbonate is the hexahydrate, ikaite, CaCO3·6H2O. Ikaite is stable only below 8 °C; the vast majority of calcium carbonate used in industry is extracted by quarrying. Pure calcium carbonate, can be produced from a pure quarried source. Alternatively, calcium carbonate is prepared from calcium oxide. Water is added to give calcium hydroxide carbon dioxide is passed through this solution to precipitate the desired calcium carbonate, referred to in the industry as precipitated calcium carbonate: CaO + H2O → Ca2 Ca2 + CO2 → CaCO3↓ + H2O The thermodynamically stable form of CaCO3 under normal conditions is hexagonal β-CaCO3. Other forms can be prepared, the denser orthorhombic λ-CaCO3 and μ-CaCO3, occurring as the mineral vaterite; the aragonite form can be prepared by precipitation at temperatures above 85 °C, the vaterite form can be prepared by precipitation at 60 °C. Calcite contains calcium atoms coordinated by six oxygen atoms, in aragonite they are coordinated by nine oxygen atoms.
The vaterite structure is not understood. Magnesium carbonate has the calcite structure, whereas strontium carbonate and barium carbonate adopt the aragonite structure, reflecting their larger ionic radii. Calcite and vaterite are pure calcium carbonate minerals. Industrially important source rocks which are predominantly calcium carbonate include limestone, chalk and travertine. Eggshells, snail shells and most seashells are predominantly calcium carbonate and can be used as industrial sources of that chemical. Oyster shells have enjoyed recent recognition as a source of dietary calcium, but are a practical industrial source. Dark green vegetables such as broccoli and kale contain dietarily significant amounts of calcium carbonate, they are not practical as an industrial source. Beyond Earth, strong evidence suggests the presence of calcium carbonate on Mars. Signs of calcium carbonate have been detected at more than one location; this provides some evidence for the past presence of liquid water.
Carbonate, is found in geologic settings and constitutes an enormous carbon reservoir. Calcium carbonate occurs as aragonite and dolomite as significant constituents of the calcium cycle; the carbonate minerals form the rock types: limestone, marble, travertine and others. In warm, clear tropical waters corals are more abundant than towards the poles where the waters are cold. Calcium carbonate contributors, including plankton, coralline algae, brachiopods, echinoderms and mollusks, are found in shallow water environments where sunlight and filterable food are more abundant. Cold-water carbonates do exist at higher latitudes but have a slow growth rate; the calcification processes are changed by ocean acidification. Where the oceanic crust is subducted under a continental plate sediments will be carried down to warmer zones in the asthenosphere and lithosphere. Under these conditions calcium carbonate decomposes to produce carbon dioxide which, along with other gases, give rise to explosive volcanic eruptions.
The carbonate compensation depth is the point in the ocean where the rate of precipitation of calcium carbonate is balanced by the rate of dissolution due to the conditions present. Deep in the ocean, the temperature pressure increases. Calcium carbonate is unusual in. Increasing pressure increases the solubility of calcium carbonate; the carbonate compensation depth can range from 4,000 to 6,000 meters below sea level. Calcium carbonate can preserve fossils through permineralization. Most of the vertebrate fossils of the Two Medicine Formation—a geologic formation known for its duck-billed dinosaur eggs—are preserved by CaCO3 permineralization; this type of preservation conserves high levels of detail down to the microscopic level. However, it leaves specimens vulnerable to weathering when exposed to the surface. Trilobite populations were once thought to have composed the majority of aquatic life during the Cambrian, due to the fact that their calcium carbonate-rich shells were more preserved than those of other species, which had purely chitinous shells.
The main use of calcium ca
In optics, the refractive index or index of refraction of a material is a dimensionless number that describes how fast light propagates through the material. It is defined as n = c v, where c is the speed of light in vacuum and v is the phase velocity of light in the medium. For example, the refractive index of water is 1.333, meaning that light travels 1.333 times as fast in vacuum as in water. The refractive index determines how much the path of light is bent, or refracted, when entering a material; this is described by Snell's law of refraction, n1 sinθ1 = n2 sinθ2, where θ1 and θ2 are the angles of incidence and refraction of a ray crossing the interface between two media with refractive indices n1 and n2. The refractive indices determine the amount of light, reflected when reaching the interface, as well as the critical angle for total internal reflection and Brewster's angle; the refractive index can be seen as the factor by which the speed and the wavelength of the radiation are reduced with respect to their vacuum values: the speed of light in a medium is v = c/n, the wavelength in that medium is λ = λ0/n, where λ0 is the wavelength of that light in vacuum.
This implies that vacuum has a refractive index of 1, that the frequency of the wave is not affected by the refractive index. As a result, the energy of the photon, therefore the perceived color of the refracted light to a human eye which depends on photon energy, is not affected by the refraction or the refractive index of the medium. While the refractive index affects wavelength, it depends on photon frequency and energy so the resulting difference in the bending angle causes white light to split into its constituent colors; this is called dispersion. It can be observed in prisms and rainbows, chromatic aberration in lenses. Light propagation in absorbing materials can be described using a complex-valued refractive index; the imaginary part handles the attenuation, while the real part accounts for refraction. The concept of refractive index applies within the full electromagnetic spectrum, from X-rays to radio waves, it can be applied to wave phenomena such as sound. In this case the speed of sound is used instead of that of light, a reference medium other than vacuum must be chosen.
The refractive index n of an optical medium is defined as the ratio of the speed of light in vacuum, c = 299792458 m/s, the phase velocity v of light in the medium, n = c v. The phase velocity is the speed at which the crests or the phase of the wave moves, which may be different from the group velocity, the speed at which the pulse of light or the envelope of the wave moves; the definition above is sometimes referred to as the absolute refractive index or the absolute index of refraction to distinguish it from definitions where the speed of light in other reference media than vacuum is used. Air at a standardized pressure and temperature has been common as a reference medium. Thomas Young was the person who first used, invented, the name "index of refraction", in 1807. At the same time he changed this value of refractive power into a single number, instead of the traditional ratio of two numbers; the ratio had the disadvantage of different appearances. Newton, who called it the "proportion of the sines of incidence and refraction", wrote it as a ratio of two numbers, like "529 to 396".
Hauksbee, who called it the "ratio of refraction", wrote it as a ratio with a fixed numerator, like "10000 to 7451.9". Hutton wrote it as a ratio with a fixed denominator, like 1.3358 to 1. Young did not use a symbol for the index of refraction, in 1807. In the next years, others started using different symbols: n, m, µ; the symbol n prevailed. For visible light most transparent media have refractive indices between 1 and 2. A few examples are given in the adjacent table; these values are measured at the yellow doublet D-line of sodium, with a wavelength of 589 nanometers, as is conventionally done. Gases at atmospheric pressure have refractive indices close to 1 because of their low density. All solids and liquids have refractive indices above 1.3, with aerogel as the clear exception. Aerogel is a low density solid that can be produced with refractive index in the range from 1.002 to 1.265. Moissanite lies at the other end of the range with a refractive index as high as 2.65. Most plastics have refractive indices in the range from 1.3 to 1.7, but some high-refractive-index polymers can have values as high as 1.76.
For infrared light refractive indices can be higher. Germanium is transparent in the wavelength region from 2 to 14 µm and has a refractive index of about 4. A type of new materials, called topological insulator, was found holding higher refractive index of up to 6 in near to mid infrared frequency range. Moreover, topological insulator material are transparent; these excellent properties make them a type of significant materials for infrared optics. According to the theory of relativity, no information can travel faster than the speed of light in vacuum, but this does not mean that the refractive index cannot be lower than 1; the refractive index measures the phase velocity of light. The phase velocity is the speed at which the crests of the wave move and can be faster than the speed of light in vacuum, thereby give a refractive index below 1; this can occur close to resonance frequencies, for absorbing media, in plasmas, for X-rays. In the X-ray regime the refractive indices are
Hexactinellid sponges are sponges with a skeleton made of four- and/or six-pointed siliceous spicules referred to as glass sponges. They are classified along with other sponges in the phylum Porifera, but some researchers consider them sufficiently distinct to deserve their own phylum, Symplasma. Glass sponges are uncommon and are found at depths from 450 to 900 metres although the species Oopsacas minuta has been found in shallow water while others have been found much deeper, they are found in all oceans of the world, although they are common in Antarctic and Northern Pacific waters. They are more-or-less cup-shaped animals, ranging from 10 to 30 centimetres in height, with sturdy lattice-like internal skeletons made up of fused spicules of silica; the body is symmetrical, with a large central cavity that, in many species, opens to the outside through a sieve formed from the skeleton. Some species of glass sponges are capable of fusing together to create bioherms, they are pale in colour, ranging from white to orange.
Much of the body is composed of extensive regions of multinucleate cytoplasm. In particular, the epidermal cells characteristic of other sponges are absent, being replaced by a syncitial net of amoebocytes, through which the spicules penetrate. Unlike other sponges, they do not possess the ability to contract. One ability they do possess is a unique system for conducting electrical impulses across their bodies, making it possible for them to respond to external stimuli. Glass sponges like "Venus' flower basket" have a tuft of fibers that extends outward like an inverted crown at the base of their skeleton; these fibers are 50 to 175 millimetres long and about the thickness of a human hair. Glass sponges are different from other sponges in a variety of other ways. For example, most of the cytoplasm is not divided into separate cells by walls but forms a syncytium or continuous mass of cytoplasm with many nuclei; these creatures are long lived. The shallow water occurrence of hexactinellids is rare world wide.
In the Antarctic two species occur as shallow as 33 meters under the ice. In the Mediterranean one species occurs as shallow as 18 metres in a cave with deep water upwelling The sponges form reefs off the coast of British Columbia and Washington State, which are studied in the Sponge Reef Project. Reefs discovered in Hecate Strait, BC have grown to up to 7 kilometres 20 metres high. Previous to these discoveries, sponge reefs were thought to have died out in the Jurassic period; the earliest known hexactinellids are from late Neoproterozoic. They are common relative to demosponges as fossils, but this is thought to be, at least in part, because their spicules are sturdier than spongin and fossilize better. Like all sponges, the hexactinellids draw water in through a series of small pores by the whip like beating of a series of hairs or flagella in chambers which in this group line the sponge wall; the class is divided into five orders, in two subclasses:Class Hexactinellida Subclass Amphidiscophora Order Amphidiscosida Subclass Hexasterophora Incertae sedis Dactylocalycidae Gray, 1867 Order Lychniscosida Order Lyssacinosida Order Sceptrulophora Sponge reef Cloud sponge Sponge Reef Project Media related to Hexactinellida at Wikimedia Commons Data related to Hexactinellida at Wikispecies
Demospongiae is the most diverse class in the phylum Porifera. They include 76.2% of all species of sponges with nearly 8,800 species worldwide. They are sponges with a soft body that covers a hard massive skeleton made of calcium carbonate, either aragonite or calcite, they are predominantly leuconoid in structure. Their "skeletons" are made of spicules consisting of fibers of the protein spongin, the mineral silica, or both. Where spicules of silica are present, they have a different shape from those in the otherwise similar glass sponges; the many diverse orders in this class include all of the large sponges. Most are marine dwellers; some species are brightly colored, with great variety in body shape. They reproduce both sexually and asexually, they are the only extant organisms that methylate sterols at the 26-position, a fact used to identify the presence of demosponges before their first known unambiguous fossils. Because of their long life span it is thought that analysis of the aragonite skeletons of these sponges could extend data regarding ocean temperature and other variables farther into the past than has been possible.
Their dense skeletons are deposited in an organized chronological manner, in concentric layers or bands. The layered skeletons look similar to reef corals. Therefore, demosponges are called coralline sponges; the Demospongiae have an ancient history. The first demosponges may have appeared during the Precambrian deposits at the end of the Cryogenian "Snowball Earth" period, their presence has been indirectly detected by fossilized steroids, called steranes, hydrocarbon markers characteristic of the cell membranes of the sponges, rather than from direct fossils of the sponges themselves. They represent a continuous chemical fossil record of demosponges through the end of the Neoproterozoic; the earliest Demospongiae fossil was discovered in the lower Cambrian of the Sirius Passet Biota of North Greenland: this single specimen had a spicule assemblage similar to that found in the subclass Heteroscleromorpha. The earliest sponge-bearing reefs date to the Early Cambrian, exemplified by a small bioherm constructed by archaeocyathids and calcified microbes at the start of the Tommotian stage about 530 Ma, found in southeast Siberia.
A major radiation occurred in the Lower Cambrian and further major radiations in the Ordovician from the middle Cambrian. The Systema Porifera book was the result of a collaboration of 45 researchers from 17 countries led by editors J. N. A. Hooper and R. W. M. van Soest. This milestone publication provided an updated comprehensive overview of sponge systematics, the largest revision of this group since the start of spongiology in the mid-19th century. In this large revision, the extant Demospongiae were organized into 14 orders that encompassed 88 families and 500 genera. Hooper and van Soest gave the following classification of demosponges into orders: Subclass Homoscleromorpha Bergquist 1978 Homosclerophorida Dendy 1905 Subclass Tetractinomorpha Astrophorida Sollas 1888 Chondrosida Boury-Esnault & Lopès 1985 Hadromerida Topsent 1894 Lithistida Sollas 1888 Spirophorida Bergquist & Hogg 1969 Subclass Ceractinomorpha Lévi 1953 Agelasida Verrill 1907 Dendroceratida Minchin 1900 Dictyoceratida Minchin 1900 Halichondrida Gray 1867 Halisarcida Bergquist 1996 Haplosclerida Topsent 1928 Poecilosclerida Topsent 1928 Verongida Bergquist 1978 Verticillitida Termier & Termier 1977 However and morphological evidence show that the Homoscleromorpha do not belong in this class.
The Homoscleromorpha was therefore taken out of the Demospongiae in 2012, became the fourth class of phylum Porifera. Morrow & Cárdenas propose a revision of the Demospongiae higher taxa classification based on molecular data of the last ten years; some demosponge subclasses and orders are polyphyletic or should be included in other orders, so that Morrow and Cárdenas propose to abandon certain names: these are the Ceractinomorpha, Halisarcida, Lithistida and Hadromerida. Instead, they recommend the use of three subclasses: Verongimorpha and Heteroscleromorpha, they retain seven of the 13 orders from Systema Porifera. They recommend to upgrade six order names, they create seven new orders. These added to the created orders make a total of 22 orders in the revised classification; these changes are now implemented in the World Porifera Database part of the World Register of Marine Species. Subclass Heteroscleromorpha Cárdenas, Pérez, Boury-Esnault, 2012 order Agelasida Verrill, 1907 order Axinellida Lévi, 1953 order Biemnida Morrow et al. 2013 order Bubarida Morrow & Cárdenas, 2015 order Clionaida Morrow & Cárdenas, 2015 order Desmacellida Morrow & Cárdenas, 2015 order Haplosclerida Topsent, 1928 order Merliida Vacelet, 1979 order Poecilosclerida Topsent, 1928 order Polymastiida Morrow & Cárdenas, 2015 order Scopalinida Morrow & Cárdenas, 2015 order Sphaerocladina Schrammen, 1924 order Spongillida Manconi & Pronzato, 2002 order Suberitida Chombard & Boury-Esnault, 1999 order Tethyida Morrow & Cárdenas, 2015 order Tetractinellida Marshall
A lens is a transmissive optical device that focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses arranged along a common axis. Lenses are made from materials such as glass or plastic, are ground and polished or molded to a desired shape. A lens can focus light to form an image, unlike a prism. Devices that focus or disperse waves and radiation other than visible light are called lenses, such as microwave lenses, electron lenses, acoustic lenses, or explosive lenses; the word lens comes from lēns, the Latin name of the lentil, because a double-convex lens is lentil-shaped. The lentil plant gives its name to a geometric figure; some scholars argue that the archeological evidence indicates that there was widespread use of lenses in antiquity, spanning several millennia. The so-called Nimrud lens is a rock crystal artifact dated to the 7th century BC which may or may not have been used as a magnifying glass, or a burning glass.
Others have suggested that certain Egyptian hieroglyphs depict "simple glass meniscal lenses". The oldest certain reference to the use of lenses is from Aristophanes' play The Clouds mentioning a burning-glass. Pliny the Elder confirms. Pliny has the earliest known reference to the use of a corrective lens when he mentions that Nero was said to watch the gladiatorial games using an emerald. Both Pliny and Seneca the Younger described the magnifying effect of a glass globe filled with water. Ptolemy wrote a book on Optics, which however survives only in the Latin translation of an incomplete and poor Arabic translation; the book was, received, by medieval scholars in the Islamic world, commented upon by Ibn Sahl, in turn improved upon by Alhazen. The Arabic translation of Ptolemy's Optics became available in Latin translation in the 12th century. Between the 11th and 13th century "reading stones" were invented; these were primitive plano-convex lenses made by cutting a glass sphere in half. The medieval rock cystal Visby lenses may not have been intended for use as burning glasses.
Spectacles were invented as an improvement of the "reading stones" of the high medieval period in Northern Italy in the second half of the 13th century. This was the start of the optical industry of grinding and polishing lenses for spectacles, first in Venice and Florence in the late 13th century, in the spectacle-making centres in both the Netherlands and Germany. Spectacle makers created improved types of lenses for the correction of vision based more on empirical knowledge gained from observing the effects of the lenses; the practical development and experimentation with lenses led to the invention of the compound optical microscope around 1595, the refracting telescope in 1608, both of which appeared in the spectacle-making centres in the Netherlands. With the invention of the telescope and microscope there was a great deal of experimentation with lens shapes in the 17th and early 18th centuries by those trying to correct chromatic errors seen in lenses. Opticians tried to construct lenses of varying forms of curvature, wrongly assuming errors arose from defects in the spherical figure of their surfaces.
Optical theory on refraction and experimentation was showing no single-element lens could bring all colours to a focus. This led to the invention of the compound achromatic lens by Chester Moore Hall in England in 1733, an invention claimed by fellow Englishman John Dollond in a 1758 patent. Most lenses are spherical lenses: their two surfaces are parts of the surfaces of spheres; each surface can be concave, or planar. The line joining the centres of the spheres making up the lens surfaces is called the axis of the lens; the lens axis passes through the physical centre of the lens, because of the way they are manufactured. Lenses may ground after manufacturing to give them a different shape or size; the lens axis may not pass through the physical centre of the lens. Toric or sphero-cylindrical lenses have surfaces with two different radii of curvature in two orthogonal planes, they have a different focal power in different meridians. This forms an astigmatic lens. An example is eyeglass lenses. More complex are aspheric lenses.
These are lenses where one or both surfaces have a shape, neither spherical nor cylindrical. The more complicated shapes allow such lenses to form images with less aberration than standard simple lenses, but they are more difficult and expensive to produce. Lenses are classified by the curvature of the two optical surfaces. A lens is biconvex. If both surfaces have the same radius of curvature, the lens is equiconvex. A lens with two concave surfaces is biconcave. If one of the surfaces is flat, the lens is plano-convex or plano-concave depending on the curvature of the other surface. A lens with one convex and one concave side is meniscus, it is this type of lens, most used in corrective lenses. If the lens is biconvex or plano-convex, a collimated beam of light passing through the lens converges to a spot behind the lens. In this case, the lens is called a
Venus' flower basket
The Venus' flower basket is a hexactinellid sponge in the phylum Porifera inhabiting the deep ocean. The sponge symbiotically houses two small spongicolid shrimp, a male and a female, who live out their lives inside the sponge; the shrimp breed, when their offspring are tiny, they escape to find a new Venus' flower basket of their own. The shrimp inside the basket clean it and, in return, the basket provides food for the shrimp by trapping it in its tissues and releasing wastes into the body of the sponge for the shrimp, it is speculated that the bioluminescent light of bacteria harnessed by the sponge may attract other small organisms which the shrimp eat. In Japan, this symbiotic relationship symbolizes the idea "till death do us part", the sponge is given as a wedding gift. Venus' flower baskets are found in a small area of the sea nearby the Philippine Islands. Similar species occur near Japan and in other parts of the western Pacific Ocean and the Indian Ocean; the body structure of these animals is a thin-walled, vase-shaped tube with a large central atrium.
The body is composed of silica in the form of 6-pointed siliceous spicules, why they are known as glass sponges. The spicules are composed of 3 perpendicular rays giving them 6 points. Spicules are microscopic, pin-like structures within the sponge's tissues that provide structural support for the sponge, it is the combination of spicule forms within a sponge's tissues. In the case of glass sponges the spicules "weave" together to form a fine mesh which gives the sponge's body a rigidity not found in other sponge species and allows glass sponges to survive at great depths in the water column; the glassy fibers that attach the sponge to the ocean floor, 5–20 cm long and thin as human hair, are of interest to fiber optics researchers. The sponge extracts silicic acid from seawater and converts it into silica forms it into an elaborate skeleton of glass fibers. Other sponges such as the orange puffball sponge can produce glass biologically; the current manufacturing process for optical fibers requires high temperatures and produces a brittle fiber.
A low-temperature process for creating and arranging such fibers, inspired by sponges, could offer more control over the optical properties of the fibers. These nano-structures are potentially useful for the creation of more efficient, low-cost solar cells; these sponges skeletons have complex geometric configurations, which have been extensively studied for their stiffness, yield strength, minimal crack propagation. An aluminum tube of equal length, effective thickness, radius, but homogeneously distributed, has 1/100th the stiffness; the Venus' flower basket and skyscrapers YouTube. William McCall, AP. "Glassy sponge has better fiber optics than man-made". Joanna Aizenberg et al. PNAS 2004. "Biological glass fibers: Correlation between optical and structural properties". Kevin Bullis. "Silicon and Sun". Technology Review. Clare Valentine. Encyclopedia of Life
The cell is the basic structural and biological unit of all known living organisms. A cell is the smallest unit of life. Cells are called the "building blocks of life"; the study of cells is called cellular biology. Cells consist of cytoplasm enclosed within a membrane, which contains many biomolecules such as proteins and nucleic acids. Organisms can be classified as multicellular; the number of cells in plants and animals varies from species to species, it has been estimated that humans contain somewhere around 40 trillion cells. Most plant and animal cells are visible only under a microscope, with dimensions between 1 and 100 micrometres. Cells were discovered by Robert Hooke in 1665, who named them for their resemblance to cells inhabited by Christian monks in a monastery. Cell theory, first developed in 1839 by Matthias Jakob Schleiden and Theodor Schwann, states that all organisms are composed of one or more cells, that cells are the fundamental unit of structure and function in all living organisms, that all cells come from pre-existing cells.
Cells emerged on Earth at least 3.5 billion years ago. Cells are of two types: eukaryotic, which contain a nucleus, prokaryotic, which do not. Prokaryotes are single-celled organisms, while eukaryotes can be either single-celled or multicellular. Prokaryotes include two of the three domains of life. Prokaryotic cells were the first form of life on Earth, characterised by having vital biological processes including cell signaling, they are simpler and smaller than eukaryotic cells, lack membrane-bound organelles such as a nucleus. The DNA of a prokaryotic cell consists of a single chromosome, in direct contact with the cytoplasm; the nuclear region in the cytoplasm is called the nucleoid. Most prokaryotes are the smallest of all organisms ranging from 0.5 to 2.0 µm in diameter. A prokaryotic cell has three architectural regions: Enclosing the cell is the cell envelope – consisting of a plasma membrane covered by a cell wall which, for some bacteria, may be further covered by a third layer called a capsule.
Though most prokaryotes have both a cell membrane and a cell wall, there are exceptions such as Mycoplasma and Thermoplasma which only possess the cell membrane layer. The envelope gives rigidity to the cell and separates the interior of the cell from its environment, serving as a protective filter; the cell wall consists of peptidoglycan in bacteria, acts as an additional barrier against exterior forces. It prevents the cell from expanding and bursting from osmotic pressure due to a hypotonic environment; some eukaryotic cells have a cell wall. Inside the cell is the cytoplasmic region that contains the genome and various sorts of inclusions; the genetic material is found in the cytoplasm. Prokaryotes can carry extrachromosomal DNA elements called plasmids, which are circular. Linear bacterial plasmids have been identified in several species of spirochete bacteria, including members of the genus Borrelia notably Borrelia burgdorferi, which causes Lyme disease. Though not forming a nucleus, the DNA is condensed in a nucleoid.
Plasmids encode additional genes, such as antibiotic resistance genes. On the outside and pili project from the cell's surface; these are structures made of proteins that facilitate communication between cells. Plants, fungi, slime moulds and algae are all eukaryotic; these cells are about fifteen times wider than a typical prokaryote and can be as much as a thousand times greater in volume. The main distinguishing feature of eukaryotes as compared to prokaryotes is compartmentalization: the presence of membrane-bound organelles in which specific activities take place. Most important among these is a cell nucleus, an organelle that houses the cell's DNA; this nucleus gives the eukaryote its name, which means "true kernel". Other differences include: The plasma membrane resembles that of prokaryotes in function, with minor differences in the setup. Cell walls may not be present; the eukaryotic DNA is organized in one or more linear molecules, called chromosomes, which are associated with histone proteins.
All chromosomal DNA is stored in the cell nucleus, separated from the cytoplasm by a membrane. Some eukaryotic organelles such as mitochondria contain some DNA. Many eukaryotic cells are ciliated with primary cilia. Primary cilia play important roles in chemosensation and thermosensation. Cilia may thus be "viewed as a sensory cellular antennae that coordinates a large number of cellular signaling pathways, sometimes coupling the signaling to ciliary motility or alternatively to cell division and differentiation." Motile eukaryotes can move using motile flagella. Motile cells are absent in flowering plants. Eukaryotic flagella are more complex than those of prokaryotes. All cells, whether prokaryotic or eukaryotic, have a membrane that envelops the cell, regulates what moves in and out, maintains the electric potential of the cell. Inside the membrane, the cytoplasm takes up most of the cell's volume. All cells possess DNA, the hereditary material of genes, RNA, containing the information necessary to build various proteins such as enzymes, the cell's primary machinery.
There are other kinds of biomolecules in cells. This article lists these primary cellular components briefly