Mohenjo-daro
Mohenjo-daro is an archaeological site in the province of Sindh, Pakistan. Built around 2500 BCE, it was one of the largest settlements of the ancient Indus Valley Civilisation, one of the world's earliest major cities, contemporaneous with the civilizations of ancient Egypt, Minoan Crete, Norte Chico. Mohenjo-daro was abandoned in the 19th century BCE as the Indus Valley Civilization declined, the site was not rediscovered until the 1920s. Significant excavation has since been conducted at the site of the city, designated a UNESCO World Heritage Site in 1980; the site is threatened by erosion and improper restoration. The city's original name is unknown. Based on his analysis of a Mohenjo-daro seal, Iravatham Mahadevan speculates that the city's ancient name could have been Kukkutarma. Cock-fighting may have had ritual and religious significance for the city, with domesticated chickens bred there for sacred purposes, rather than as a food source. Mohenjo-daro may have been a point of diffusion for the eventual worldwide domestication of chickens.
Mohenjo-daro, the modern name for the site, has been variously interpreted as "Mound of the Dead Men" in Sindhi, as "Mound of Mohan". Mohenjo-daro is located west of the Indus River in Larkana District, Pakistan, in a central position between the Indus River and the Ghaggar-Hakra River, it is situated on a Pleistocene ridge in the middle of the flood plain of the Indus River Valley, around 28 kilometres from the town of Larkana. The ridge was prominent during the time of the Indus Valley Civilization, allowing the city to stand above the surrounding flood, but subsequent flooding has since buried most of the ridge in silt deposits; the Indus still flows east of the site, but the Ghaggar-Hakra riverbed on the western side is now dry. Mohenjo-daro was built in the 26th century BCE, it was one of the largest cities of the ancient Indus Valley Civilization known as the Harappan Civilization, which developed around 3,000 BCE from the prehistoric Indus culture. At its height, the Indus Civilization spanned much of what is now Pakistan and North India, extending westwards to the Iranian border, south to Gujarat in India and northwards to an outpost in Bactria, with major urban centers at Harappa, Mohenjo-daro, Kalibangan and Rakhigarhi.
Mohenjo-daro was the most advanced city of its time, with remarkably sophisticated civil engineering and urban planning. When the Indus civilization went into sudden decline around 1900 BCE, Mohenjo-daro was abandoned; the ruins of the city remained undocumented for around 3,700 years until R. D. Banerji, an officer of the Archaeological Survey of India, visited the site in 1919–20 identifying what he thought to be a Buddhist stupa known to be there and finding a flint scraper which convinced him of the site's antiquity; this led to large-scale excavations of Mohenjo-daro led by Kashinath Narayan Dikshit in 1924–25, John Marshall in 1925–26. In the 1930s major excavations were conducted at the site under the leadership of Marshall, D. K. Dikshitar and Ernest Mackay. Further excavations were carried out in 1945 by his trainee, Ahmad Hasan Dani; the last major series of excavations were conducted in 1965 by George F. Dales. After 1965 excavations were banned due to weathering damage to the exposed structures, the only projects allowed at the site since have been salvage excavations, surface surveys, conservation projects.
In the 1980s, German and Italian survey groups led by Dr. Michael Jansen and Dr. Maurizio Tosi used less invasive archeological techniques, such as architectural documentation, surface surveys, localized probing, to gather further information about Mohenjo-daro. A dry core drilling conducted in 2015 by Pakistan's National Fund for Mohenjo-daro revealed that the site is larger than the unearthed area. Mohenjo-daro has a planned layout with rectilinear buildings arranged on a grid plan. Most were built of mortared brick; the covered area of Mohenjo-daro is estimated at 300 hectares. The Oxford Handbook of Cities in World History offers a "weak" estimate of a peak population of around 40,000; the sheer size of the city, its provision of public buildings and facilities, suggests a high level of social organization. The city is divided into the so-called Citadel and the Lower City; the Citadel – a mud-brick mound around 12 metres high – is known to have supported public baths, a large residential structure designed to house about 5,000 citizens, two large assembly halls.
The city had a central marketplace, with a large central well. Individual households or groups of households obtained their water from smaller wells. Waste water was channeled to covered drains; some houses those of more prestigious inhabitants, include rooms that appear to have been set aside for bathing, one building had an underground furnace for heated bathing. Most houses had inner courtyards, with doors; some buildings had two stories. In 1950, Sir Mortimer Wheeler identified one large building in Mohenjo-daro as a "Great Granary". Certain wall-divisions in its massive wooden superstructure appeared to be grain storage-bays, complete with air-ducts to dry the grain. According to Wheeler, carts would have brought grain from the countryside and unloaded them directly into the bays. However, Jonathan Mark Kenoyer noted the complete lack of evidence for grain at the "granary", which, he argued, might therefore be bett
Plant
Plants are multicellular, predominantly photosynthetic eukaryotes of the kingdom Plantae. Plants were treated as one of two kingdoms including all living things that were not animals, all algae and fungi were treated as plants. However, all current definitions of Plantae exclude the fungi and some algae, as well as the prokaryotes. By one definition, plants form the clade Viridiplantae, a group that includes the flowering plants and other gymnosperms and their allies, liverworts and the green algae, but excludes the red and brown algae. Green plants obtain most of their energy from sunlight via photosynthesis by primary chloroplasts that are derived from endosymbiosis with cyanobacteria, their chloroplasts contain b, which gives them their green color. Some plants are parasitic or mycotrophic and have lost the ability to produce normal amounts of chlorophyll or to photosynthesize. Plants are characterized by sexual reproduction and alternation of generations, although asexual reproduction is common.
There are about 320 thousand species of plants, of which the great majority, some 260–290 thousand, are seed plants. Green plants provide a substantial proportion of the world's molecular oxygen and are the basis of most of Earth's ecosystems on land. Plants that produce grain and vegetables form humankind's basic foods, have been domesticated for millennia. Plants have many cultural and other uses, as ornaments, building materials, writing material and, in great variety, they have been the source of medicines and psychoactive drugs; the scientific study of plants is known as a branch of biology. All living things were traditionally placed into one of two groups and animals; this classification may date from Aristotle, who made the distincton between plants, which do not move, animals, which are mobile to catch their food. Much when Linnaeus created the basis of the modern system of scientific classification, these two groups became the kingdoms Vegetabilia and Animalia. Since it has become clear that the plant kingdom as defined included several unrelated groups, the fungi and several groups of algae were removed to new kingdoms.
However, these organisms are still considered plants in popular contexts. The term "plant" implies the possession of the following traits multicellularity, possession of cell walls containing cellulose and the ability to carry out photosynthesis with primary chloroplasts; when the name Plantae or plant is applied to a specific group of organisms or taxon, it refers to one of four concepts. From least to most inclusive, these four groupings are: Another way of looking at the relationships between the different groups that have been called "plants" is through a cladogram, which shows their evolutionary relationships; these are not yet settled, but one accepted relationship between the three groups described above is shown below. Those which have been called "plants" are in bold; the way in which the groups of green algae are combined and named varies between authors. Algae comprise several different groups of organisms which produce food by photosynthesis and thus have traditionally been included in the plant kingdom.
The seaweeds range from large multicellular algae to single-celled organisms and are classified into three groups, the green algae, red algae and brown algae. There is good evidence that the brown algae evolved independently from the others, from non-photosynthetic ancestors that formed endosymbiotic relationships with red algae rather than from cyanobacteria, they are no longer classified as plants as defined here; the Viridiplantae, the green plants – green algae and land plants – form a clade, a group consisting of all the descendants of a common ancestor. With a few exceptions, the green plants have the following features in common, they undergo closed mitosis without centrioles, have mitochondria with flat cristae. The chloroplasts of green plants are surrounded by two membranes, suggesting they originated directly from endosymbiotic cyanobacteria. Two additional groups, the Rhodophyta and Glaucophyta have primary chloroplasts that appear to be derived directly from endosymbiotic cyanobacteria, although they differ from Viridiplantae in the pigments which are used in photosynthesis and so are different in colour.
These groups differ from green plants in that the storage polysaccharide is floridean starch and is stored in the cytoplasm rather than in the plastids. They appear to have had a common origin with Viridiplantae and the three groups form the clade Archaeplastida, whose name implies that their chloroplasts were derived from a single ancient endosymbiotic event; this is the broadest modern definition of the term'plant'. In contrast, most other algae not only have different pigments but have chloroplasts with three or four surrounding membranes, they are not close relatives of the Archaeplastida having acquired chloroplasts separately from ingested or symbiotic green and red algae. They are thus not included in the broadest modern definition of the plant kingdom, although they were in the past; the green plants or Viridiplantae were traditionally divided into the green algae (including
Bark (botany)
Bark is the outermost layers of stems and roots of woody plants. Plants with bark include trees, woody vines, shrubs. Bark is a nontechnical term, it consists of the inner bark and the outer bark. The inner bark, which in older stems is living tissue, includes the innermost area of the periderm; the outer bark in older stems includes the dead tissue on the surface of the stems, along with parts of the innermost periderm and all the tissues on the outer side of the periderm. The outer bark on trees which lies external to the last formed periderm is called the rhytidome. Products derived from bark include: bark shingle siding and wall coverings and other flavorings, tanbark for tannin, latex, poisons, various hallucinogenic chemicals and cork. Bark has been used to make cloth and ropes and used as a surface for paintings and map making. A number of plants are grown for their attractive or interesting bark colorations and surface textures or their bark is used as landscape mulch. What is called bark includes a number of different tissues.
Cork is an external, secondary tissue, impermeable to water and gases, is called the phellem. The cork is produced by the cork cambium, a layer of meristematically active cells which serve as a lateral meristem for the periderm; the cork cambium, called the phellogen, is only one cell layer thick and it divides periclinally to the outside producing cork. The phelloderm, not always present in all barks, is a layer of cells formed by and interior to the cork cambium. Together, the phellem and phelloderm constitute the periderm. Cork cell walls contain suberin, a waxy substance which protects the stem against water loss, the invasion of insects into the stem, prevents infections by bacteria and fungal spores; the cambium tissues, i.e. the cork cambium and the vascular cambium, are the only parts of a woody stem where cell division occurs. Phloem is a nutrient-conducting tissue composed of sieve tubes or sieve cells mixed with parenchyma and fibers; the cortex is the primary tissue of roots. In stems the cortex is between the epidermis layer and the phloem, in roots the inner layer is not phloem but the pericycle.
From the outside to the inside of a mature woody stem, the layers include: Bark Periderm Cork, includes the rhytidome Cork cambium Phelloderm Cortex Phloem Vascular cambium Wood Sapwood Heartwood Pith In young stems, which lack what is called bark, the tissues are, from the outside to the inside: Epidermis, which may be replaced by periderm Cortex Primary and secondary phloem Vascular cambium Secondary and primary xylem. As the stem ages and grows, changes occur that transform the surface of the stem into the bark; the epidermis is a layer of cells that cover the plant body, including the stems, leaves and fruits, that protects the plant from the outside world. In old stems the epidermal layer and primary phloem become separated from the inner tissues by thicker formations of cork. Due to the thickening cork layer these cells die; this dead layer is the rough corky bark that forms around other stems. A secondary covering called the periderm forms on small woody stems and many non-woody plants, composed of cork, the cork cambium, the phelloderm.
The periderm forms from the phellogen. The periderm replaces the epidermis, acts as a protective covering like the epidermis. Mature phellem cells have suberin in their walls to protect the stem from desiccation and pathogen attack. Older phellem cells are dead; the skin on the potato tuber constitutes the cork of the periderm. In woody plants the epidermis of newly grown stems is replaced by the periderm in the year; as the stems grow a layer of cells form under the epidermis, called the cork cambium, these cells produce cork cells that turn into cork. A limited number of cell layers may form interior to the cork cambium, called the phelloderm; as the stem grows, the cork cambium produces new layers of cork which are impermeable to gases and water and the cells outside the periderm, namely the epidermis and older secondary phloem die. Within the periderm are lenticels, which form during the production of the first periderm layer. Since there are living cells within the cambium layers that need to exchange gases during metabolism, these lenticels, because they have numerous intercellular spaces, allow gaseous exchange with the outside atmosphere.
As the bark develops, new lenticels are formed within the cracks of the cork layers. The rhytidome is the most familiar part of bark, being the outer layer that covers the trunks of trees, it is composed of dead cells and is produced by the formation of multiple layers of suberized periderm and phloem tissue. The rhytidome is well developed in older stems and roots of trees. In shrubs, older bark is exfoliated and thick rhytidome accumulates, it is thickest and most distinctive at the trunk or bole of the tree. Bark tissues make up by weight between 10–20% of woody vascular plants and consists of various biopolymers, lignin, suberin and polysaccharides. Up to 40% of the bark tissue is made of lignin which forms an important part of a plant providing stru
Goa
Goa is a state on the south-western coast of India within the coastal region known as the Konkan, separated from the Deccan highlands of the state of Karnataka by the Western Ghats. It is bounded by Maharashtra to the north and Karnataka to the east and south, with the Arabian Sea forming its western coast, it is the fourth-smallest by population. Goa has the highest GDP per capita among all Indian states, two and a half times that of the country, it was ranked the best-placed state by the Eleventh Finance Commission for its infrastructure and ranked on top for the best quality of life in India by the National Commission on Population based on the 12 Indicators. Panaji is the state's capital; the historic city of Margao still exhibits the cultural influence of the Portuguese, who first landed in the early 16th century as merchants and conquered it soon thereafter. Goa is a former Portuguese province. Goa is visited by large numbers of international and domestic tourists each year for its white sand beaches, places of worship and World Heritage-listed architecture.
It has rich flora and fauna, owing to its location on the Western Ghats range, a biodiversity hotspot. In ancient literature, Goa was known by many names, such as Gomanchala, Gopakapattam, Govapuri and Gomantak. Other historical names for Goa are Sindapur and Mahassapatam. Prehistory Rock art engravings found in Goa exhibit the earliest traces of human life in India. Goa, situated within the Shimoga-Goa Greenstone Belt in the Western Ghats, yields evidence for Acheulean occupation. Rock art engravings are present on laterite platforms and granite boulders in Usgalimal near the west flowing Kushavati river and in Kajur. In Kajur, the rock engravings of animals and other designs in granite have been associated with what is considered to be a megalithic stone circle with a round granite stone in the centre. Petroglyphs, stone-axe, choppers dating to 10,000 years ago have been found in various locations in Goa, including Kazur and the Mandovi-Zuari basin. Evidence of Palaeolithic life is visible at Dabolim, Shigao, Arli, Diwar, Sanguem and Aquem-Margaon.
Difficulty in carbon dating the laterite rock compounds poses a problem for determining the exact time period. Early Goan society underwent radical change when Indo-Aryan and Dravidian migrants amalgamated with the aboriginal locals, forming the base of early Goan culture. Early History In the 3rd century BC, Goa was part of the Maurya Empire, ruled by the Buddhist emperor, Ashoka of Magadha. Buddhist monks laid the foundation of Buddhism in Goa. Between the 2nd century BC and the 6th century AD, Goa was ruled by the Bhojas of Goa. Chutus of Karwar ruled some parts as feudatories of the Satavahanas of Kolhapur, Western Kshatrapas, the Abhiras of Western Maharashtra, Bhojas of the Yadav clans of Gujarat, the Konkan Mauryas as feudatories of the Kalachuris; the rule passed to the Chalukyas of Badami, who controlled it between 578 and 753, the Rashtrakutas of Malkhed from 753 to 963. From 765 to 1015, the Southern Silharas of Konkan ruled Goa as the feudatories of the Chalukyas and the Rashtrakutas.
Over the next few centuries, Goa was successively ruled by the Kadambas as the feudatories of the Chalukyas of Kalyani. They patronised Jainism in Goa. In 1312, Goa came under the governance of the Delhi Sultanate; the kingdom's grip on the region was weak, by 1370 it was forced to surrender it to Harihara I of the Vijayanagara empire. The Vijayanagara monarchs held on to the territory until 1469, when it was appropriated by the Bahmani sultans of Gulbarga. After that dynasty crumbled, the area fell into the hands of the Adil Shahis of Bijapur, who established as their auxiliary capital the city known under the Portuguese as Velha Goa. Portuguese period In 1510, the Portuguese defeated the ruling Bijapur sultan Yusuf Adil Shah with the help of a local ally, Timayya, they set up a permanent settlement in Velha Goa. This was the beginning of Portuguese rule in Goa that would last for four and a half centuries, until its annexation in 1961; the Goa Inquisition, a formal tribunal, was established in 1560, was abolished in 1812.
In 1843 the Portuguese moved the capital to Panaji from Velha Goa. By the mid-18th century, Portuguese Goa had expanded to most of the present-day state limits; the Portuguese lost other possessions in India until their borders stabilised and formed the Estado da Índia Portuguesa or State of Portuguese India, of which Goa was the largest territory. Contemporary period After India gained independence from the British in 1947, India requested that Portuguese territories on the Indian subcontinent be ceded to India. Portugal refused to negotiate on the sovereignty of its Indian enclaves. On 19 December 1961, the Indian Army invaded with Operation Vijay resulting in the annexation of Goa, of Daman and Diu islands into the Indian union. Goa, along with Diu, was organised as a centrally administered union territory of India. On 30 May 1987, the union territory was split, Goa was made India's twenty-fifth state, with Daman and Diu remaining a union territory. Goa encompasses an area of 3,702 km2, it lies between the latitudes 14°53′54″ N and 15°40′00″ N and longitudes 73°40′33″ E and 74°20′13″ E. Goa is a part of the coastal country known as the Konkan, an escarpment rising up to the Western Ghats
Pakistan
Pakistan the Islamic Republic of Pakistan, is a country in South Asia. It is the world’s sixth-most populous country with a population exceeding 212,742,631 people. In area, it is the 33rd-largest country. Pakistan has a 1,046-kilometre coastline along the Arabian Sea and Gulf of Oman in the south and is bordered by India to the east, Afghanistan to the west, Iran to the southwest, China in the far northeast, it is separated narrowly from Tajikistan by Afghanistan's Wakhan Corridor in the northwest, shares a maritime border with Oman. The territory that now constitutes Pakistan was the site of several ancient cultures and intertwined with the history of the broader Indian subcontinent; the ancient history involves the Neolithic site of Mehrgarh and the Bronze Age Indus Valley Civilisation, was home to kingdoms ruled by people of different faiths and cultures, including Hindus, Indo-Greeks, Turco-Mongols and Sikhs. The area has been ruled by numerous empires and dynasties, including the Persian Achaemenid Empire, Alexander III of Macedon, the Seleucid Empire, the Indian Maurya Empire, the Gupta Empire, the Arab Umayyad Caliphate, the Delhi Sultanate, the Mongol Empire, the Mughal Empire, the Afghan Durrani Empire, the Sikh Empire and, most the British Empire.
Pakistan is the only country to have been created in the name of Islam. It is an ethnically and linguistically diverse country, with a diverse geography and wildlife. A dominion, Pakistan adopted a constitution in 1956, becoming an Islamic republic. An ethnic civil war and Indian military intervention in 1971 resulted in the secession of East Pakistan as the new country of Bangladesh. In 1973, Pakistan adopted a new constitution which stipulated that all laws are to conform to the injunctions of Islam as laid down in the Quran and Sunnah. A regional and middle power, Pakistan has the sixth-largest standing armed forces in the world and is a nuclear power as well as a declared nuclear-weapons state, the second in South Asia and the only nation in the Muslim world to have that status. Pakistan has a semi-industrialised economy with a well-integrated agriculture sector and a growing services sector, it is ranked among the emerging and growth-leading economies of the world, is backed by one of the world's largest and fastest-growing middle class.
Pakistan's political history since independence has been characterized by periods of military rule, political instability and conflicts with India. The country continues to face challenging problems, including overpopulation, poverty and corruption. Pakistan is a member of the UN, the Shanghai Cooperation Organisation, the OIC, the Commonwealth of Nations, the SAARC and the Islamic Military Counter Terrorism Coalition; the name Pakistan means "land of the pure" in Urdu and Persian. It alludes to the word pāk meaning pure in Pashto; the suffix ـستان is a Persian word meaning the place of, recalls the synonymous Sanskrit word sthāna स्थान. The name of the country was coined in 1933 as Pakstan by Choudhry Rahmat Ali, a Pakistan Movement activist, who published it in his pamphlet Now or Never, using it as an acronym referring to the names of the five northern regions of British India: Punjab, Kashmir and Baluchistan; the letter i was incorporated to ease pronunciation. Some of the earliest ancient human civilisations in South Asia originated from areas encompassing present-day Pakistan.
The earliest known inhabitants in the region were Soanian during the Lower Paleolithic, of whom stone tools have been found in the Soan Valley of Punjab. The Indus region, which covers most of present day Pakistan, was the site of several successive ancient cultures including the Neolithic Mehrgarh and the Bronze Age Indus Valley Civilisation at Harappa and Mohenjo-Daro; the Vedic period was characterised by an Indo-Aryan culture. Multan was an important Hindu pilgrimage centre; the Vedic civilisation flourished in the ancient Gandhāran city of Takṣaśilā, now Taxila in the Punjab, founded around 1000 BCE. Successive ancient empires and kingdoms ruled the region: the Persian Achaemenid Empire, Alexander the Great's empire in 326 BCE and the Maurya Empire, founded by Chandragupta Maurya and extended by Ashoka the Great, until 185 BCE; the Indo-Greek Kingdom founded by Demetrius of Bactria included Gandhara and Punjab and reached its greatest extent under Menander, prospering the Greco-Buddhist culture in the region.
Taxila had one of the earliest universities and centres of higher education in the world, established during the late Vedic period in 6th century BCE. The school consisted of several monasteries without large dormitories or lecture halls where the religious instruction was provided on an individualistic basis; the ancient university was documented by the invading forces of Alexander the Great, "the like of which had not been seen in Greece," and was recorded by Chinese pilgrims in the 4th or 5th century CE. At its zenith, the Rai Dynasty of Sindh ruled the surrounding territories; the Pala Dynasty was the last Buddhist empire, under Dharmapala and Devapala, stretched across South Asia from what is now Bangladesh through Northern India to Pakistan. The Arab conqueror Muhammad bin Qasim conquered Sindh in 711 CE; the Pakistan government's official chronol
Redox
Redox is a chemical reaction in which the oxidation states of atoms are changed. Any such reaction involves both a reduction process and a complementary oxidation process, two key concepts involved with electron transfer processes. Redox reactions include all chemical reactions; the chemical species from which the electron is stripped is said to have been oxidized, while the chemical species to which the electron is added is said to have been reduced. It can be explained in simple terms: Oxidation is the loss of electrons or an increase in oxidation state by a molecule, atom, or ion. Reduction is a decrease in oxidation state by a molecule, atom, or ion; as an example, during the combustion of wood, oxygen from the air is reduced, gaining electrons from carbon, oxidized. Although oxidation reactions are associated with the formation of oxides from oxygen molecules, oxygen is not included in such reactions, as other chemical species can serve the same function; the reaction can occur slowly, as with the formation of rust, or more in the case of fire.
There are simple redox processes, such as the oxidation of carbon to yield carbon dioxide or the reduction of carbon by hydrogen to yield methane, more complex processes such as the oxidation of glucose in the human body. "Redox" is a portmanteau of the words "reduction" and "oxidation". The word oxidation implied reaction with oxygen to form an oxide, since dioxygen was the first recognized oxidizing agent; the term was expanded to encompass oxygen-like substances that accomplished parallel chemical reactions. The meaning was generalized to include all processes involving loss of electrons; the word reduction referred to the loss in weight upon heating a metallic ore such as a metal oxide to extract the metal. In other words, ore was "reduced" to metal. Antoine Lavoisier showed. Scientists realized that the metal atom gains electrons in this process; the meaning of reduction became generalized to include all processes involving a gain of electrons. Though "reduction" seems counter-intuitive when speaking of the gain of electrons, it might help to think of reduction as the loss of oxygen, its historical meaning.
Since electrons are negatively charged, it is helpful to think of this as reduction in electrical charge. The electrochemist John Bockris has used the words electronation and deelectronation to describe reduction and oxidation processes when they occur at electrodes; these words are analogous to protonation and deprotonation, but they have not been adopted by chemists worldwide. The term "hydrogenation" could be used instead of reduction, since hydrogen is the reducing agent in a large number of reactions in organic chemistry and biochemistry. But, unlike oxidation, generalized beyond its root element, hydrogenation has maintained its specific connection to reactions that add hydrogen to another substance; the word "redox" was first used in 1928. The processes of oxidation and reduction occur and cannot happen independently of one another, similar to the acid–base reaction; the oxidation alone and the reduction alone are each called a half-reaction, because two half-reactions always occur together to form a whole reaction.
When writing half-reactions, the gained or lost electrons are included explicitly in order that the half-reaction be balanced with respect to electric charge. Though sufficient for many purposes, these general descriptions are not correct. Although oxidation and reduction properly refer to a change in oxidation state — the actual transfer of electrons may never occur; the oxidation state of an atom is the fictitious charge that an atom would have if all bonds between atoms of different elements were 100% ionic. Thus, oxidation is best defined as an increase in oxidation state, reduction as a decrease in oxidation state. In practice, the transfer of electrons will always cause a change in oxidation state, but there are many reactions that are classed as "redox" though no electron transfer occurs. In redox processes, the reductant transfers electrons to the oxidant. Thus, in the reaction, the reductant or reducing agent loses electrons and is oxidized, the oxidant or oxidizing agent gains electrons and is reduced.
The pair of an oxidizing and reducing agent that are involved in a particular reaction is called a redox pair. A redox couple is a reducing species and its corresponding oxidizing form, e.g. Fe2+/Fe3+ Substances that have the ability to oxidize other substances are said to be oxidative or oxidizing and are known as oxidizing agents, oxidants, or oxidizers; that is, the oxidant removes electrons from another substance, is thus itself reduced. And, because it "accepts" electrons, the oxidizing agent is called an electron acceptor. Oxygen is the quintessential oxidizer. Oxidants are chemical substances with elements in high oxidation states, or else electronegative elements that can gain extra electrons by oxidizing another substance. Substances that have the ability to reduce other substances are said to be reductive or reducing and are known as
Alizarin
Alizarin or 1,2-dihydroxyanthraquinone is an organic compound with formula C14H8O4, used throughout history as a prominent red dye, principally for dyeing textile fabrics. It was derived from the roots of plants of the madder genus. In 1869, it became the first natural dye to be produced synthetically. Alizarin is the main ingredient for the manufacture of the madder lake pigments known to painters as Rose madder and Alizarin crimson. Alizarin in the most common usage of the term has a deep red color, but the term is part of the name for several related non-red dyes, such as Alizarine Cyanine Green and Alizarine Brilliant Blue. A notable use of alizarin in modern times is as a staining agent in biological research because it stains free calcium and certain calcium compounds a red or light purple color. Alizarin continues to be used commercially as a red textile dye, but to a lesser extent than in the past. Madder has been cultivated as a dyestuff since antiquity in central Asia and Egypt, where it was grown as early as 1500 BC.
Cloth dyed with madder root pigment was found in the tomb of the Pharaoh Tutankhamun and in the ruins of Pompeii and ancient Corinth. In the Middle Ages, Charlemagne encouraged madder cultivation. Madder was used as a dye in Western Europe in the Late Medieval centuries. In 17th century England, alizarin was used as a red dye for the clothing of the parliamentary New Model Army; the distinctive red color would continue to be worn for centuries, giving English and British soldiers the nickname of "redcoats". The madder dyestuff is combined with a dye mordant. According to which mordant used, the resulting color may be anywhere from pink through purple to dark brown. In the 18th century the most valued color was a bright red known as "Turkey Red"; the combination of mordants and overall technique used to obtain the Turkey Red originated in the Middle East or Turkey. It was a complex and multi-step technique in its Middle Eastern formulation, some parts of which were unnecessary; the process was simplified in late 18th-century Europe.
By 1804, a dye maker George Field in Britain had refined a technique to make lake madder by treating it with alum, an alkali, that converts the water-soluble madder extract into a solid, insoluble pigment. This resulting madder lake has a longer-lasting color, can be used more efficaciously, for example by blending it into a paint. Over the following years, it was found that other metal salts, including those containing iron and chromium, could be used in place of alum to give madder-based pigments of various other colors; this general method of preparing lakes has been known for centuries but was simplified in the late 18th and early 19th centuries. In 1826, the French chemist Pierre-Jean Robiquet found that madder root contained two colorants, the red alizarin and the more fading purpurin; the alizarin component became the first natural dye to be synthetically duplicated in 1868 when the German chemists Carl Graebe and Carl Liebermann, working for BASF, found a way to produce it from anthracene.
About the same time, the English dye chemist William Henry Perkin independently discovered the same synthesis, although the BASF group filed their patent before Perkin by one day. The subsequent discovery that anthracene could be abstracted from coal tar further advanced the importance and affordability of alizarin's artificial synthesis; the synthetic alizarin could be produced for a fraction of the cost of the natural product, the market for madder collapsed overnight. The principal synthesis entailed oxidation of anthraquinone-2-sulfonic acid with sodium nitrate in concentrated sodium hydroxide. Alizarin itself has been in turn replaced today by the more light-resistant quinacridone pigments developed at DuPont in 1958. Alizarin is one of ten dihydroxyanthraquinone isomers, its molecular structure can be viewed as being derived from anthraquinone by replacement of two neighboring hydrogen atoms by hydroxyl groups. It is soluble in hexane and chloroform, can be obtained from the latter as red-purple crystals, melting point 277–278 °C.
Alizarin changes color depending on the pH of the solution it is in, thereby making it a pH indicator. Alizarin Red is used in a biochemical assay to determine, quantitatively by colorimetry, the presence of calcific deposition by cells of an osteogenic lineage; as such it is an early stage marker of matrix mineralization, a crucial step towards the formation of calcified extracellular matrix associated with true bone. Alizarin's abilities as a biological stain were first noted in 1567, when it was observed that when fed to animals, it stained their teeth and bones red; the chemical is now used in medical studies involving calcium. Free calcium forms precipitates with alizarin, tissue block containing calcium stain red when immersed in alizarin. Thus, both pure calcium and calcium in bones and other tissues can be stained; these alizarin-stained elements can be better visualized under fluorescent lights, excited by 440–460 nm. The process of staining calcium with alizarin works best. In clinical practice, it is used to stain synovial fluid to assess for basic calcium phosphate crystals.
Alizarin has been used in studies involving bone growth, bone marrow, calcium deposits in the vascular system, cellular signaling, gene expression, tissue engineering, mesenchymal stem cells. In geology, it is used as a stain to indicate the calcium carbonate minerals and aragonite. Madder lake
