Yet another example is the community of micro-organisms in soil that live off leaf litter. Leaves typically last one year and are then replaced by new ones. These micro-organisms mineralize the discarded leaves and release nutrients that are taken up by the plant. Such relationships are called reciprocal syntrophy because the plant lives off the products of micro-organisms.
Syntrophic microbial food webs can play an integral role in the breakdown of organic pollutants such as oils, aromatic compounds, and amino acids. Environmental contamination with oil is of high ecological importance, but can be mediated through syntrophic degradation.
These intermediates can then be taken up and metabolized syntrophically by methanogens to make the whole process more thermodynamically favourable. Species like Desulfovibrio employ this method. The main motive behind a syntrophic relation between two bacterial organisms is generalized as a relationship where each participant's metabolic activity cannot independently overcome the thermodynamic pressure of the reaction under standard conditions even when a cosubstrate or nutrient is added into the environment.
Therefore, the cooperation of the other participant is required to reduce the intermediate pool size. Dutch cities such as Amsterdam and Haarlem are structured as a central square surrounded by concentric canals marking every expansion.
In cities such as and also Moscow , this pattern is still clearly visible. A system of rectilinear city streets and land plots, known as the grid plan , has been used for millennia in Asia, Europe, and the Americas.
The Indus Valley Civilisation built Mohenjo-Daro , Harappa and other cities on a grid pattern, using ancient principles described by Kautilya , and aligned with the compass points. Urban-type settlement extends far beyond the traditional boundaries of the city proper  in a form of development sometimes described critically as urban sprawl. Metropolitan areas include suburbs and exurbs organized around the needs of commuters , and sometimes edge cities characterized by a degree of economic and political independence.
In the US these are grouped into metropolitan statistical areas for purposes of demography and marketing. Some cities are now part of a continuous urban landscape called urban agglomeration , conurbation , or megalopolis exemplified by the BosWash corridor of the Northeastern United States. Cities, characterized by population density , symbolic function, and urban planning , have existed for thousands of years. In the fourth and third millennium BC , complex civilizations flourished in the river valleys of Mesopotamia , India , China , and Egypt.
Some had large, dense populations , but others carried out urban activities in the realms of politics or religion without having large associated populations. Among the early Old World cities, Mohenjo-daro of the Indus Valley Civilization in present-day Pakistan , existing from about BC, was one of the largest, with a population of 50, or more and a sophisticated sanitation system. These sites appear planned in a highly regimented and stratified fashion, with a minimalistic grid of rooms for the workers and increasingly more elaborate housing available for higher classes.
In Mesopotamia, the civilization of Sumer , followed by Assyria and Babylon , gave rise to numerous cities, governed by kings and fostering multiple languages written in cuneiform. In the following centuries, independent city-states of Greece developed the polis , an association of male landowning citizens who collectively constituted the city. Under the authority of its empire , Rome transformed and founded many cities coloniae , and with them brought its principles of urban architecture, design, and society.
In the ancient Americas, early urban traditions developed in the Andes and Mesoamerica. In the Andes, the first urban centers developed in the Norte Chico civilization , Chavin and Moche cultures, followed by major cities in the Huari , Chimu and Inca cultures. The Norte Chico civilization included as many as 30 major population centers in what is now the Norte Chico region of north-central coastal Peru.
It is the oldest known civilization in the Americas, flourishing between the 30th century BC and the 18th century BC. Later cultures such as the Aztec , Andean civilization , Mayan , Mound Builders , and Pueblo peoples drew on these earlier urban traditions. Many of their ancient cities continue to be inhabited, including major metropolitan cities such as Mexico City , in the same location as Tenochtitlan ; while ancient continuously inhabited Pueblos are near modern urban areas in New Mexico , such as Acoma Pueblo near the Albuquerque metropolitan area and Taos Pueblo near Taos ; while others like Lima are located nearby ancient Peruvian sites such as Pachacamac.
In the first millennium AD, Angkor in the Khmer Empire grew into one of the most extensive cities in the world   and may have supported up to one million people.
In the remnants of the Roman Empire , cities of late antiquity gained independence but soon lost population and importance. In the Holy Roman Empire , beginning in the 12th. By , these cities, as far as still part of the empire, became part of the Imperial Estates governing the empire with the emperor through the Imperial Diet.
By the thirteenth and fourteenth centuries, some cities become powerful states, taking surrounding areas under their control or establishing extensive maritime empires. In Italy medieval communes developed into city-states including the Republic of Venice and the Republic of Genoa. Their power was later challenged and eclipsed by the Dutch commercial cities of Ghent , Ypres , and Amsterdam. In the West, nation-states became the dominant unit of political organization following the Peace of Westphalia in the seventeenth century.
However, most towns remained small. During the Spanish colonization of the Americas the old Roman city concept was extensively used. Cities were founded in the middle of the newly conquered territories, and were bound to several laws regarding administration, finances and urbanism. The growth of modern industry from the late 18th century onward led to massive urbanization and the rise of new great cities, first in Europe and then in other regions, as new opportunities brought huge numbers of migrants from rural communities into urban areas.
England led the way as London became the capital of a world empire and cities across the country grew in locations strategic for manufacturing. Industrialized cities became deadly places to live, due to health problems resulting from overcrowding , occupational hazards of industry, contaminated water and air, poor sanitation , and communicable diseases such as typhoid and cholera.
Factories and slums emerged as regular features of the urban landscape. In the second half of the twentieth century, deindustrialization or " economic restructuring " in the West led to poverty , homelessness , and urban decay in formerly prosperous cities.
America's "Steel Belt" became a " Rust Belt " and cities such as Detroit , Michigan, and Gary, Indiana began to shrink , contrary to the global trend of massive urban expansion. Amidst these economic changes, high technology and instantaneous telecommunication enable select cities to become centers of the knowledge economy.
Urbanization is the process of migration from rural into urban areas, driven by various political, economic, and cultural factors. Until the 18th century, an equilibrium existed between the rural agricultural population and towns featuring markets and small-scale manufacturing.
Urbanization rapidly spread across the Europe and the Americas and since the s has taken hold in Asia and Africa as well. The Population Division of the United Nations Department of Economic and Social Affairs , reported in that for the first time more than half of the world population lives in cities.
Latin America is the most urban continent, with four fifths of its population living in cities, including one fifth of the population said to live in shantytowns favelas , poblaciones callampas , etc. Asia is home to by far the greatest absolute number of city-dwellers: over two billion and counting.
Megacities , cities with population in the multi-millions, have proliferated into the dozens, arising especially in Asia, Africa, and Latin America. Cities around the world have expanded physically as they grow in population, with increases in their surface extent, with the creation of high-rise buildings for residential and commercial use, and with development underground. Urbanization can create rapid demand for water resources management , as formerly good sources of freshwater become overused and polluted, and the volume of sewage begins to exceed manageable levels.
Local government of cities takes different forms including prominently the municipality especially in England , in the United States , in India , and in other British colonies ; legally, the municipal corporation ;  municipio in Spain and in Portugal , and, along with municipalidad , in most former parts of the Spanish and Portuguese empires and the commune in France and in Chile ; or comune in Italy.
The chief official of the city has the title of mayor. Whatever their true degree of political authority, the mayor typically acts as the figurehead or personification of their city. This hierarchy of law is not enforced rigidly in practice—for example in conflicts between municipal regulations and national principles such as constitutional rights and property rights.
Technologies, techniques, and laws governing these areas—developed in cities—have become ubiquitous in many areas. Cities typically provide municipal services such as education , through school systems ; policing , through police departments; and firefighting , through fire departments ; as well as the city's basic infrastructure. These are provided more or less routinely, in a more or less equal fashion. The traditional basis for municipal finance is local property tax levied on real estate within the city.
Local government can also collect revenue for services, or by leasing land that it owns. This situation has become acute in deindustrialized cities and in cases where businesses and wealthier citizens have moved outside of city limits and therefore beyond the reach of taxation. Governance includes government but refers to a wider domain of social control functions implemented by many actors including nongovernmental organizations.
The biggest investors and real estate developers act as the city's de facto urban planners. The related concept of good governance places more emphasis on the state, with the purpose of assessing urban governments for their suitability for development assistance.
Urban planning , the application of forethought to city design, involves optimizing land use, transportation, utilities, and other basic systems, in order to achieve certain objectives. Urban planners and scholars have proposed overlapping theories as ideals for how plans should be formed.
Planning tools, beyond the original design of the city itself, include public capital investment in infrastructure and land-use controls such as zoning.
The continuous process of comprehensive planning involves identifying general objectives as well as collecting data to evaluate progress and inform future decisions. Government is legally the final authority on planning but in practice the process involves both public and private elements. The legal principle of eminent domain is used by government to divest citizens of their property in cases where its use is required for a project.
The history of urban planning dates to some of the earliest known cities, especially in the Indus Valley and Mesoamerican civilizations, which built their cities on grids and apparently zoned different areas for different purposes. Urban society is typically stratified. Spatially, cities are formally or informally segregated along ethnic, economic and racial lines. People living relatively close together may live, work, and play, in separate areas, and associate with different people, forming ethnic or lifestyle enclaves or, in areas of concentrated poverty, ghettoes.
While in the US and elsewhere poverty became associated with the inner city , in France it has become associated with the banlieues , areas of urban development which surround the city proper. Meanwhile, across Europe and North America, the racially white majority is empirically the most segregated group. Suburbs in the west, and, increasingly, gated communities and other forms of "privatopia" around the world, allow local elites to self-segregate into secure and exclusive neighborhoods.
Landless urban workers, contrasted with peasants and known as the proletariat , form a growing stratum of society in the age of urbanization. In Marxist doctrine, the proletariat will inevitably revolt against the bourgeoisie as their ranks swell with disenfranchised and disaffected people lacking all stake in the status quo. Historically, cities rely on rural areas for intensive farming to yield surplus crops , in exchange for which they provide money, political administration, manufactured goods, and culture.
As hubs of trade cities have long been home to retail commerce and consumption through the interface of shopping. In the 20th century, department stores using new techniques of advertising , public relations , decoration , and design , transformed urban shopping areas into fantasy worlds encouraging self-expression and escape through consumerism. In general, the density of cities expedites commerce and facilitates knowledge spillovers , helping people and firms exchange information and generate new ideas.
Population density enables also sharing of common infrastructure and production facilities, however in very dense cities, increased crowding and waiting times may lead to some negative effects. Although manufacturing fueled the growth of cities, many now rely on a tertiary or service economy.
The services in question range from tourism , hospitality , entertainment , housekeeping and prostitution to grey-collar work in law , finance , and administration. Cities are typically hubs for education and the arts , supporting universities , museums , temples , and other cultural institutions. Density makes for effective mass communication and transmission of news , through heralds , printed proclamations , newspapers , and digital media. These communication networks, though still using cities as hubs, penetrate extensively into all populated areas.
In the age of rapid communication and transportation, commentators have described urban culture as nearly ubiquitous    or as no longer meaningful.
Today, a city's promotion of its cultural activities dovetails with place branding and city marketing , public diplomacy techniques used to inform development strategy; to attract businesses, investors, residents, and tourists; and to create a shared identity and sense of place within the metropolitan area.
Elvis lovers visit Memphis to pay their respects at Graceland. Bread and circuses among other forms of cultural appeal, attract and entertain the masses.
Cities play a crucial strategic role in warfare due to their economic, demographic, symbolic, and political centrality. For the same reasons, they are targets in asymmetric warfare. Many cities throughout history were founded under military auspices, a great many have incorporated fortifications , and military principles continue to influence urban design.
Powers engaged in geopolitical conflict have established fortified settlements as part of military strategies, as in the case of garrison towns, America's Strategic Hamlet Program during the Vietnam War , and Israeli settlements in Palestine. During World War II , national governments on occasion declared certain cities open , effectively surrendering them to an advancing enemy in order to avoid damage and bloodshed.
Urban warfare proved decisive, however, in the Battle of Stalingrad , where Soviet forces repulsed German occupiers, with extreme casualties and destruction. In an era of low-intensity conflict and rapid urbanization, cities have become sites of long-term conflict waged both by foreign occupiers and by local governments against insurgency. Although capture is the more common objective, warfare has in some cases spelt complete destruction for a city.
Mesopotamian tablets and ruins attest to such destruction,  as does the Latin motto Carthago delenda est. Cities are responsible for a substantial portion of the emissions responsible for global warming. Over half of the world population is in cities, and cities have outside influence on construction and transportation-- two of the key contributors to global warming emissions.
Urban infrastructure involves various physical networks and spaces necessary for transportation, water use, energy, recreation, and public functions. Infrastructure in general if not every infrastructure project plays a vital role in a city's capacity for economic activity and expansion, underpinning the very survival of the city's inhabitants, as well as technological, commercial, industrial, and social activities.
Megaprojects such as the construction of airports , power plants , and railways require large upfront investments and thus tend to require funding from national government or the private sector. Urban infrastructure ideally serves all residents equally but in practice may prove uneven—with, in some cities, clear first-class and second-class alternatives. Public utilities literally, useful things with general availability include basic and essential infrastructure networks, chiefly concerned with the supply of water, electricity, and telecommunications capability to the populace.
Sanitation , necessary for good health in crowded conditions, requires water supply and waste management as well as individual hygiene. Urban water systems include principally a water supply network and a network for wastewater including sewage and stormwater. Historically , either local governments or private companies have administered urban water supply , with a tendency toward government water supply in the 20th century and a tendency toward private operation at the turn of the twenty-first.
Modern urban life relies heavily on the energy transmitted through electricity for the operation of electric machines from household appliances to industrial machines to now-ubiquitous electronic systems used in communications, business, and government and for traffic lights , streetlights and indoor lighting. Cities rely to a lesser extent on hydrocarbon fuels such as gasoline and natural gas for transportation, heating , and cooking.
Telecommunications infrastructure such as telephone lines and coaxial cables also traverse cities, forming dense networks for mass and point-to-point communications. Because cities rely on specialization and an economic system based on wage labour , their inhabitants must have the ability to regularly travel between home, work, commerce, and entertainment. Cities also rely on long-distance transportation truck, rail , and airplane for economic connections with other cities and rural areas.
Historically, city streets were the domain of horses and their riders and pedestrians , who only sometimes had sidewalks and special walking areas reserved for them. Mitel Mobile App. Keeping employees connected — anytime, anywhere. Business on the Move? Keep your numbers wherever you go. Learn more. Network Connectivity Connect with your customers cost effectively using the latest technologies. Data Cabling Data cabling infrastructure holds your entire communications system together.