annualsExamples of true annuals include corn, lettuce, pea, cauliflower, watermelon, bean, zinnia and marigold ^[2].    Biennials example. beets, Brussels sprouts, cabbage, carrots, celery, lettuce, parsley, and Swiss chard.  PerennialsExamples of evergreen perennials include Begonia and banana.Examples of deciduous perennials include goldenrod and mint.Examples of monocarpic perennials include Agave and some species of Streptocarpus.Examples of woody perennials include maple, pine, and apple trees.Examples of herbaceous perennials used in agriculture include alfalfa, Thinopyrum intermedium, and Red clover.

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minicomputer (colloquially, mini) is a class of multi-user computers that lies in the middle range of the computing spectrum, in between the largest multi-user systems (mainframe computers) and the smallest single-user systems (microcomputers or personal computers). The class at one time formed a distinct group with its own hardware and operating systems, but the contemporary term for this class of system is midrange computer, such as the higher-end SPARC, POWER and Itanium -based systems from Sun Microsystems, IBM and Hewlett-Packard.

//

[edit] History

[edit] 1960s: Origin; 1970s: Market entrenchment

The term “mini computer” evolved in the 1960s to describe the “small” third generation computers that became possible with the use of transistor and core memory technologies. They usually took up one or a few cabinets the size of a large refrigerator or two, compared with mainframes that would usually fill a room. The first successful minicomputer was IBM’s 16-bit IBM 1130^[1], which cost from US$32,280 upwards when launched February 11, 1965. The IBM 1130 was used by many companies until the mid 70’s and was upgraded to support timesharing via a 3rd party backplane modification. Clones followed with the Digital Scientific (Meta-4), Computer Hardware Incorporated (CHI-2130) and Data-General (GA-2130) until the late 80’s. A timesharing system, which was used with the clones, was offered by DNA Systems, Inc, Saginaw, MI and implemented the first notion of clustering.

Digital Equipment Corporation’s 12-bit PDP-8, which cost from US$16,000 upwards was launched March 22, 1965. The important precursors of the PDP-8 include the PDP-5, LINC, the TX-0, the TX-2, and the PDP-1. Digital Equipment gave rise to a number of minicomputer companies along Massachusetts Route 128, including Data General, Wang Laboratories, Apollo Computer, and Prime Computer.

The 7400 series of TTL integrated circuits started appearing in minicomputers in the late 1960s. The 74181 arithmetic logic unit (ALU) was commonly used in the CPU data paths. Each 74181 had a bus width of four bits, hence the popularity of bit-slice architecture. The 7400 series offered data-selectors, multiplexers, three-state buffers, memories, etc. in dual in-line packages with one-tenth inch spacing, making major system components and architecture evident to the naked eye. (Starting in the 1980s, many minicomputers used VLSI circuits (Very Large Scale Integration), often making the hardware organization much less apparent.)

As microcomputers developed in the 1970s and 80s, minicomputers filled the mid-range area between low powered microcomputers and high capacity mainframes. At the time microcomputers were single-user, relatively simple machines running simple program-launcher operating systems like CP/M or MS-DOS, while minis were much more powerful systems that ran full multi-user, multitasking operating systems like VMS and Unix, often with timesharing versions of BASIC for application development (MAI Basic Four systems being very popular in that regard). The classical mini was a 16-bit computer, while the emerging higher performance 32-bit minis were often referred to as superminis.

At the launch of the MITS Altair 8800 in 1975, Radio Electronics magazine referred to the system as a “minicomputer”, although it would properly be called a microcomputer; as it was the first commercially available personal computer based on the single-chip microprocessor from Intel.

[edit] Mid-1980s, 1990s: The minis give way to the micros

The decline of the minis happened due to the lower cost of microprocessor based hardware, the emergence of inexpensive and easily deployable local area network systems, the emergence of the 80286 and the 80386 microprocessors, and the desire of end-users to be less reliant on inflexible minicomputer manufacturers and IT departments/“data centers” — with the result that minicomputers and dumb terminals were replaced by networked workstations and servers and PCs in the latter half of the 1980s.

During the 1990s the change from minicomputers to inexpensive PC networks was cemented by the development of several versions of Unix to run on the Intel x86 microprocessor architecture, including Solaris, FreeBSD, NetBSD and OpenBSD. Also, the Microsoft Windows series of operating systems, beginning with Windows NT, now included server versions that supported pre-emptive multitasking and other features required for servers.

As microprocessors have become more powerful, CPUs built up from multiple components — once the distinguishing feature differentiating mainframes and midrange systems from microcomputers — have become increasingly obsolete, even in the largest mainframe computers.

Digital Equipment Corporation was the leading minicomputer manufacturer, at one time the 2nd largest computer company after IBM. But as the minicomputer declined in the face of generic UNIX servers and Intel based PCs, not only DEC, but almost every other minicomputer company including Data General, Prime, Computervision, Honeywell and Wang Laboratories, many based in New England also collapsed. DEC was sold to Compaq in 1998.

[edit] The minicomputer’s industrial impact and heritage

Several pioneering computer companies first built minicomputers, such as DEC, Data General, and Hewlett-Packard (HP) (who now refers to its HP3000 minicomputers as “servers” rather than “minicomputers”). And although today’s PCs and servers are clearly microcomputers physically, architecturally their CPUs and operating systems have evolved largely by integrating features from minicomputers.

In the software context, the relatively simple OSes for early microcomputers were usually inspired by minicomputer OSes (such as CP/M’s similarity to Digital’s RSTS) and multiuser OSs of today are often either inspired by or directly descended from minicomputer OSs (UNIX was originally a minicomputer OS, while Windows NT — the foundation for all current versions of Microsoft Windows — borrowed design ideas liberally from VMS and UNIX). Many of the first generation of PC programmers were educated on minicomputer systems.^{[citation needed]}

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Reception

Businesses’ intended purchase rates and satisfaction ratings for Windows Vista, Windows XP, and Mac OS X Leopard, based on information from ChangeWave collected in February 2008^[95]

Initially it was thought that the adoption of Vista has been generally low, due to largely poor reviews and harsh criticism, but a later Gartner research report predicted that Vista business adoption in 2008 will actually beat that of XP during the same time frame (21.3% vs. 16.9%)^[96] while IDC had indicated that the launch of Windows Server 2008 served as a catalyst for the stronger adoption rates.^[97][98] As of January 2009, Forrester Research had indicated that almost one third of North American and European corporations have started deploying Vista.^[99] Earlier, PC World rated it as the biggest tech disappointment of 2007,^[100] and it was rated by InfoWorld as #2 of Tech’s all-time 25 flops.^[101] The internet-usage market share for Windows Vista, taking the latest statistic, was 22.48% as of January 2009.^[102] This figure combined with World Internet Users and Population Stats yields a user base of roughly 350 million^[12] which exceeded Microsoft’s two-year post launch expectations by 150 million.^[10]

Within its first month, 20 million copies of Vista were sold, double the amount of Windows XP sales within its first month in October 2001, five years earlier.^[103] In China, only 244 genuine retail copies were sold within the first two weeks, leading authorities to believe that software piracy left many copies unaccounted for.^[104][105] However, PC World indicated that the visitor base of Windows Vista was increasing at a much slower rate compared to that of Windows XP. Within the first year of its release, the percentage of Windows XP users visiting PC World’s website reached 36%; in the same time frame, however, Windows Vista visitors reached only 14%, with 71% of users still running XP.^[106] In November 2006, PC World had expected the overall first-year adoption rate to be 15% (vs. 12-14% for XP).^[107] Due to Vista’s relatively low adoption rates and continued demand for Windows XP, Microsoft continued to sell Windows XP until June 30, 2008 instead of the previously planned date of January 31, 2008.^[108] There were reports of Vista users downgrading their operating systems, as well as reports of businesses planning to skip Vista.^[109][110] A study conducted by ChangeWave in March 2008 showed that the percentage of corporate users who are “very satisfied” with Vista was dramatically lower than other operating systems, with Vista at 8%, compared to the 40% who said they were “very satisfied” with Windows XP.^[9]

Although business adoption of Vista has been far higher than Apple or Linux platforms, it has been slower than expected; while businesses do tend to delay upgrading their operating systems, there have been reports that Vista is installed on considerably fewer enterprise PCs than previously projected.^[111] According to InformationWeek, in December 2006, 6% of business enterprises were expected to employ Vista within the first year, yet as of October 2007, only about 1% of enterprise PCs were actually using Vista.^[112] While a large number of businesses had bought early-adopter licenses to run Windows Vista, many of these companies delayed deployment.^[113]

There have been a number of organizations who have denounced Vista due to its problems. For example, in October 2007, The Dutch Consumers’ Association called for a boycott of Windows Vista after Microsoft refused to offer free copies of Windows XP to users who had problems with Vista.^[114]

Amid the negative reviews and reception, there have also been significant positive reviews of Vista, most notable among PC gamers and the advantages brought about with DirectX 10, which allows for better gaming performance and more realistic graphics, as well as support for many new capabilities brought about in new video cards and GPUs.^[115] However, many DirectX 9 games showed a drop in frame rate compared to that experienced in Windows XP.^{[116][117][118]} These results were largely the consequence of Vista’s immature graphics processing units drivers, and higher system requirements for Vista itself.^[119][120] Recent benchmarks suggest that, as of mid-2008, Vista SP1 is now on par with Windows XP in terms of game performance.^[121] A February 2009 survey by Valve Corporation indicated that 36.01% of gamers are running Windows Vista (26.49% 32-bit, 9.52% 64-bit).^[122]

On February 29, 2008, Microsoft announced that it will lower the price of the Vista operating system sold at retail outlets in order to aid in its adoption.^[123] These price cuts only apply to the retail versions sold in shops, which account for less than 10% of total Vista sales. Vista Ultimate, for example, will see a 20% drop in its price, from US$399 to $319.^[124]

On July 17, 2008, Microsoft announced that it had sold 180 million licenses,^[125] which would amount to between 36 and 57 billion dollars in gross retail sale price using February 29, 2008 price tags of the various versions. Initial development of the software was claimed to be 6 billion dollars. However, according to HP, Microsoft’s sales figures include business systems that ship with Vista licences but are “downgraded” and preloaded with XP.^[126][127]

On July 30, 2008, Microsoft indicated that Vista appears to be causing a shift in the PC industry from 32-bit to 64-bit. The installed base of 64-bit editions of Windows Vista, as a percentage of all Windows Vista systems, had more than tripled in the United States in the previous three months, while worldwide adoption had more than doubled during the same period. Another view showed that 20% of new Windows Vista PCs in the United States connecting to Windows Update in June were 64-bit PCs, up from 3% in March. Microsoft stated that the falling price of RAM and increased use of multitasking are benefits from SuperFetch, which accelerates performance with the installation of more RAM allowed 64-bit editions of Windows Vista.^[128] This has also been confirmed by Brad Brooks in an interview on October 13, 2008.^[129]

[edit] Competition with Windows XP

In July 2008, according to a marketing manager working for HP Australia, Windows XP was still being chosen over Windows Vista for the majority of business computer sales. As all customers of OEM versions of Vista Business and Ultimate are eligible for a free downgrade to Windows XP Professional,^[130] these Windows XP licenses are sold as Vista Business licenses, thus increasing Vista’s sales figures.^[131] Some computer manufacturers have chosen to ship Windows XP restore disks along with new computers with Vista Business and Ultimate editions pre-installed,^[132] as well as new computers with XP instead of Vista.

By February, 2009, more than two years after Vista’s release, its uptake among corporations was below 10%.^[133]

A desktop computer is a person single location, as opposed to a mobile laptop or portable computer. Prior to the wide spread of microprocessors a computer that could fit on a desk was considered remarkably small. Today the phrase usually indicates a particular style of computer case. Desktop computers come in a variety of styles ranging from large vertical tower cases to small form factor models that can be tucked behind an LCD monitor. In this sense, the term ‘desktop’ refers specifically to a horizontally-oriented case, usually intended to have the display screen placed on top to save space on the desk top. Most modern desktop computers have separate screens and keyboards. A specialized form of desktop case is used for home theater PC systems, incorporating front-panel mounted controls for audio and video.al computer (PC) in a form intended for regular use at a

[edit] All-in-One
All-in-One computers are desktop computers that combine the monitor into the same case as the CPU. Apple has manufactured several popular examples of all-in-one computers, such as the original Macintosh of the mid-1980s and the iMac of the late 1990s and 2000s. Some older 8-bit computers, such as the Commodore PET 2001 or Kaypro II, also fit into this category. All-in-One PCs are typically more portable than other desktop PCs and many have been built with carrying handles integrated into the case. They can simply be unplugged and transported to a new location.

However, like laptops, All-in-One desktop computers tend to suffer from a comparative lack of upgradeability or hardware customization, as internal hardware is often placed in the back of the visual display unit. Furthermore, in the case of the iMac line since 2002, the CPU and other internal hardware units are, more or less, permanently glued to the motherboard due to space constraints.

[edit] Comparison with laptops
 This section requires expansion.

Main article: Laptop#Disadvantages
Desktops have the advantage over laptops that the spare parts and extensions tend to be standardized, resulting in lower prices and greater availability. For example, the form factor of the motherboard is standardized, like the ATX form factor. Desktops have several standardized expansion slots, like PCI or PCI express, while laptops only tend to have one mini PCI slot and one PC card slot (or ExpressCard slot). This means that a desktop can be customized and upgraded to a greater extent than laptops. Procedures for (dis-)assembly of desktops tend to be simple and standardized to a great extent too. This tends not to be the case for laptops, though adding or replacing some parts, like the optical drive, rechargeable battery, hard disk, and adding an extra memory module is often quite simple.

Another advantage of desktop is, that power consumption is not as critical as in laptop computers because the desktop is powered from the wall socket. Desktop computers also provides more space for heat to escape. The two large microprocessor manufacturers Intel and AMD develop special CPUs for mobile computers (i.e. laptops) that consume less power and lower heat, but with lower performance levels.

[edit] Gigabit Ethernet Cards
The current state of desktop computing has left the issue of desktop scalability in limbo in that many legacy desktops are Ethernet challenged in that they can only achieve speeds from 10mb to 100mb per second. It is time that we focus and address the use of PC PCI based Gigabit Ethernet (Gig E) capable network interface cards.² Many manufacturers are currently installing them in all new model desktop computers. The inclusion of gigabit speed Ethernet adapters has increased by leaps and bounds the switch to Server extension of the way we think about LAN and Wan networking. The uses of Gig E adapters have allowed many businesses to expand their infrastructures removing the bottlenecks of standard Ethernet connectivity. The combined use of Gig switching, category 6 or fiber cabling to the desktop has given IT enthusiasts cause to smile. Previously cost was the overriding factor in the hesitation to bring gigabit speeds to the desktop. This barrier has been broken with the introduction of lower cost drivers, new protocols and hardware that is designed to fit the PCI bus of newer model computers. Manufacturers have made standard gigabit internal and expansion cards truly bringing fiber speeds from the switch to the desktop. Faster access times add up to reduced costs on bandwidth and less time in accessing server resources modern servers come with Gigabit Ethernet ports to allow fast link to switching resources. Computers using these adapters show a marked increase in access to applications, handling and resolution of graphical interfaces and increased access up time bordering on 99.5% on average. Currently Gigabit adapters provide connectivity to network systems in the 1000BASE-T standard for Gigabit Ethernet over copper, Gigabit speeds can be widely deployed at less expense using standard Category 5 cabling also in 1998 Gigabit Ethernet over fiber optic cable as IEEE 802.3z¹

Photosynthesis[α] is a metabolic pathway that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight.[1] Photosynthesis occurs in plants, algae, and many species of Bacteria, but not in Archaea. Photosynthetic organisms are called photoautotrophs, but not all organisms that use light as a source of energy carry out photosynthesis, since photoheterotrophs use organic compounds, rather than carbon dioxide, as a source of carbon.[2] In plants, algae and cyanobacteria photosynthesis uses carbon dioxide and water, releasing oxygen as a waste product. Photosynthesis is crucially important for life on Earth, since as well as it maintaining the normal level of oxygen in the atmosphere, nearly all life either depends on it directly as a source of energy, or indirectly as the ultimate source of the energy in their food.[2][β] The amount of energy trapped by photosynthesis is immense, approximately 100 terawatts per year:[3] which is about seven times larger than the yearly power consumption of human civilization.[4] In all, photosynthetic organisms convert around 100,000,000,000 tonnes of carbon into biomass per year.[5]

Although photosynthesis can occur in different ways in different species, some features are always the same. For example, the process always begins when energy from light is absorbed by proteins called photosynthetic reaction centers that contain chlorophyll. In plants these proteins are held inside organelles called chloroplasts, while in bacteria they are embedded in the plasma membrane. Some of the light energy gathered by chlorophylls is stored in the form of adenosine triphosphate (ATP). The rest of the energy is used to remove electrons from a substance such as water. These electrons are then used in the reactions that turn carbon dioxide into organic compounds. In plants, algae and cyanobacteria these reactions are called the Calvin cycle, but different sets of reactions can be found in bacteria, such as the reverse Krebs cycle in Chlorobium. Many photosynthetic organisms have adaptations that concentrate or store carbon dioxide, this helps reduce a wasteful process called photorespiration that would otherwise consume part of the sugar produced during photosynthesis.

Photosynthesis evolved early in the evolutionary history of life, when all forms of life on Earth were microorganisms. Although the dates are difficult to estimate with any accuracy, the first photosynthetic organisms probably evolved about 3,500 million years ago, and used hydrogen or hydrogen sulfide as sources of electrons, rather than water.[6] Cyanobacteria appeared later, around 3,000 million years ago, and changed the Earth forever when they began to oxygenate the atmosphere, beginning about 2,400 million years ago.[7] This new atmosphere allowed the evolution of complex life such as protists. Eventually, about 550 million years ago, one of these protists formed a symbiotic relationship with a cyanobacterium, producing the ancestor of the plants and algae.[8] The chloroplasts in modern plants are the descendants of these ancient symbiotic cyanobacteria.

Contents [hide]
1 Overview
2 Photosynthetic membranes and organelles
3 Light reactions
3.1 Z scheme
3.2 Water photolysis
3.3 Oxygen and photosynthesis
4 Light-independent reactions
4.1 The Calvin Cycle
4.2 C4 and C3 photosynthesis and CAM
5 Order and kinetics
6 Efficiency
7 Evolution
7.1 Symbiosis and the origin of chloroplasts
7.2 Cyanobacteria and the evolution of photosynthesis
8 Discovery
9 Factors
9.1 Light intensity (irradiance), wavelength and temperature
9.2 Carbon dioxide levels and photorespiration
10 See also
11 Footnotes
12 References
13 Further reading
14 External links
 

Overview
 
Photosynthesis splits water to liberate O2 and fixes CO2 into sugarPhotosynthetic organisms are photoautotrophs, which means that they are able to synthesize food directly from carbon dioxide using energy from light. In plants, algae and cyanobacteria, photosynthesis releases oxygen, this is called oxygenic photosynthesis. Although there are some differences between oxygenic photosynthesis in plants, algae and cyanobacteria, the overall process is quite similar in these organisms. However, there are some types of bacteria that carry out anoxygenic photosynthesis, which consumes carbon dioxide but does not release oxygen.

Carbon dioxide is converted into sugars in a process called carbon fixation. Carbon fixation is a redox reaction, so photosynthesis needs to supply both a source of energy to drive this process, and also the electrons needed to convert carbon dioxide into carbohydrate, which is a reduction reaction. In general outline, photosynthesis is the opposite of cellular respiration, where glucose and other compounds are oxidized to produce carbon dioxide, water, and release chemical energy. However, the two processes take place through a different sequence of chemical reactions and in different cellular compartments.

The general equation for photosynthesis is therefore:

CO2 + 2 H2A + photons → (CH2O)n + H2O + 2A
carbon dioxide + electron donor + light energy → carbohydrate + oxygen + oxidized electron donor
Since water is used as the electron donor in oxygenic photosynthesis, the equation for this process is:

CO2 + 2 H2O + photons → (CH2O)n + H2O + O2
carbon dioxide + water + light energy → carbohydrate + oxygen + water
Photosynthesis occurs in two stages. In the first stage, light-dependent reactions or light reactions capture the energy of light and use it to make the energy-storage molecules ATP and NADPH. During the second stage, the light-independent reactions use these products to capture and reduce carbon dioxide.

Photosynthetic membranes and organelles
 
Chloroplast ultrastructure:
1. outer membrane
2. intermembrane space
3. inner membrane (1+2+3: envelope)
4. stroma (aqueous fluid)
5. thylakoid lumen (inside of thylakoid)
6. thylakoid membrane
7. granum (stack of thylakoids)
8. thylakoid (lamella)
9. starch
10. ribosome
11. plastidial DNA
12. plastoglobule (drop of lipids)Main articles: Chloroplast and Thylakoid
The proteins that gather light for photosynthesis are embedded within cell membranes. The simplest way these are arranged is in photosynthetic bacteria, where these proteins are held within the plasma membrane.[9] However, this membrane may be tightly-folded into cylindrical sheets called thylakoids,[10] or bunched up into round vesicles called intracytoplasmic membranes.[11] These structures can fill most of the interior of a cell, giving the membrane a very large surface area and therefore increasing the amount of light that the bacteria can absorb.[10]

In plants and algae, photosynthesis takes place in organelles called chloroplasts. A chloroplast is contained by an envelope that consists of an inner and an outer phospholipid membrane. Between these two layers is the intermembrane space. A typical plant cell contains about 10 to 100 chloroplasts. Within the stroma are stacks of thylakoids, the sub-organelles which are the site of photosynthesis. The thylakoids are arranged in stacks called grana (singular: granum). A thylakoid has a flattened disk shape. Inside it is an empty area called the thylakoid space or lumen. The thylakoid membrane contains many integral and peripheral membrane proteins. The proteins complexes which contain special pigments absorbing light energy are called photosystems.

Plants absorb light primarily using the pigment chlorophyll, which is the reason that most plants have a green color. Besides chlorophyll plants also use pigments such as carotenes and xanthophylls.[12] Algae also use chlorophyll, but various other pigments are present as phycocyanin, carotenes, and xanthophylls in green algae, phycoerythrin in red algae (rhodophytes) and fucoxanthol in brown algae and diatoms resulting in a wide variety of colors.

These pigments are embedded in plants and algae in special antenna-proteins. In such proteins all the pigments are ordered to work well together. Such a protein is also called a light-harvesting complex.

Although all cells in the green parts of a plant have chloroplasts, most of the energy is captured in the leaves. The cells in the interior tissues of a leaf, called the mesophyll, can contain between 450,000 and 800,000 chloroplasts for every square millimeter of leaf. The surface of the leaf is uniformly coated with a water-resistant waxy cuticle that protects the leaf from excessive evaporation of water and decreases the absorption of ultraviolet or blue light to reduce heating. The transparent epidermis layer allows light to pass through to the palisade mesophyll cells where most of the photosynthesis takes place.

Light reactions
 
Light-dependent reactions of photosynthesis at the thylakoid membraneMain article: Light-dependent reaction
In the light reactions, one molecule of the pigment chlorophyll absorbs one photon and loses one electron. This electron is passed to a modified form of chlorophyll called pheophytin, which passes the electron to a quinone molecule, allowing the start of a flow of electrons down an electron transport chain that leads to the ultimate reduction of NADP to NADPH. In addition, this creates a proton gradient across the chloroplast membrane; its dissipation is used by ATP Synthase for the concomitant synthesis of ATP. The chlorophyll molecule regains the lost electron from a water molecule through a process called photolysis, which releases a dioxygen (O2) molecule. The overall equation for the light-dependent reactions under the conditions of non-cyclic electron flow in green plants is:[13]

2 H2O + 2 NADP+ + 2 ADP + 2 Pi + light → 2 NADPH + 2 H+ + 2 ATP + O2
Not all wavelengths of light can support photosynthesis. The photosynthetic action spectrum depends on the type of accessory pigments present. For example, in green plants, the action spectrum resembles the absorption spectrum for chlorophylls and carotenoids with peaks for violet-blue and red light. In red algae, the action spectrum overlaps with the absorption spectrum of phycobilins for blue-green light, which allows these algae to grow in deeper waters that filter out the longer wavelengths used by green plants. The non-absorbed part of the light spectrum is what gives photosynthetic organisms their color (e.g., green plants, red algae, purple bacteria) and is the least effective for photosynthesis in the respective organisms.

Z scheme
 
A Photosystem: A light-harvesting cluster of photosynthetic pigments present in the thylakoid membrane of chloroplasts.
The “Z scheme”In plants, light-dependent reactions occur in the thylakoid membranes of the chloroplasts and use light energy to synthesize ATP and NADPH. The light-dependent reaction has two forms; cyclic and non-cyclic reaction. In the non-cyclic reaction, the photons are captured in the light-harvesting antenna complexes of photosystem II by chlorophyll and other accessory pigments (see diagram at right). When a chlorophyll molecule at the core of the photosystem II reaction center obtains sufficient excitation energy from the adjacent antenna pigments, an electron is transferred to the primary electron-acceptor molecule, Pheophytin, through a process called Photoinduced charge separation. These electrons are shuttled through an electron transport chain, the so called Z-scheme shown in the diagram, that initially functions to generate a chemiosmotic potential across the membrane. An ATP synthase enzyme uses the chemiosmotic potential to make ATP during photophosphorylation, whereas NADPH is a product of the terminal redox reaction in the Z-scheme. The electron enters the Photosystem I molecule. The electron is excited due to the light absorbed by the photosystem. A second electron carrier accepts the electron, which again is passed down lowering energies of electron acceptors. The energy created by the electron acceptors is used to move hydrogen ions across the thylakoid membrane into the lumen. The electron is used to reduce the co-enzyme NADP, which has functions in the light-independent reaction. The cyclic reaction is similar to that of the non-cyclic, but differs in the form that it generates only ATP, and no reduced NADP (NADPH) is created. The cyclic reaction takes place only at photosystem I. Once the electron is displaced from the photosystem, the electron is passed down the electron acceptor molecules and returns back to photosystem I, from where it was emitted, hence the name cyclic reaction.

Water photolysis
Main articles: Photodissociation and Oxygen evolution
The NADPH is the main reducing agent in chloroplasts, providing a source of energetic electrons to other reactions. Its production leaves chlorophyll with a deficit of electrons (oxidized), which must be obtained from some other reducing agent. The excited electrons lost from chlorophyll in photosystem I are replaced from the electron transport chain by plastocyanin. However, since photosystem II includes the first steps of the Z-scheme, an external source of electrons is required to reduce its oxidized chlorophyll a molecules. The source of electrons in green-plant and cyanobacterial photosynthesis is water. Two water molecules are oxidized by four successive charge-separation reactions by photosystem II to yield a molecule of diatomic oxygen and four hydrogen ions; the electron yielded in each step is transferred to a redox-active tyrosine residue that then reduces the photoxidized paired-chlorophyll a species called P680 that serves as the primary (light-driven) electron donor in the photosystem II reaction center. The oxidation of water is catalyzed in photosystem II by a redox-active structure that contains four manganese ions and a calcium ion; this oxygen-evolving complex binds two water molecules and stores the four oxidizing equivalents that are required to drive the water-oxidizing reaction. Photosystem II is the only known biological enzyme that carries out this oxidation of water. The hydrogen ions contribute to the transmembrane chemiosmotic potential that leads to ATP synthesis. Oxygen is a waste product of light-dependent reactions, but the majority of organisms on Earth use oxygen for cellular respiration, including photosynthetic organisms.[14][15]

Oxygen and photosynthesis
With respect to oxygen and photosynthesis, there are two important concepts.

Plant and cyanobacterial (blue-green algae) cells also use oxygen for cellular respiration, although they have a net output of oxygen since much more is produced during photosynthesis.
Oxygen is a product of the light-driven water-oxidation reaction catalyzed by photosystem II; it is not generated by the fixation of carbon dioxide. Consequently, the source of oxygen during photosynthesis is water, not carbon dioxide.
The concept that oxygen production is not directly associated with the fixation of carbon dioxide was first proposed by Cornelis Van Niel in the 1930s, who studied photosynthetic bacteria. Aside from the cyanobacteria, bacteria only have one photosystem and use reducing agents other than water. They get electrons from a variety of different inorganic chemicals including sulfide or hydrogen, so for most of these bacteria oxygen is not produced.

Others, such as the halophiles (an Archaea), produced so-called purple membranes where the bacteriorhodopsin could harvest light and produce energy. The purple membranes was one of the first to be used to demonstrate the chemiosmotic theory: light hit the membranes and the pH of the solution that contained the purple membranes dropped as protons were pumping out of the membrane.

Light-independent reactions

The Calvin Cycle
Main articles: Carbon fixation and Light-independent reaction
In the Light-independent or dark reactions the enzyme RuBisCO captures CO2 from the atmosphere and in a process that requires the newly formed NADPH, called the Calvin-Benson Cycle, releases three-carbon sugars, which are later combined to form sucrose and starch. The overall equation for the light-independent reactions in green plants is:[13]

3 CO2 + 9 ATP + 6 NADPH + 6 H+ → C3H6O3-phosphate + 9 ADP + 8 Pi + 6 NADP+ + 3 H2O
 
Overview of the Calvin cycle and carbon fixationTo be more specific, carbon fixation produces an intermediate product, which is then converted to the final carbohydrate products. The carbon skeletons produced by photosynthesis are then variously used to form other organic compounds, such as the building material cellulose, as precursors for lipid and amino acid biosynthesis, or as a fuel in cellular respiration. The latter occurs not only in plants but also in animals when the energy from plants gets passed through a food chain.

The fixation or reduction of carbon dioxide is a process in which carbon dioxide combines with a five-carbon sugar, ribulose 1,5-bisphosphate (RuBP), to yield two molecules of a three-carbon compound, glycerate 3-phosphate (GP), also known as 3-phosphoglycerate (PGA). GP, in the presence of ATP and NADPH from the light-dependent stages, is reduced to glyceraldehyde 3-phosphate (G3P). This product is also referred to as 3-phosphoglyceraldehyde (PGAL) or even as triose phosphate. Triose is a 3-carbon sugar (see carbohydrates). Most (5 out of 6 molecules) of the G3P produced is used to regenerate RuBP so the process can continue (see Calvin-Benson cycle). The 1 out of 6 molecules of the triose phosphates not “recycled” often condense to form hexose phosphates, which ultimately yield sucrose, starch and cellulose. The sugars produced during carbon metabolism yield carbon skeletons that can be used for other metabolic reactions like the production of amino acids and lipids.

C4 and C3 photosynthesis and CAM
 
Overview of C4 carbon fixationIn hot and dry conditions, plants will close their stomata to prevent loss of water. Under these conditions, CO2 will decrease, and dioxygen gas, produced by the light reactions of photosynthesis, will increase in the leaves, causing an increase of photorespiration by the oxygenase activity of ribulose-1,5-bisphosphate carboxylase/oxygenase and decrease in carbon fixation. Some plants have evolved mechanisms to increase the CO2 concentration in the leaves under these conditions.

Main article: C4 carbon fixation
C4 plants chemically fix carbon dioxide in the cells of the mesophyll by adding it to the three-carbon molecule phosphoenolpyruvate (PEP), a reaction catalyzed by an enzyme called PEP carboxylase and which creates the four-carbon organic acid, oxaloacetic acid. Oxaloacetic acid or malate synthesized by this process is then translocated to specialized bundle sheath cells where the enzyme, rubisco, and other Calvin cyle enzymes are located, and where CO2 released by decarboxylation of the four-carbon acids is then fixed by rubisco activity to the three-carbon sugar 3 phosphoglyceric acids. The physical separation of rubisco from the oxygen-generating light reactions reduces photorespiration and increases CO2 fixation and thus photosynthetic capacity of the leaf. [16] C4 plants can produce more sugar than C3 plants in conditions of high light and temperature. Many important crop plants are C4 plants including maize, sorghum, sugarcane, and millet. Plants lacking PEP-carboxylase are called C3 plants because the primary carboxylation reaction, catalyzed by Rubiso, produces the three-carbon sugar 3 phosphoglyceric acids directly in the Calvin-Benson Cycle.

Main article: CAM photosynthesis
Xerophytes such as cacti and most succulents also use PEP carboxylase to capture carbon dioxide in a process called Crassulacean acid metabolism (CAM). In contrast to C4 metabolism, which physically separates the CO2 fixation to PEP from the Calvin cycle, CAM only temporally separates these two processes. CAM plants have a different leaf anatomy than C4 plants, and fix the CO2 at night, when their stomata are open. CAM plants store the CO2 mostly in the form of malic acid via carboxylation of phosphoenolpyruvate to oxaloacetate, which is then reduced to malate. Decarboxylation of malate during the day releases CO2 inside the leaves thus allowing carbon fixation to 3-phosphoglycerate by rubisco.

Order and kinetics
The overall process of photosynthesis takes place in four stages. The first, energy transfer in antenna chlorophyll takes place in the femtosecond [1 femtosecond (fs) = 10,−15 s] to picosecond [1 picosecond (ps) = 10−12 s] time scale. The next phase, the transfer of electrons in photochemical reactions, takes place in the picosecond to nanosecond time scale [1 nanosecond (ns) = 10−9 s]. The third phase, the electron transport chain and ATP synthesis, takes place on the microsecond [1 microsecond (μs) = 10−6 s] to millisecond [1 millisecond (ms) = 10−3 s) time scale. The final phase is carbon fixation and export of stable products and takes place in the millisecond to second time scale. The first three stages occur in the thylakoid membranes.

Efficiency
Plants convert light into chemical energy with a maximum photosynthetic efficiency of approximately 6%.[17] By comparison solar panels convert light into electric energy at a photosynthetic efficiency of approximately 10-20%. Actual plant’s photosynthetic efficiency varies with the frequency of the light being converted, light intensity, temperature and proportion of CO2 in atmosphere.

Evolution
 
Plant cells with visible chloroplasts.Early photosynthetic systems, such as those from green and purple sulfur and green and purple non-sulfur bacteria, are thought to have been anoxygenic, using various molecules as electron donors. Green and purple sulfur bacteria are thought to have used hydrogen and sulfur as an electron donor. Green nonsulfur bacteria used various amino and other organic acids. Purple nonsulfur bacteria used a variety of non-specific organic molecules. The use of these molecules is consistent with the geological evidence that the atmosphere was highly reduced at that time.[citation needed]

Fossils of what are thought to be filamentous photosynthetic organisms have been dated at 3.4 billion years old.[18]

The main source of oxygen in the atmosphere is oxygenic photosynthesis, and its first appearance is sometimes referred to as the oxygen catastrophe. Geological evidence suggests that oxygenic photosynthesis, such as that in cyanobacteria, became important during the Paleoproterozoic era around 2 billion years ago. Modern photosynthesis in plants and most photosynthetic prokaryotes is oxygenic. Oxygenic photosynthesis uses water as an electron donor which is oxidized to molecular dioxygen (O2) in the photosynthetic reaction center.

Symbiosis and the origin of chloroplasts
Several groups of animals have formed symbiotic relationships with photosynthetic algae. These are most common in corals, sponges and sea anemones, possibly due to these animals having particularly simple body plans and large surface areas compared to their volumes.[19] In addition, a few marine molluscs Elysia viridis and Elysia chlorotica also maintain a symbiotic relationship with chloroplasts that they capture from the algae in their diet and then store in their bodies. This allows the molluscs to survive solely by photosynthesis for several months at a time.[20][21] Some of the genes from the plant cell nucleus have even been transferred to the slugs, so that the chloroplasts can be supplied with proteins that they need to survive.[22]

An even closer form of symbiosis may explain the origin of chloroplasts. Chloroplasts have many similarities with photosynthetic bacteria including a circular chromosome, prokaryotic-type ribosomes, and similar proteins in the photosynthetic reaction center.[23][24] The endosymbiotic theory suggests that photosynthetic bacteria were acquired (by endocytosis) by early eukaryotic cells to form the first plant cells. Therefore, chloroplasts may be photosynthetic bacteria that adapted to life inside plant cells. Like mitochondria, chloroplasts still possess their own DNA, separate from the nuclear DNA of their plant host cells and the genes in this chloroplast DNA resemble those in cyanobacteria.[25] DNA in chloroplasts codes for redox proteins such as photosynthetic reaction centers. The CoRR Hypothesis proposes that this Co-location is required for Redox Regulation.

Cyanobacteria and the evolution of photosynthesis
The biochemical capacity to use water as the source for electrons in photosynthesis evolved once, in a common ancestor of extant cyanobacteria. The geological record indicates that this transforming event took place early in Earth’s history, at least 2450-2320 million years ago (Ma), and possibly much earlier.[citation needed] Available evidence from geobiological studies of Archean (>2500 Ma) sedimentary rocks indicates that life existed 3500 Ma, but the question of when oxygenic photosynthesis evolved is still unanswered. A clear paleontological window on cyanobacterial evolution opened about 2000 Ma, revealing an already-diverse biota of blue-greens. Cyanobacteria remained principal primary producers throughout the Proterozoic Eon (2500-543 Ma), in part because the redox structure of the oceans favored photoautotrophs capable of nitrogen fixation.[citation needed] Green algae joined blue-greens as major primary producers on continental shelves near the end of the Proterozoic, but only with the Mesozoic (251-65 Ma) radiations of dinoflagellates, coccolithophorids, and diatoms did primary production in marine shelf waters take modern form. Cyanobacteria remain critical to marine ecosystems as primary producers in oceanic gyres, as agents of biological nitrogen fixation, and, in modified form, as the plastids of marine algae.[26]

Discovery
Although some of the steps in photosynthesis are still not completely understood, the overall photosynthetic equation has been known since the 1800s.

Jan van Helmont began the research of the process in the mid-1600s when he carefully measured the mass of the soil used by a plant and the mass of the plant as it grew. After noticing that the soil mass changed very little, he hypothesized that the mass of the growing plant must come from the water, the only substance he added to the potted plant. His hypothesis was partially accurate—much of the gained mass also comes from carbon dioxide as well as water. However, this was a signaling point to the idea that the bulk of a plant’s biomass comes from the inputs of photosynthesis, not the soil itself.

Joseph Priestley, a chemist and minister, discovered that when he isolated a volume of air under an inverted jar, and burned a candle in it, the candle would burn out very quickly, much before it ran out of wax. He further discovered that a mouse could similarly “injure” air. He then showed that the air that had been “injured” by the candle and the mouse could be restored by a plant.

In 1778, Jan Ingenhousz, court physician to the Austrian Empress, repeated Priestley’s experiments. He discovered that it was the influence of sunlight on the plant that could cause it to rescue a mouse in a matter of hours.

In 1796, Jean Senebier, a Swiss pastor, botanist, and naturalist, demonstrated that green plants consume carbon dioxide and release oxygen under the influence of light. Soon afterwards, Nicolas-Théodore de Saussure showed that the increase in mass of the plant as it grows could not be due only to uptake of CO2, but also to the incorporation of water. Thus the basic reaction by which photosynthesis is used to produce food (such as glucose) was outlined.

Cornelis Van Niel made key discoveries explaining the chemistry of photosynthesis. By studying purple sulfur bacteria and green bacteria he was the first scientist to demonstrate that photosynthesis is a light-dependent redox reaction, in which hydrogen reduces carbon dioxide.

Robert Emerson discovered two light reactions by testing plant productivity using different wavelengths of light. With the red alone, the light reactions were suppressed. When blue and red were combined, the output was much more substantial. Thus, there were two photosystems, one aborbing up to 600 nm wavelengths, the other up to 700. The former is known as PSII, the latter is PSI. PSI contains only chlorophyll a, PSII contains primarily chlorophyll a with most of the available chlorophyll b, among other pigments.[27]

Further experiments to prove that the oxygen developed during the photosynthesis of green plants came from water, were performed by Robert Hill in 1937 and 1939. He showed that isolated chloroplasts give off oxygen in the presence of unnatural reducing agents like iron oxalate, ferricyanide or benzoquinone after exposure to light. The Hill reaction is as follows:

2 H2O + 2 A + (light, chloroplasts) → 2 AH2 + O2
where A is the electron acceptor. Therefore, in light the electron acceptor is reduced and oxygen is evolved. Cyt b6, now known as a plastoquinone, is one electron acceptor.

Samuel Ruben and Martin Kamen used radioactive isotopes to determine that the oxygen liberated in photosynthesis came from the water.

Melvin Calvin and Andrew Benson, along with James Bassham, elucidated the path of carbon assimilation (the photosynthetic carbon reduction cycle) in plants. The carbon reduction cycle is known as the Calvin cycle, which inappropriately ignores the contribution of Bassham and Benson. Many scientists refer to the cycle as the Calvin-Benson Cycle, Benson-Calvin, and some even call it the Calvin-Benson-Bassham (or CBB) Cycle.

A Nobel Prize winning scientist, Rudolph A. Marcus, was able to discover the function and significance of the electron transport chain.

Factors
There are three main factors affecting photosynthesis and several corollary factors. The three main are:

Light irradiance and wavelength
Carbon dioxide concentration
Temperature.

Light intensity (irradiance), wavelength and temperature
In the early 1900s Frederick Frost Blackman along with Gabrielle Matthaei investigated the effects of light intensity (irradiance) and temperature on the rate of carbon assimilation.

At constant temperature, the rate of carbon assimilation varies with irradiance, initially increasing as the irradiance increases. However at higher irradiance this relationship no longer holds and the rate of carbon assimilation reaches a plateau.
At constant irradiance, the rate of carbon assimilation increases as the temperature is increased over a limited range. This effect is only seen at high irradiance levels. At low irradiance, increasing the temperature has little influence on the rate of carbon assimilation.
These two experiments illustrate vital points: firstly, from research it is known that photochemical reactions are not generally affected by temperature. However, these experiments clearly show that temperature affects the rate of carbon assimilation, so there must be two sets of reactions in the full process of carbon assimilation. These are of course the light-dependent ‘photochemical’ stage and the light-independent, temperature-dependent stage. Second, Blackman’s experiments illustrate the concept of limiting factors. Another limiting factor is the wavelength of light. Cyanobacteria, which reside several meters underwater, cannot receive the correct wavelengths required to cause photoinduced charge separation in conventional photosynthetic pigments. To combat this problem, a series of proteins with different pigments surround the reaction center.This unit is called a phycobilisome.

Carbon dioxide levels and photorespiration
As carbon dioxide concentrations rise, the rate at which sugars are made by the light-independent reactions increases until limited by other factors. RuBisCO, the enzyme that captures carbon dioxide in the light-independent reactions, has a binding affinity for both carbon dioxide and oxygen. When the concentration of carbon dioxide is high, RuBisCO will fix carbon dioxide. However, if the oxygen concentration is high, RuBisCO will bind oxygen instead of carbon dioxide. This process, called photorespiration, uses energy, but does not make sugar.

RuBisCO oxygenase activity is disadvantageous to plants for several reasons:

One product of oxygenase activity is phosphoglycolate (2 carbon) instead of 3-phosphoglycerate (3 carbon). Phosphoglycolate cannot be metabolized by the Calvin-Benson cycle and represents carbon lost from the cycle. A high oxygenase activity, therefore, drains the sugars that are required to recycle ribulose 5-bisphosphate and for the continuation of the Calvin-Benson cycle.
Phosphoglycolate is quickly metabolized to glycolate that is toxic to a plant at a high concentration; it inhibits photosynthesis.
Salvaging glycolate is an energetically expensive process that uses the glycolate pathway and only 75% of the carbon is returned to the Calvin-Benson cycle as 3-phosphoglycerate.
A highly-simplified summary is:
2 glycolate + ATP → 3-phosphoglycerate + carbon dioxide + ADP +NH3
The salvaging pathway for the products of RuBisCO oxygenase activity is more commonly known as photorespiration, since it is characterized by light-dependent oxygen consumption and the release of carbon dioxide

Motherboard

The ASUS A8N VM CSM
Connects to	Microprocessors via one of: sockets Slots (on older motherboards) Main memory via one of: Slots Sockets for individual chips (on old motherboards) Peripherals via one of: External ports Internal cables Expansion cards via one of: PCI bus AGP bus PCI Express bus ISA bus (on older motherboards) Others
Form factors	ATX microATX AT (on older motherboards) Baby AT (on older motherboards) Others
Common manufacturers	ASUS Foxconn Intel Others

A motherboard is the central printed circuit board (PCB) in some complex electronic systems, such as modern personal computers. The motherboard is sometimes alternatively known as the mainboard, system board, or, on Apple computers, the logic board.^[1] It is also sometimes casually shortened to mobo.^[2]

//

[ History

Prior to the advent of the microprocessor, a computer was usually built in a card-cage case or mainframe with components connected by a backplane consisting of a set of slots themselves connected with wires; in very old designs the wires were discrete connections between card connector pins, but printed-circuit boards soon became the standard practice. The central processing unit, memory and peripherals were housed on individual printed circuit boards which plugged into the backplane.

During the late 1980s and 1990s, it became economical to move an increasing number of peripheral functions onto the motherboard (see above). In the late 1980s, motherboards began to include single ICs (called Super I/O chips) capable of supporting a set of low-speed peripherals: keyboard, mouse, floppy disk drive, serial ports, and parallel ports. As of the late 1990s, many personal computer motherboards support a full range of audio, video, storage, and networking functions without the need for any expansion cards at all; higher-end systems for 3D gaming and computer graphics typically retain only the graphics card as a separate component.

The early pioneers of motherboard manufacturing were Micronics, Mylex, AMI, DTK, Hauppauge, Orchid Technology, Elitegroup, DFI, and a number of Taiwan-based manufacturers.

Popular personal computers such as the Apple II and IBM PC had published schematic diagrams and other documentation which permitted rapid reverse-engineering and third-party replacement motherboards. Usually intended for building new computers compatible with the exemplars, many motherboards offered additional performance or other features and were used to upgrade the manufacturer’s original equipment.

The term mainboard is archaically applied to devices with a single board and no additional expansions or capability. In modern terms this would include embedded systems, and controlling boards in televisions, washing machines etc. A motherboard specifically refers to a printed circuit with the capability to add/extend its performance/capabailities with the addition of “daughterboards”.

[] Overview

An Acer E360 motherboard made by Foxconn, from 2005, with a large number of integrated peripherals. This board’s nForce3 chipset lacks a traditional northbridge.

Most computer motherboards produced today are designed for IBM-compatible computers, which currently account for around 90% of global PC sales^{[citation needed]}. A motherboard, like a backplane, provides the electrical connections by which the other components of the system communicate, but unlike a backplane, it also hosts the central processing unit, and other subsystems and devices.

Motherboards are also used in many other electronics devices such as mobile phones,stop-watches,clocks,and other small electronc devices.

A typical desktop computer has its microprocessor, main memory, and other essential components on the motherboard. Other components such as external storage, controllers for video display and sound, and peripheral devices may be attached to the motherboard as plug-in cards or via cables, although in modern computers it is increasingly common to integrate some of these peripherals into the motherboard itself.

An important component of a motherboard is the microprocessor’s supporting chipset, which provides the supporting interfaces between the CPU and the various buses and external components. This chipset determines, to an extent, the features and capabilities of the motherboard.

Modern motherboards include, at a minimum:

• sockets (or slots) in which one or more microprocessors are installed^[3]

• slots into which the system’s main memory is installed (typically in the form of DIMM modules containing DRAM chips)

• a chipset which forms an interface between the CPU’s front-side bus, main memory, and peripheral buses

• non-volatile memory chips (usually Flash ROM in modern motherboards) containing the system’s firmware or BIOS

• a clock generator which produces the system clock signal to synchronize the various components

• slots for expansion cards (these interface to the system via the buses supported by the chipset)

• power connectors flickers, which receive electrical power from the computer power supply and distribute it to the CPU, chipset, main memory, and expansion cards.^[4]

The Octek Jaguar V motherboard from 1993.^[5] This board has 6 ISA slots but few onboard peripherals, as evidenced by the lack of external connectors.

Additionally, nearly all motherboards include logic and connectors to support commonly-used input devices, such as PS/2 connectors for a mouse and keyboard. Early personal computers such as the Apple II or IBM PC included only this minimal peripheral support on the motherboard. Occasionally video interface hardware was also integrated into the motherboard; for example on the Apple II, and rarely on IBM-compatible computers such as the IBM PC Jr. Additional peripherals such as disk controllers and serial ports were provided as expansion cards.

Given the high thermal design power of high-speed computer CPUs and components, modern motherboards nearly always include heatsinks and mounting points for fans to dissipate excess heat.

[edit] CPU sockets

Main article: CPU socket

[edit] Integrated peripherals

Block diagram of a modern motherboard, which supports many on-board peripheral functions as well as several expansion slots.

With the steadily declining costs and size of integrated circuits, it is now possible to include support for many peripherals on the motherboard. By combining many functions on one PCB, the physical size and total cost of the system may be reduced; highly-integrated motherboards are thus especially popular in small form factor and budget computers.

For example, the ECS RS485M-M,^[6] a typical modern budget motherboard for computers based on AMD processors, has on-board support for a very large range of peripherals:

• disk controllers for a floppy disk drive, up to 2 PATA drives, and up to 6 SATA drives (including RAID 0/1 support)

• integrated ATI Radeon graphics controller supporting 2D and 3D graphics, with VGA and TV output

• integrated sound card supporting 8-channel (7.1) audio and S/PDIF output

• fast Ethernet network controller for 10/100 Mbit networking

• USB 2.0 controller supporting up to 12 USB ports

• IrDA controller for infrared data communication (e.g. with an IrDA enabled Cellular Phone or Printer)

• temperature, voltage, and fan-speed sensors that allow software to monitor the health of computer components

Expansion cards to support all of these functions would have cost hundreds of dollars even a decade ago, however as of April 2007^[update] such highly-integrated motherboards are available for as little as $30 in the USA.

[edit] Peripheral card slots

A typical motherboard of 2009 will have a different number of connections depending on its standard. A standard ATX motherboard will typically have 1x PCI-E 16x connection for a graphics card, 2x PCI slots for various expansion cards and 1x PCI-E 1x which will eventually supersede PCI.

A standard Super ATX motherboard will have 1x PCI-E 16x connection for a graphics card. It will also have a varying number of PCI and PCI-E 1x slots. It can sometimes also have a PCI-E 4x slot. This varies between brands and models.

Some motherboards have 2x PCI-E 16x slots, to allow more than 2 monitors without special hardware or to allow use of a special graphics technology called SLI (for Nvidia) and Crossfire (for ATI). These allow 2 graphics cards to be linked together, to allow better performance in intensive graphical computing tasks, such as gaming and video-editing.

As of 2007^[update], virtually all motherboards come with at least 4x USB ports on the rear, with at least 2 connections on the board internally for wiring additional front ports that are built into the computer’s case. Ethernet is also included now. This is a standard networking cable for connecting the computer to a network or a modem. A sound chip is always included on the motherboard, to allow sound to be output without the need for any extra components. This allows computers to be far more multimedia-based than before. Cheaper machines now often have their graphics chip built into the motherboard rather than a separate card.

[edit] Temperature and reliability

Motherboards are generally air cooled with heat sinks often mounted on larger chips, such as the northbridge, in modern motherboards. If the motherboard is not cooled properly, then this can cause its computer to crash. Passive cooling, or a single fan mounted on the power supply, was sufficient for many desktop computer CPUs until the late 1990s; since then, most have required CPU fans mounted on their heatsinks, due to rising clock speeds and power consumption. Most motherboards have connectors for additional case fans as well. Newer motherboards have integrated temperature sensors to detect motherboard and CPU temperatures, and controllable fan connectors which the BIOS or operating system can use to regulate fan speed. Some higher-powered computers (which typically have high-performance processors and large amounts of RAM, as well as high-performance video cards) use a water-cooling system instead of many fans.

Some small form factor computers and home theater PCs designed for quiet and energy-efficient operation boast fan-less designs. This typically requires the use of a low-power CPU, as well as careful layout of the motherboard and other components to allow for heat sink placement.

A 2003 study^[7] found that some spurious computer crashes and general reliability issues, ranging from screen image distortions to I/O read/write errors, can be attributed not to software or peripheral hardware but to aging capacitors on PC motherboards. Ultimately this was shown to be the result of a faulty electrolyte formulation.^[8]

For more information on premature capacitor failure on PC motherboards, see capacitor plague.

Motherboards use electrolytic capacitors to filter the DC power distributed around the board. These capacitors age at a temperature-dependent rate, as their water based electrolytes slowly evaporate. This can lead to loss of capacitance and subsequent motherboard malfunctions due to voltage instabilities. While most capacitors are rated for 2000 hours of operation at 105 °C,^[9] their expected design life roughly doubles for every 10 °C below this. At 45 °C a lifetime of 15 years can be expected. This appears reasonable for a computer motherboard, however many manufacturers have delivered substandard capacitors,^{[citation needed]} which significantly reduce life expectancy. Inadequate case cooling and elevated temperatures easily exacerbate this problem. It is possible, but tedious and time-consuming, to find and replace failed capacitors on PC motherboards; it is less expensive to buy a new motherboard than to pay for such a repair.^{[citation needed]}

[edit] Form factor

Main article: Comparison of computer form factors

microATX form factor motherboard

Motherboards are produced in a variety of sizes and shapes (”form factors“), some of which are specific to individual computer manufacturers. However, the motherboards used in IBM-compatible commodity computers have been standardized to fit various case sizes. As of 2007^[update], most desktop computer motherboards use one of these standard form factors—even those found in Macintosh and Sun computers which have not traditionally been built from commodity components.

Laptop computers generally use highly integrated, miniaturized, and customized motherboards. This is one of the reasons that laptop computers are difficult to upgrade and expensive to repair. Often the failure of one laptop component requires the replacement of the entire motherboard, which is usually more expensive than a desktop motherboard due to the large number of integrated components.

[edit] Nvidia SLI and ATI Crossfire

Nvidia SLI and ATI Crossfire technology allows 2 or more of the same series graphics cards to be linked together to allow a faster graphics experience. Almost all medium to high end Nvidia cards and most high end ATI cards support the technology.

They both require compatible motherboards. There is an obvious need for 2x PCI-E 8x slots to allow 2 cards to be inserted into the computer. The same function can be achieved in 650i motherboards by NVIDIA, with a pair of x8 slots. Originally, tri-Crossfire was achieved at 8x speeds with 2 16x slots and 1 8x slot albeit at a slower speed. ATI opened the technology up to Intel in 2006 and such all new Intel chipsets support Crossfire.

SLI is a little more proprietary in its needs. It requires a motherboard with Nvidia’s own NForce chipset series to allow it to run (exception: Intel X58 chipset).

It is important to note that SLI and Crossfire will not usually scale to 2x the performance of a single card when using a dual setup. They also do not double the effective amount of VRAM or memory bandwidth.

[edit] Bootstrapping using the BIOS

Main article: booting

Motherboards contain some non-volatile memory to initialize the system and load an operating system from some external peripheral device. Microcomputers such as the Apple II and IBM PC used read-only memory chips, mounted in sockets on the motherboard. At power-up, the central processor would load its program counter with the address of the boot ROM, and start executing ROM instructions, displaying system information on the screen and running memory checks, which would in turn start loading memory from an external or peripheral device (disk drive). If none is available, then the computer can perform tasks from other memory stores or display an error message, depending on the model and design of the computer and version of the BIOS.

Most modern motherboard designs use a BIOS, stored in an EEPROM chip soldered to the motherboard, to bootstrap the motherboard. (Socketed BIOS chips are widely used, also.) By booting the motherboard, the memory, circuitry, and peripherals are tested and configured. This process is known as a computer Power-On Self Test (POST) and may include testing some of the following devices:

• floppy drive

• network controller

• CD-ROM drive

• DVD-ROM drive

• SCSI hard drive

• IDE, EIDE, or SATA hard drive

• External USB memory storage device

Any of the above devices can be stored with machine code instructions to load an operating system or a program

Random-access memory (usually known by its acronym, RAM) is a form of computer data storage. Today it takes the form of integrated circuits that allows the stored data to be accessed in any order (i.e., at random). The word random thus refers to the fact that any piece of data can be returned in a constant time, regardless of its physical location and whether or not it is related to the previous piece of data.[1]

This contrasts with storage mechanisms such as tapes, magnetic discs and optical discs, which rely on the physical movement of the recording medium or a reading head. In these devices, the movement takes longer than the data transfer, and the retrieval time varies depending on the physical location of the next item.

The word RAM is mostly associated with volatile types of memory (such as DRAM memory modules), where the information is lost after the power is switched off. However, many other types of memory are RAM as well (i.e., Random Access Memory), including most types of ROM and a kind of flash memory called NOR-Flash.

An early type of widespread writable random access memory was the magnetic core memory, developed from 1949 to 1952, and subsequently used in most computers up until the development of the static and dynamic integrated RAM circuits in the late 1960s and early 1970s. Before this, computers used relays, delay line memory or various kinds of vacuum tube arrangements to implement “main” memory functions (i.e., hundreds or thousands of bits), some of which were random access, some not. Latches built out of vacuum tube triodes, and later, out of discrete transistors, were used for smaller and faster memories such as registers and (random access) register banks. Prior to the development of integrated ROM circuits, permanent (or read-only) random access memory was often constructed using semiconductor diode matrices driven by address decoders.

Manchester United Football Club is an English football club, based at Old Trafford in Trafford, Greater Manchester, and is one of the most popular football clubs in the world,^[3] with over 330 million supporters worldwide^[4][5] – almost 5% of the world’s population.^[6] The club was a founding member of the Premier League in 1992, and has played in the top division of English football since 1938, with the exception of the 1974–75 season. Average attendances at the club have been higher than any other team in English football for all but six seasons since 1964–65.^[7]

Manchester United are the reigning English, European, and World Champions having won the 2007–08 Premier League, the 2007–08 UEFA Champions League, and the 2008 FIFA Club World Cup. The club is the second most successful in the history of English football and by far the most successful of recent times, having won 20 major honours since the start of Alex Ferguson’s reign as manager in November 1986. In 1968, they became the first English club to win the European Cup, beating Benfica 4–1. They won a second European Cup as part of an unprecedented Treble in 1999, before winning their third in 2008, 40 years almost to the day after their first. The club also holds the record for the most FA Cup titles with 11.^[8]

Since the late 1990s, the club has been one of the richest in the world with the highest revenue of any football club,^[9] and is currently ranked as the richest and most valuable club in any sport, with an estimated value of £897 million (€1.333 billion / $1.8 billion) as of September 2008.^[10] Manchester United was a founding member of the now defunct G-14 group of Europe’s leading football clubs,^[11] and its replacement, the European Club Association.^[12]

Alex Ferguson has been manager of the club since 6 November 1986, joining from Aberdeen after the sacking of Ron Atkinson.^[13] The current club captain is Gary Neville, who succeeded Roy Keane in November 2005

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