document_title: List of Special A episodes

question_text: when does kei confess to hikari in the anime

text: List of Special A episodes

text: The following is the list of episodes for the Japanese anime series Special A. The episodes are produced by AIC and GONZO, directed by Yoshikazu Miyao and composed by Jukki Hanada. Characters are adapted from the manga drawn by Maki Minami. It is currently airing in Japan on Chiba TV and various other stations. Broadcast on Chiba TV began on 6 April 2008 and continued for 24 episodes, ending on 14 September 2008.

text: The series has been given four different theme songs, two opening themes and two ending themes.  All the theme songs were performed by the seven principal cast members: Yūko Gotō, Jun Fukuyama, Hitomi Nabatame, Hiro Shimono, Tsubasa Yonaga, Kazuma Horie, and Ayahi Takagaki.

text: The anime is licensed by Mighty Media Co., Ltd in China and Taiwan and Sentai Filmworks in America and Canada.  Special A premiered in the Philippines on February 26, 2009 and ended on March 31, 2009 through TV5. The show premiered 5 months after it ended in Japan making it the fastest acquired anime title on Philippine free TV history.[1]  It is the first country in Southeast Asia to air the series outside Japan. This anime was aired in South Korea via Animax, marking it as the first country in East Asia and second in Asia to air the series outside Japan.

text: Episodes[edit]

text:
#
Title
Original air date

01
"Hikari ~ Kei""Hikari·Kei" (光·彗) 6 April 2008
At six years old, Hikari Hanazono was unbeaten at pro-wrestling until she was easily defeated by Kei Takishima.  Since that day, she has vowed to defeat Kei, who proves to be remarkably talented at whatever he attempts, and has convinced her father to allow her to attend the same expensive elite schools as her rival' Kei Takishima!
02
"Pride ~ Pro-Wrestling""Puraido·Puroresu" (プライド·プロレス) 13 April 2008
The S.A is challenged to a pro-wrestling competition by the Student Council President, Hajime Kakei.  Hikari and Kei accept the challenge, but run into trouble when the President decides to use underhanded tactics to beat them and Kei begins to worry about Hikari's safety without considering her feelings.
03
"Rice Balls ~ Sincerity""Onigiri･Magokoro" (おにぎり･まごころ) 20 April 2008
Hikari, having lost a bet to Kei in the previous episode, agrees to make a bento for him.  However, despite how hard she tries, all Hikari's attempts to cook something wonderful for him to eat prove to be disastrous.
04
"Brothers ~ Teacher""Kyōdai･Sensei" (兄弟･先生) 27 April 2008
Kei invites Hikari to his house because his father wants to talk about pro-wrestling with someone.  She unexpectedly becomes a tutor to Kei's younger brother, Sui, who has a rather strained relationship with Kei, and tries to bring the brothers closer together.
05
"Festival ~ Contest""Matsuri･Shōbu" (祭り･勝負) 4 May 2008
The Student Council and S.A challenge each other to see who will organize the upcoming school festival, but Hikari and Kei end up arguing; Hikari joins the Student Council's side and Akira soon follows.  While Kei takes them all on single-handedly, Hikari learns that he has always done everything alone.
06
"Invitation ~ Saiga's Mansion""Shōtaijō·Saiga Yakushi" (招待状･雑賀邸) 11 May 2008
Because of some intervention from Sui, Hikari ends up being Kei's partner at a birthday party for Yahiro Saiga.  While wondering how Yahiro fits into Kei's past, Hikari is shocked to hear from Yahiro that Kei is in love with her.
07
"Sensitive ~ Thickheaded""Binkan·Donkan" (敏感·鈍感) 18 May 2008
Unable to believe that Kei is in love with her, Hikari behaves strangely around Kei after Yahiro's party and refuses to tell him why.  To determine where the S.A will go for their vacation, Kei suggests a contest where the winner decides - with the condition that if he wins, Hikari will tell him what Yahiro said to her at the party.
08
"Journey ~ Dog""Tabi·Inu" (旅·犬) 25 May 2008
The S.A head off to Hawaii for their summer vacation, but end up playing "family" to Chitose, the spoiled and demanding son of a client for Ryuu's family.  He is particularly harsh with Hikari, but she is determined to make sure he has a good time, even if it means being the dog in Chitose's family.
09
"Villa ~ Letter""Bessō·Tegami" (別荘·手紙) 1 June 2008
Hikari joins Yahiro and Chitose at their villa in order to find out why Yahiro's relationship with Kei and Akira is so strained, but ends up becoming his prisoner so that Yahiro can force Akira to see him again. At that moment Kei was worried of Hikari. At the end of the episode, Hikari kisses Kei on the cheek.
10
"Anchor ~ Ryuu""Shingari·Ryū" (殿·竜) 8 June 2008
The greenhouse is vandalized and it seems that Hikari has a hunch on who may have committed the act. When she comes face to face with the suspect, he challenges the S.A - however, it must be Ryuu who completes the challenge.  Old memories and feelings resurface for Ryuu while Hikari learns more about her friends.
11
"Girlfriend ~ Boyfriend""Kanojo·Kareshi" (彼女·彼氏) 15 June 2008
When Tadashi lies to his mother about having a girlfriend to avoid attending a matchmaking session, he asks Hikari to pretend to be his girlfriend in hopes of fooling his mother, the director of Hakusenkan.
12
"High Fever ~ Passion""Kōnetsu·Jōnetsu" (高熱·情熱) 22 June 2008
After hearing about Hikari's date with Tadashi, Kei arrives from Shanghai with the intention of having her spend the day with him, as stipulated in a challenge he proposed to her earlier.  However, he winds up getting a fever and Hikari takes care of him; as she does, Kei reflects on his feelings and relationship with her.
13
"Magic ~ Friends""Mahō·Tomodachi" (魔法·友達) 29 June 2008
The S.A must spend three days in separate classes at an ordinary high school as punishment for Tadashi's attempt to deceive his mother.  However, the experience becomes difficult for Akira when she encounters a girl who resembles a childhood friend whose friendship with Akira ended badly.
14
"Protecting ~ Apologizing""Mamoritai·Gomen ne" (守りたい·ごめんね) 6 July 2008
Akira's determination to protect her new friend, Yui, from Yahiro ends up pushing Yui away instead.  Meanwhile, Yahiro's attempt to separate Akira from Yui comes with well-meaning intentions that Akira may never realize.
15
"Moral Code ~ Excellent""Jingi·Jōtō" (仁義·上等) 13 July 2008
Hikari must go on a date with Kei on Sunday, as promised when he was recovering from a fever.  However, their "date" is interrupted by Sakura Ushikubo, Kei's future fiancée, who takes Hikari away and befriends her.  When she agrees to help Sakura find out more about Kei, Hikari begins to feel some unexplainable feelings regarding Kei.
16
"Love ~ Kiss""Suki·Kisu" (好き·キス) 20 July 2008
The S.A is invited to a barbecue by Sakura, but they quickly discover that she planned the event so that she can spend some time with Jun, whom she has fallen in love with.  While Sakura does everything she can to get a kiss from him, a less than enthusiastic Jun is forced to reveal a secret he has kept from most of his friends.
17
"Restraint ~ Smile""Enryo·Egao" (遠慮·笑顔) 27 July 2008
The S.A. is invited to attend and plan a festival at Kokusen Academy by Sakura and Yahiro. Yahiro tries to tell Akira he cares about her, but decides against it when he realizes that she has feelings for Tadashi.
18
"Toudou ~ Karino""Tōdō·Karino" (東堂·狩野) 3 August 2008
Upset that Tadashi does not seem to care about her, Akira goes to a matchmaking party with Sakura.  When Tadashi apologizes to her, she becomes angry at herself for causing trouble. Yahiro helps Tadashi find a way to comfort her, and Tadashi decides to bring Akira to a viewpoint that they had been to when they were children.
19
"Singing Voice ~ Bad Guy""Utakoe·Warumono" (歌声·悪者) 10 August 2008
Megumi asks Yahiro out, claiming to have a crush on him, but he thinks there's more to it and will only believe her if she can win a "game" where she must make him say he liked their date. After he helps her with her voice, Megumi realizes she feels more than just pity for him. Meanwhile, Hikari catches a cold and the rest of the SA go to see her.  When Hikari's parents leave her friends to take care of her, they find the feverish Hikari doing outrageous things.
20
"Switch ~ At the Edge""Suitchi·Gakueppuchi" (スイッチ·崖っぷち) 17 August 2008
Hikari, Kei, and Jun attend the opening of a new athletic park and run into Sakura.  Jun avoids Sakura, who chases after him in hopes of discovering his true feelings for her, while Hikari and Kei must deal with the scores of girls that Jun's alter ego keeps attracting.
21
"To Match Up ~ To Come True""Kanau·Kanau" (敵う·叶う) 24 August 2008
Hikari meets Aoi Ogata, a member of the Takishima Group who finds Kei important to him and is willing to do anything for Kei.  When Aoi decides Kei is better off studying in London, Hikari must prove she's worthy of being Kei's rival.  However, this meeting with Aoi causes Hikari to think about her true feelings for Kei.
22
"Love ~ Strange""Koi·Hen" (恋·変) 31 August 2008
Hikari struggles to figure out her feelings for Kei and finds it increasingly awkward to be around him.  After spending the day together, she realizes she loves him - but stops short of a confession.  Meanwhile, Aoi has approached the Director of Hakusenkan and leaves her with no choice but to lock up the S.A's greenhouse for reasons unknown.
23
"SA ~ FA""SA·FA"  7 September 2008
Thanks to Aoi, the S.A class has been disbanded in order to force Kei to go to London.  So that Kei and his friends can remain together, Kei proposes a challenge to Hikari that if she wins, he will bring back the S.A.  However, Hikari learns too late what Kei has done in order to bring back the S.A - he has already left for London for her sake.
24
"Hanazono Hikari ~ Takishima Kei""Hanazono Hikari·Takishima Kei" (華園 光·滝島 彗) 14 September 2008
Hikari and S.A head to London to find Kei and bring him back to Japan while Kei resigns himself to not seeing Hikari and his friends again.  Everyone, including Yahiro and Sakura, overcome the obstacles standing in the way of reaching their friend.  When Hikari meets with Kei at last, they finish the challenge he issued her and Hikari lost. They express their true feelings for one another and eventually share a kiss in the air in front of the big clock. When they get back to Japan, Kei and the rest of the S.A members successfully return to school along with getting their greenhouse back.


document_title: Sand

question_text: what type of rock is made of particles .05 cm

root

text: Sand

text: Composition

text: The exact definition of sand varies. The scientific Unified Soil Classification System used in engineering and geology corresponds to US Standard Sieves,[3] and defines sand as particles with a diameter of between 0.074 and 4.75 millimeters. By another definition, in terms of particle size as used by geologists, sand particles range in diameter from 0.0625 mm (or 1⁄16 mm) to 2 mm. An individual particle in this range size is termed a sand grain. Sand grains are between gravel (with particles ranging from 2 mm up to 64 mm by the latter system, and from 4.75 mm up to 75 mm in the former) and silt (particles smaller than 0.0625 mm down to 0.004 mm).  The size specification between sand and gravel has remained constant for more than a century, but particle diameters as small as 0.02 mm were considered sand under the Albert Atterberg standard in use during the early 20th century. The grains of sand in Archimedes Sand Reckoner written around 240 BCE, were 0.02 mm in diameter. A 1953 engineering standard published by the American Association of State Highway and Transportation Officials set the minimum sand size at 0.074 mm. A 1938 specification of the United States Department of Agriculture was 0.05 mm.[4]  Sand feels gritty when rubbed between the fingers. Silt, by comparison, feels like flour).

tag: p

text: ISO 14688 grades sands as fine, medium, and coarse with ranges 0.063 mm to 0.2 mm to 0.63 mm to 2.0 mm.  In the United States, sand is commonly divided into five sub-categories based on size: very fine sand (1⁄16 – 1⁄8 mm diameter), fine sand (1⁄8 mm – 1⁄4 mm), medium sand (1⁄4 mm – 1⁄2 mm), coarse sand (1⁄2 mm – 1 mm), and very coarse sand (1 mm – 2 mm). These sizes are based on the Krumbein phi scale, where size in Φ = -log2D; D being the particle size in mm. On this scale, for sand the value of Φ varies from −1 to +4, with the divisions between sub-categories at whole numbers.

text: The most common constituent of sand, in inland continental settings and non-tropical coastal settings, is silica (silicon dioxide, or SiO2), usually in the form of quartz, which, because of its chemical inertness and considerable hardness, is the most common mineral resistant to weathering.

text: The composition of mineral sand is highly variable, depending on the local rock sources and conditions. The bright white sands found in tropical and subtropical coastal settings are eroded limestone and may contain coral and shell fragments in addition to other organic or organically derived fragmental material, suggesting sand formation depends on living organisms, too.[5] The gypsum sand dunes of the White Sands National Monument in New Mexico are famous for their bright, white color. Arkose is a sand or sandstone with considerable feldspar content, derived from  weathering and erosion of a (usually nearby) granitic rock outcrop. Some sands contain magnetite, chlorite, glauconite or gypsum. Sands rich in magnetite are dark to black in color, as are sands derived from volcanic basalts and obsidian. Chlorite-glauconite bearing sands are typically green in color, as are sands derived from basaltic lava with a high olivine content. Many sands, especially those found extensively in Southern Europe, have iron impurities within the quartz crystals of the sand, giving a deep yellow color. Sand deposits in some areas contain garnets and other resistant minerals, including some small gemstones.

text: Study

text: The study of individual grains can reveal much historical information as to the origin and kind of transport of the grain.[6] Quartz sand that is recently weathered from granite or gneiss quartz crystals will be angular. It is called grus in geology or sharp sand in the building trade where it is preferred for concrete, and in gardening where it is used as a soil amendment to loosen clay soils. Sand that is transported long distances by water or wind will be rounded, with characteristic abrasion patterns on the grain surface. Desert sand is typically rounded.

text: People who collect sand as a hobby are known as arenophiles.  Organisms that thrive in sandy environments are psammophiles.[7]

text: Uses

text: This section needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (January 2018) (Learn how and when to remove this template message)

tag: table

text:
Agriculture: Sandy soils are ideal for crops such as watermelons, peaches and peanuts, and their excellent drainage characteristics make them suitable for intensive dairy farming.
Aquaria: Sand makes a low cost aquarium base material which some believe is better than gravel for home use. It is also a necessity for saltwater reef tanks, which emulate environments composed largely of aragonite sand broken down from coral and shellfish.
Artificial reefs: Geotextile bagged sand can serve as the foundation for new reefs.
Artificial islands in the Persian Gulf.
Beach nourishment: Governments move sand to beaches where tides, storms or deliberate changes to the shoreline erode the original sand.[8]
Brick: Manufacturing plants add sand to a mixture of clay and other materials for manufacturing bricks.[9]
Cob: Coarse sand makes up as much as 75% of cob.
Concrete: Sand is often a principal component of this critical construction material.
Glass: Sand rich in silica is the principal component in common glasses.
Hydraulic fracturing: A drilling technique for natural gas, which uses rounded silica sand as a "proppant", a material to hold open cracks that are caused by the hydraulic fracturing process.
Landscaping: Sand makes small hills and slopes (golf courses would be an example).
Mortar: Sand is mixed with masonry cement or Portland cement and lime to be used in masonry construction.
Paint: Mixing sand with paint produces a textured finish for walls and ceilings or non-slip floor surfaces.
Railroads: Engine drivers and rail transit operators use sand to improve the traction of wheels on the rails.
Recreation. Playing with sand is a favorite beach time activity. One of the most beloved uses of sand is to make sometimes intricate, sometimes simple structures known as sand castles. Such structures are well known for their impermanence. Sand is also used in children's play. Special play areas enclosing a significant area of sand, known as sandboxes, are common on many public playgrounds, and even at some single family homes. Sand dunes are also popular among climbers, motorcyclists and beach buggy drivers.
Roads: Sand improves traction (and thus traffic safety) in icy or snowy conditions.
Sand animation: Performance artists draw images in sand. Makers of animated films use the same term to describe their use of sand on frontlit or backlit glass.
Sand casting: Casters moisten or oil molding sand, also known as foundry sand and then shape it into molds into which they pour molten material. This type of sand must be able to withstand high temperatures and pressure, allow gases to escape, have a uniform, small grain size and be non-reactive with metals.
Sand castles: Shaping sand into castles or other miniature buildings is a popular beach activity.
Sandbags: These protect against floods and gunfire. The inexpensive bags are easy to transport when empty, and unskilled volunteers can quickly fill them with local sand in emergencies.
Sandblasting: Graded sand serves as an abrasive in cleaning, preparing, and polishing.
Thermal weapon: While not in widespread use anymore, sand used to be heated and poured on invading troops in the classical and medieval time periods.
Water filtration: Media filters use sand for filtering water.
Wuḍūʾ: the Islamic procedure for washing parts of the body.
Zoanthid "skeletons": Animals in this order of marine benthic cnidarians related to corals and sea anemones, incorporate sand into their mesoglea for structural strength, which they need because they lack a true skeleton.

tag: ul

text: Resources and environmental concerns

text: Only some sands are suitable for the construction industry, for example for making concrete. Because of the growth of population and of cities and the consequent construction activity there is a huge demand for these special kinds of sand, and natural sources are running low. In 2012 French director Denis Delestrac made a documentary called "Sand Wars" about the impact of the lack of construction sand. It shows the ecological and economic effects of both legal and illegal trade in construction sand.[10][11][12]

text: Sand's many uses require a significant dredging industry, raising environmental concerns over fish depletion, landslides, and flooding.[13] Countries such as China, Indonesia, Malaysia and Cambodia ban sand exports, citing these issues as a major factor.[14]  It is estimated that the annual consumption of sand and gravel is 40 billion tons and sand is a US$70 billion global industry.[15]

text: Hazards

text: While sand is generally non-toxic, sand-using activities such as sandblasting require precautions. Bags of silica sand used for sandblasting now carry labels warning the user to wear respiratory protection to avoid breathing the resulting fine silica dust. Safety data sheets for silica sand state that "excessive inhalation of crystalline silica is a serious health concern".[16]

text: In areas of high pore water pressure, sand and salt water can form quicksand, which is a colloid hydrogel that behaves like a liquid. Quicksand produces a considerable barrier to escape for creatures caught within, who often die from exposure (not from submersion) as a result.


document_title: Photosynthesis

question_text: what is the site of photosynthesis in plant

question_tokens: what is the site of photosynthesis in plant

text: Photosynthesis

text: Overview

text: Photosynthetic organisms are photoautotrophs, which means that they are able to synthesize food directly from carbon dioxide and water using energy from light. However, not all organisms that use light as a source of energy carry out photosynthesis; photoheterotrophs use organic compounds, rather than carbon dioxide, as a source of carbon.[4] In plants, algae, and cyanobacteria, photosynthesis releases oxygen. This is called oxygenic photosynthesis and is by far the most common type of photosynthesis used by living organisms. Although there are some differences between oxygenic photosynthesis in plants, algae, and cyanobacteria, the overall process is quite similar in these organisms. There are also many varieties of anoxygenic photosynthesis, used mostly by certain types of bacteria, which consume carbon dioxide but do not release oxygen.

text: Carbon dioxide is converted into sugars in a process called carbon fixation. Photosynthesis provides the energy in the form of free electrons that are used to split carbon from carbon dioxide that is then used to fix that carbon once again as carbohydrate. Carbon fixation is an endothermic redox reaction, so photosynthesis supplies the energy that drives both process. In general outline, photosynthesis is the opposite of cellular respiration, in which glucose and other compounds are oxidized to produce carbon dioxide and water, and to release chemical energy (an exothermic reaction) to drive the organism's metabolism. The two processes, reduction of carbon dioxide to carbohydrate and then later oxidation of the carbohydrate, are distinct: photosynthesis and cellular respiration take place through a different sequence of chemical reactions and in different cellular compartments.

text: The general equation for photosynthesis as first proposed by Cornelius van Niel is therefore:[14]

text: Since water is used as the electron donor in oxygenic photosynthesis, the equation for this process is:

text: This equation emphasizes that water is both a reactant in the light-dependent reaction and a product of the light-independent reaction, but canceling n water molecules from each side gives the net equation:

text: Other processes substitute other compounds (such as arsenite) for water in the electron-supply role; for example some microbes use sunlight to oxidize arsenite to arsenate:[15] The equation for this reaction is:

text: 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.

text: Most organisms that utilize oxygenic photosynthesis use visible light for the light-dependent reactions, although at least three use shortwave infrared or, more specifically, far-red radiation.[17]

text: Some organisms employ even more radical variants of photosynthesis. Some archea use a simpler method that employs a pigment similar to those used for vision in animals. The bacteriorhodopsin changes its configuration in response to sunlight, acting as a proton pump. This produces a proton gradient more directly, which is then converted to chemical energy. The process does not involve carbon dioxide fixation and does not release oxygen, and seems to have evolved separately from the more common types of photosynthesis.[18][19]

text: Photosynthetic membranes and organelles

text: In photosynthetic bacteria, the proteins that gather light for photosynthesis are embedded in cell membranes. In its simplest form, this involves the membrane surrounding the cell itself.[20] However, the membrane may be tightly folded into cylindrical sheets called thylakoids,[21] or bunched up into round vesicles called intracytoplasmic membranes.[22] 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.[21]

text: In plants and algae, photosynthesis takes place in organelles called chloroplasts. A typical plant cell contains about 10 to 100 chloroplasts. The chloroplast is enclosed by a membrane. This membrane is composed of a phospholipid inner membrane, a phospholipid outer membrane, and an intermembrane space. Enclosed by the membrane is an aqueous fluid called the stroma. Embedded within the stroma are stacks of thylakoids (grana), which are the site of photosynthesis. The thylakoids appear as flattened disks. The thylakoid itself is enclosed by the thylakoid membrane, and within the enclosed volume is a lumen or thylakoid space. Embedded in the thylakoid membrane are integral and peripheral membrane protein complexes of the photosynthetic system.

text: Plants absorb light primarily using the pigment chlorophyll. The green part of the light spectrum is not absorbed but is reflected which is the reason that most plants have a green color. Besides chlorophyll, plants also use pigments such as carotenes and xanthophylls.[23] Algae also use chlorophyll, but various other pigments are present, such as phycocyanin, carotenes, and xanthophylls in green algae, phycoerythrin in red algae (rhodophytes) and fucoxanthin in brown algae and diatoms resulting in a wide variety of colors.

text: These pigments are embedded in plants and algae in complexes called antenna proteins. In such proteins, the pigments are arranged to work together. Such a combination of proteins is also called a light-harvesting complex.

text: Although all cells in the green parts of a plant have chloroplasts, the majority of those are found in specially adapted structures called leaves. Certain species adapted to conditions of strong sunlight and aridity, such as many Euphorbia and cactus species, have their main photosynthetic organs in their stems. 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 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.

text: Light-dependent reactions

text: In the light-dependent 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, starting the 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 (energy gradient) across the chloroplast membrane, which is used by ATP synthase in the synthesis of ATP. The chlorophyll molecule ultimately regains the electron it lost when a water molecule is split in a process called photolysis, which releases a dioxygen (O2) molecule as a waste product.

text: The overall equation for the light-dependent reactions under the conditions of non-cyclic electron flow in green plants is:[24]

text: 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 absorption peaks in violet-blue and red light. In red algae, the action spectrum is blue-green light, which allows these algae to use the blue end of the spectrum to grow in the deeper waters that filter out the longer wavelengths (red light) used by above ground 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.

text: Z scheme

text: In plants, light-dependent reactions occur in the thylakoid membranes of the chloroplasts where they drive the synthesis of ATP and NADPH. The light-dependent reactions are of two forms: cyclic and non-cyclic.

text: 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). The absorption of a photon by the antenna complex frees an electron by a process called photoinduced charge separation. The antenna system is at the core of the chlorophyll molecule of the photosystem II reaction center. That freed electron is transferred to the primary electron-acceptor molecule, pheophytin. As the electrons are shuttled through an electron transport chain (the so-called Z-scheme shown in the diagram), it initially functions to generate a chemiosmotic potential by pumping proton cations (H+) across the membrane and into the thylakoid space. An ATP synthase enzyme uses that chemiosmotic potential to make ATP during photophosphorylation, whereas NADPH is a product of the terminal redox reaction in the Z-scheme. The electron enters a chlorophyll molecule in Photosystem I. There it is further excited by the light absorbed by that photosystem. The electron is then passed along a chain of electron acceptors to which it transfers some of its energy. The energy delivered to the electron acceptors is used to move hydrogen ions across the thylakoid membrane into the lumen. The electron is eventually used to reduce the co-enzyme NADP with a H+ to NADPH (which has functions in the light-independent reaction); at that point, the path of that electron ends.

text: The cyclic reaction is similar to that of the non-cyclic, but differs in 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 to photosystem I, from where it was emitted, hence the name cyclic reaction.

text: Water photolysis

text: The NADPH is the main reducing agent produced by chloroplasts, which then goes on to provide a source of energetic electrons in other cellular reactions. Its production leaves chlorophyll in photosystem I with a deficit of electrons (chlorophyll has been oxidized), which must be balanced by some other reducing agent that will supply the missing electron. The excited electrons lost from chlorophyll from photosystem I are supplied from the electron transport chain by plastocyanin. However, since photosystem II is the first step 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 electrons yielded are transferred to a redox-active tyrosine residue that then reduces the oxidized chlorophyll a (called P680) that serves as the primary light-driven electron donor in the photosystem II reaction center. That photo receptor is in effect reset and is then able to repeat the absorption of another photon and the release of another photo-dissociated electron. 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 contains the four oxidizing equivalents that are used to drive the water-oxidizing reaction. Photosystem II is the only known biological enzyme that carries out this oxidation of water. The hydrogen ions released 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.[25][26]

text: Light-independent reactions

text: Calvin cycle

text: In the light-independent (or "dark") reactions, the enzyme RuBisCO captures CO2 from the atmosphere and, in a process called the Calvin-Benson cycle, it uses the newly formed NADPH and 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[24]:128

text: Carbon fixation produces the intermediate three-carbon sugar product, which is then converted to the final carbohydrate products. The simple carbon sugars produced by photosynthesis are then used in the forming of other organic compounds, such as the building material cellulose, the 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 is passed through a food chain.

text: The fixation or reduction of carbon dioxide is a process in which carbon dioxide combines with a five-carbon sugar, ribulose 1,5-bisphosphate, to yield two molecules of a three-carbon compound, glycerate 3-phosphate, also known as 3-phosphoglycerate. Glycerate 3-phosphate, in the presence of ATP and NADPH produced during the light-dependent stages, is reduced to glyceraldehyde 3-phosphate. This product is also referred to as 3-phosphoglyceraldehyde (PGAL) or, more generically, as triose phosphate. Most (5 out of 6 molecules) of the glyceraldehyde 3-phosphate produced is used to regenerate ribulose 1,5-bisphosphate so the process can continue. The triose phosphates not thus "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.

text: Carbon concentrating mechanisms

text: On land

text: In hot and dry conditions, plants close their stomata to prevent water loss. Under these conditions, CO2 will decrease and oxygen gas, produced by the light reactions of photosynthesis, will increase, 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.[27]

text: Plants that use the C4 carbon fixation process 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, creating 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 cycle enzymes are located, and where CO2 released by decarboxylation of the four-carbon acids is then fixed by RuBisCO activity to the three-carbon 3-phosphoglyceric acids. The physical separation of RuBisCO from the oxygen-generating light reactions reduces photorespiration and increases CO2 fixation and, thus, the photosynthetic capacity of the leaf.[28] 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 that do not use PEP-carboxylase in carbon fixation are called C3 plants because the primary carboxylation reaction, catalyzed by RuBisCO, produces the three-carbon 3-phosphoglyceric acids directly in the Calvin-Benson cycle. Over 90% of plants use C3 carbon fixation, compared to 3% that use C4 carbon fixation;[29] however, the evolution of C4 in over 60 plant lineages makes it a striking example of convergent evolution.[27]

text: 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 spatially separates the CO2 fixation to PEP from the Calvin cycle, CAM temporally separates these two processes. CAM plants have a different leaf anatomy from C3 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. Sixteen thousand species of plants use CAM.[30]

text: In water

text:
Cyanobacteria possess carboxysomes, which increase the concentration of CO2 around RuBisCO to increase the rate of photosynthesis. An enzyme, carbonic anhydrase, located within the carboxysome releases CO2 from the dissolved hydrocarbonate ions (HCO−
3). Before the CO2 diffuses out it is quickly sponged up by RuBisCO, which is concentrated within the carboxysomes. HCO−
3 ions are made from CO2 outside the cell by another carbonic anhydrase and are actively pumped into the cell by a membrane protein. They cannot cross the membrane as they are charged, and within the cytosol they turn back into CO2 very slowly without the help of carbonic anhydrase. This causes the HCO−
3 ions to accumulate within the cell from where they diffuse into the carboxysomes.[31] Pyrenoids in algae and hornworts also act to concentrate CO2 around rubisco.[32]

text: Order and kinetics

text: The overall process of photosynthesis takes place in four stages:[13]

text:
Stage
Description
Time scale

1
Energy transfer in antenna chlorophyll (thylakoid membranes)
femtosecond to picosecond

2
Transfer of electrons in photochemical reactions (thylakoid membranes)
picosecond to nanosecond

3
Electron transport chain and ATP synthesis (thylakoid membranes)
microsecond to millisecond

4
Carbon fixation and export of stable products
millisecond to second

text: Efficiency

text: Plants usually convert light into chemical energy with a photosynthetic efficiency of 3–6%.[33] Absorbed light that is unconverted is dissipated primarily as heat, with a small fraction (1–2%)[34] re-emitted as chlorophyll fluorescence at longer (redder) wavelengths. A fact that allows measurement of the light reaction of photosynthesis by using chlorophyll fluorometers.[35]

text: Actual plants' photosynthetic efficiency varies with the frequency of the light being converted, light intensity, temperature and proportion of carbon dioxide in the atmosphere, and can vary from 0.1% to 8%.[36] By comparison, solar panels convert light into electric energy at an efficiency of approximately 6–20% for mass-produced panels, and above 40% in laboratory devices.

text: The efficiency of both light and dark reactions can be measured but the relationship between the two can be complex.[37] For example, the ATP and NADPH energy molecules, created by the light reaction, can be used for carbon fixation or for photorespiration in C3 plants.[37] Electrons may also flow to other electron sinks.[38][39][40] For this reason, it is not uncommon for authors to differentiate between work done under non-photorespiratory conditions and under photorespiratory conditions.[41][42][43]

text: Chlorophyll fluorescence of photosystem II can measure the light reaction, and Infrared gas analyzers can measure the dark reaction.[44] It is also possible to investigate both at the same time using an integrated chlorophyll fluorometer and gas exchange system, or by using two separate systems together.[45] Infrared gas analyzers and some moisture sensors are sensitive enough to measure the photosynthetic assimilation of CO2, and of ΔH2O using reliable methods[46] CO2 is commonly measured in μmols/m2/s−1, parts per million or volume per million and H20 is commonly measured in mmol/m2/s−1 or in mbars.[46] By measuring CO2 assimilation, ΔH2O, leaf temperature, barometric pressure, leaf area, and photosynthetically active radiation or PAR, it becomes possible to estimate, “A” or carbon assimilation, “E” or transpiration, “gs” or stomatal conductance, and Ci or intracellular CO2.[46] However, it is more common to used chlorophyll fluorescence for plant stress measurement, where appropriate, because the most commonly used measuring parameters FV/FM and Y(II) or F/FM’ can be made in a few seconds, allowing the measurement of larger plant populations.[43]

text: Gas exchange systems that offer control of CO2 levels, above and below ambient, allow the common practice of measurement of A/Ci curves, at different CO2 levels, to characterize a plant’s photosynthetic response.[46]

text: Integrated chlorophyll fluorometer – gas exchange systems allow a more precise measure of photosynthetic response and mechanisms.[44][47] While standard gas exchange photosynthesis systems can measure Ci, or substomatal CO2 levels, the addition of integrated chlorophyll fluorescence measurements allows a more precise measurement of CC to replace Ci.[45][48] The estimation of CO2 at the site of carboxylation in the chloroplast, or CC, becomes possible with the measurement of mesophyll conductance or gm using an integrated system.[44][45][49]

text: Photosynthesis measurement systems are not designed to directly measure the amount of light absorbed by the leaf. But analysis of chlorophyll-fluorescence, P700- and P515-absorbance and gas exchange measurements reveal detailed information about e.g. the photosystems, quantum efficiency and the CO2 assimilation rates. With some instruments even wavelength-dependency of the photosynthetic efficiency can be analyzed.[50]

text: A phenomenon known as quantum walk increases the efficiency of the energy transport of light significantly. In the photosynthetic cell of an algae, bacterium, or plant, there are light-sensitive molecules called chromophores arranged in an antenna-shaped structure named a photocomplex. When a photon is absorbed by a chromophore, it is converted into a quasiparticle referred to as an exciton, which jumps from chromophore to chromophore towards the reaction center of the photocomplex, a collection of molecules that traps its energy in a chemical form that makes it accessible for the cell's metabolism. The exciton's wave properties enable it to cover a wider area and try out several possible paths simultaneously, allowing it to instantaneously "choose" the most efficient route, where it will have the highest probability of arriving at its destination in the minimum possible time. Because that quantum walking takes place at temperatures far higher than quantum phenomena usually occur, it is only possible over very short distances, due to obstacles in the form of destructive interference that come into play. These obstacles cause the particle to lose its wave properties for an instant before it regains them once again after it is freed from its locked position through a classic "hop". The movement of the electron towards the photo center is therefore covered in a series of conventional hops and quantum walks.[51][52][53]

text: Evolution

text: Early photosynthetic systems, such as those in green and purple sulfur and green and purple nonsulfur bacteria, are thought to have been anoxygenic, and used various other molecules as electron donors rather than water. Green and purple sulfur bacteria are thought to have used hydrogen and sulfur as electron donors. Green nonsulfur bacteria used various amino and other organic acids as an electron donor. Purple nonsulfur bacteria used a variety of nonspecific organic molecules. The use of these molecules is consistent with the geological evidence that Earth's early atmosphere was highly reducing at that time.[54]

text: Fossils of what are thought to be filamentous photosynthetic organisms have been dated at 3.4 billion years old.[55][56]

text:
The main source of oxygen in the Earth's atmosphere derives from 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 oxygen (O
2) in the photosynthetic reaction center.

text: Symbiosis and the origin of chloroplasts

text: Several groups of animals have formed symbiotic relationships with photosynthetic algae. These are most common in corals, sponges and sea anemones. It is presumed that this is due to the particularly simple body plans and large surface areas of these animals compared to their volumes.[57] In addition, a few marine mollusks Elysia viridis and Elysia chlorotica also maintain a symbiotic relationship with chloroplasts they capture from the algae in their diet and then store in their bodies. This allows the mollusks to survive solely by photosynthesis for several months at a time.[58][59] 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.[60]

text: 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 ribosome, and similar proteins in the photosynthetic reaction center.[61][62] 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 possess their own DNA, separate from the nuclear DNA of their plant host cells and the genes in this chloroplast DNA resemble those found in cyanobacteria.[63] DNA in chloroplasts codes for redox proteins such as those found in the photosynthetic reaction centers. The CoRR Hypothesis proposes that this Co-location is required for Redox Regulation.[clarification needed]

text: Cyanobacteria and the evolution of photosynthesis

text: 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, it is speculated, much earlier.[64][65] Because the Earth's atmosphere contained almost no oxygen during the estimated development of photosynthesis, it is believed that the first photosynthetic cyanobacteria did not generate oxygen.[66] 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-green algae. Cyanobacteria remained the principal primary producers of oxygen 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-green algae as the major primary producers of oxygen on continental shelves near the end of the Proterozoic, but it was only with the Mesozoic (251–65 Ma) radiations of dinoflagellates, coccolithophorids, and diatoms did the primary production of oxygen in marine shelf waters take modern form. Cyanobacteria remain critical to marine ecosystems as primary producers of oxygen in oceanic gyres, as agents of biological nitrogen fixation, and, in modified form, as the plastids of marine algae.[67]

text: Discovery

text: Although some of the steps in photosynthesis are still not completely understood, the overall photosynthetic equation has been known since the 19th century.

text: Jan van Helmont began the research of the process in the mid-17th century 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.

text: 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.

text: In 1778, Jan Ingenhousz, repeated Priestley's experiments. He discovered that it was the influence of sunlight on the plant that could cause it to revive a mouse in a matter of hours.

text: 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 afterward, 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.

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

text: 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 absorbing up to 600 nm wavelengths, the other up to 700 nm. 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 pigment. These include phycobilins, which are the red and blue pigments of red and blue algae respectively, and fucoxanthol for brown algae and diatoms. The process is most productive when the absorption of quanta are equal in both the PSII and PSI, assuring that input energy from the antenna complex is divided between the PSI and PSII system, which in turn powers the photochemistry.[13]

text: Robert Hill thought that a complex of reactions consisting of an intermediate to cytochrome b6 (now a plastoquinone), another is from cytochrome f to a step in the carbohydrate-generating mechanisms. These are linked by plastoquinone, which does require energy to reduce cytochrome f for it is a sufficient reductant. Further experiments to prove that the oxygen developed during the photosynthesis of green plants came from water, were performed by 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[68] is as follows:

text: where A is the electron acceptor. Therefore, in light, the electron acceptor is reduced and oxygen is evolved.

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

text: 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 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.

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

text: Otto Heinrich Warburg and Dean Burk discovered the I-quantum photosynthesis reaction that splits the CO2, activated by the respiration.[69]

text: Louis N.M. Duysens and Jan Amesz discovered that chlorophyll a will absorb one light, oxidize cytochrome f, chlorophyll a (and other pigments) will absorb another light, but will reduce this same oxidized cytochrome, stating the two light reactions are in series.

text: Development of the concept

text: In 1893, Charles Reid Barnes proposed two terms, photosyntax and photosynthesis, for the biological process of synthesis of complex carbon compounds out of carbonic acid, in the presence of chlorophyll, under the influence of light. Over time, the term photosynthesis came into common usage as the term of choice. Later discovery of anoxygenic photosynthetic bacteria and photophosphorylation necessitated redefinition of the term.[70]

text: C3 : C4 photosynthesis research

text: After WWII at late 1940 at the University of California, Berkeley, the details of photosynthetic carbon metabolism were sorted out by the chemists Melvin Calvin, Andrew Benson, James Bassham and a score of students and researchers utilizing the carbon-14 isotope and paper chromatography techniques.[71] The pathway of CO2 fixation by the algae Chlorella in a fraction of a second in light resulted in a 3 carbon molecule called phosphoglyceric acid (PGA). For that original and ground-breaking work, a Nobel Prize in Chemistry was awarded to Melvin Calvin 1961. In parallel, plant physiologists studied leaf gas exchanges using the new method of infrared gas analysis and a leaf chamber where the net photosynthetic rates ranged from 10 to 13 u mole CO2/square metere.sec., with the conclusion that all terrestrial plants having the same photosynthetic capacities that were light saturated at less than 50% of sunlight.[72][73] These rates were determined in potted plants grown indoors under low light intensity.

text: Later in 1958-1963 at Cornell University, field grown maize was reported to have much greater leaf photosynthetic rates of 40 u mol CO2/square meter.sec and was not saturated at near full sunlight.[74][75] This higher rate in maize was almost double those observed in other species such as wheat and soybean, indicating that large differences in photosynthesis exist among higher plants. At the University of Arizona, detailed gas exchange research on more than 15 species of monocot and dicot uncovered for the first time that differences in leaf anatomy are crucial factors in differentiating photosynthetic capacities among species.[76][77] In tropical grasses, including maize, sorghum, sugarcane, Bermuda grass and in the dicot amaranthus, leaf photosynthetic rates were around 38−40 u mol CO2/square meter.sec., and the leaves have two types of green cells, i. e. outer layer of mesophyll cells surrounding a tightly packed cholorophyllous vascular bundle sheath cells. This type of anatomy was termed Kranz anatomy in the 19th century by the botanist Gottlieb Haberlandt while studying leaf anatomy of sugarcane.[78] Plant species with the greatest photosynthetic rates and Kranz anatomy showed no apparent photorespiration, very low CO2 compensation point, high optimum temperature, high stomatal resistances and lower mesophyll resistances for gas diffusion and rates never saturated at full sun light.[79] The research at Arizona was designated Citation Classic by the ISI 1986.[77] These species was later termed C4 plants as the first stable compound of CO2 fixation in light has 4 carbon as malate and aspartate.[80][81][82] Other species that lack Kranz anatomy were termed C3 type such as cotton and sunflower, as the first stable carbon compound is the 3-carbon PGA acid. At 1000 ppm CO2 in measuring air, both the C3 and C4 plants had similar leaf photosynthetic rates around 60 u mole CO2/square meter.sec. indicating the suppression of photorespiration in C3 plants.[76][77]

text: Factors

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

text:
Light irradiance and wavelength
Carbon dioxide concentration
Temperature.

text: Light intensity (irradiance), wavelength and temperature

text: The process of photosynthesis provides the main input of free energy into the biosphere, and is one of four main ways in which radiation is important for plant life.[83]

text: The radiation climate within plant communities is extremely variable, with both time and space.

text: In the early 20th century, Frederick Blackman and Gabrielle Matthaei investigated the effects of light intensity (irradiance) and temperature on the rate of carbon assimilation.

text:
At constant temperature, the rate of carbon assimilation varies with irradiance, increasing as the irradiance increases, but reaching a plateau at higher irradiance.
At low irradiance, increasing the temperature has little influence on the rate of carbon assimilation. At constant high irradiance, the rate of carbon assimilation increases as the temperature is increased.

text: These two experiments illustrate several important points: First, it is known that, in general, photochemical reactions are not 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' temperature-independent 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.[clarification needed]

text: Carbon dioxide levels and photorespiration

text: 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 carbon dioxide concentration is low, RuBisCO will bind oxygen instead of carbon dioxide. This process, called photorespiration, uses energy, but does not produce sugars.

text: RuBisCO oxygenase activity is disadvantageous to plants for several reasons:

text:
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. The reactions also produce ammonia (NH3), which is able to diffuse out of the plant, leading to a loss of nitrogen.

tag: ol

text: 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.


document_title: The Book of Eli

question_text: what year did the book of eli take place

text: The Book of Eli

text: Plot

text: Thirty years after a nuclear apocalypse,[5] Eli (Denzel Washington) travels on foot toward the west coast of the former United States. Along the way he demonstrates uncanny survival and fighting skills, hunting wildlife and swiftly defeating a group of desert bandits who try to ambush him. Searching for water, he arrives in a ramshackle town rebuilt and overseen by Carnegie (Gary Oldman). Carnegie dreams of building more towns and of controlling the people by using the power of a certain book. His henchmen scour the desolate landscape daily in search of it, but to no avail.

text: In town Eli barters with a store owner, the Engineer (Tom Waits), to recharge the battery of his portable music player. While waiting, he goes across the street to the town bar, where a gang of bikers attack, but he quickly kills all fifteen. Realizing Eli is a literate man like himself, Carnegie asks Eli to stay, making it clear that the offer is non-negotiable. After Carnegie's blind mistress Claudia (Jennifer Beals) gives Eli some food and water, Carnegie orders Claudia's daughter Solara (Mila Kunis) to seduce Eli, but he turns her down. While killing time, she finds his book. Unperturbed, Eli offers to share his food with her, and before they eat, he says grace. The following day, Solara repeats the prayer, but Carnegie overhears them and realizes Solara's words relate to the book he has been seeking. He soon forces Solara to tell him if Eli was reading a book, and realizes Eli's book is the one he wants—a Bible.

text: Eli sneaks out of his room and Carnegie has his henchmen shoot at Eli, but the bullets seemingly just graze him, as if he is being protected. Eli shoots most of Carnegie's men and hits Carnegie in the leg with a shotgun blast. Solara, having left the town, leads Eli to a cave which is the town's water supply, hoping she can accompany him on his travels. Eli traps her inside and continues on alone. Solara escapes and soon finds herself ambushed by two bandits who attempt to rape her, but Eli reappears and kills them. As they continue on, Eli explains his mission to Solara. According to Eli, his book is the last remaining copy of the Bible, as all other copies were intentionally destroyed following the nuclear war thirty years ago. He says he was led to the book by a voice in his head, which then directed him to travel westward to a place where it would be safe. The voice assured him that he would be protected and guided on his journey.

text: Eventually, Eli and Solara investigate an isolated house. They fall into a trap, but manage to allay the suspicions of the residents, George (Michael Gambon) and Martha (Frances de la Tour), who invite them in for tea. When Eli realizes that the couple are cannibals, they attempt to leave just as Carnegie and his henchmen arrive. In the ensuing shootout, George, Martha and many of Carnegie's men are killed. While Eli and Solara are captured, Carnegie threatens to kill Solara unless Eli surrenders the Bible. With the Bible now in his possession, Carnegie shoots Eli, leaving him for dead. Solara escapes and drives back to help Eli. Rather than chase her, Carnegie returns to the town, since his sole remaining vehicle is running low on fuel. Solara finds Eli, and they drive until they reach the Golden Gate Bridge, and then row to Alcatraz Island, where they find a group intent on preserving what they can of literature and music. Eli tells the guards that he has a copy of the King James Version of the Bible. Once inside, Eli, revealed to be blind, begins to dictate the Bible from memory to Lombardi (Malcolm McDowell), the leader of the group.[6][5]

text: Meanwhile, back in the town, the Engineer opens the locked Bible where Carnegie discovers, to his dismay, that it is in Braille, and orders Claudia to read it to him. She pretends that she cannot remember how to read it, telling Carnegie his leg wound has become infected and his men have begun to run amok. At the sanctuary, Eli has died, but not before he finished reciting the entire book. The printing press at Alcatraz begins producing copies of the Bible, and Lombardi places one on a bookshelf between the Torah and Tanakh on one side and the Quran on the other. Solara, though offered sanctuary, chooses to return home, taking with her Eli's sword and other possessions.

text: Cast

text:
Denzel Washington as Eli
Gary Oldman as Carnegie
Mila Kunis as Solara
Ray Stevenson as Redridge
Jennifer Beals as Claudia
Evan Jones as Martz
Joe Pingue as Hoyt
Frances de la Tour as Martha
Michael Gambon as George
Tom Waits as Engineer
Chris Browning as Hijack Leader
Malcolm McDowell as Lombardi (uncredited)

text: Production

text: In May 2007, Columbia Pictures and Warner Bros. signed the Hughes brothers to direct The Book of Eli, based on a script by Gary Whitta. (The brothers had last directed the 2001 film From Hell.)[7] Subsequently, Anthony Peckham rewrote the script, and in September 2008 Denzel Washington won the lead role.[8] The following October, Gary Oldman was cast to star alongside Washington.[9] Principal photography began in February 2009 and took place in New Mexico.[10] Alcon Entertainment financed the film and co-produced with Silver Pictures.[11]

text: Jeff Imada choreographed the complex fight scenes, which feature the Filipino martial art of Kali. Washington trained for months with Dan Inosanto and Imada for his role.[12]

text: Reception

text: Box office

text: The film was released in North America on January 15, 2010 in 3,111 theaters. It took in $11,672,970—$3,752 per theater, its opening day.[13] By the end of its opening four-day holiday weekend it grossed $38,437,553—$12,355 per theater. It ranked number two, behind Avatar.[14] On its second weekend, it placed third with Legion taking its number two place and grossed $15,732,493—$5,057 per theater.[15] By its third weekend it dropped down to number five and made $8,908,286—$2,897 per theater.[16] The film has come to gross $94,835,059 in the United States and Canada, and $62,256,659 in other markets, with an estimated worldwide total of $157,091,718.[3]

text: Critical reviews

text: The film has received mixed reviews from critics. Review aggregator Rotten Tomatoes reports that 48% of 188 critics have given the film a positive review, with a rating average of 5.5 out of 10. The site's consensus is that "It's certainly uneven, and many viewers will find that its reach exceeds its grasp, but The Book of Eli finds the Hughes brothers injecting some fresh stylish fun into the kind of post-apocalyptic wasteland filmgoers have seen more than enough of lately."[17] Based on 33 critics reviews, Metacritic (another review aggregator) has assigned the film a weighted average score of 53 out of 100, indicating "mixed or average reviews".[18]

text: Todd McCarthy of Variety predicted "this will not be one of ... Denzel Washington's bigger grossers."[19] Chicago Sun-Times critic Roger Ebert gave the film 3 of 4 stars and said of the film: "You won't be sorry you went. It grips your attention, and then at the end throws in several WTF! moments, which are a bonus."[20] Reviewing the film for The A.V. Club, Scott Tobias graded the film a B, and wrote "At a time when theaters are experiencing a glut of doomsday scenarios, the Hughes' ashen, bombed-out future world looks a little too familiar, no matter how crisply they present it. But the showdown between Washington and a deliciously hammy Oldman complicates the film's overt religiosity...".[21] Owen Gleiberman of Entertainment Weekly gave the film a D, calling it "a ponderous dystopian bummer that might be described as The Road Warrior without car chases, or The Road without humanity.[22]

text: Home media

text: The film was released on DVD and Blu-ray Disc on May 31, 2010 in the UK and on June 15, 2010 in the United States and Canada. The DVD took the top spot on all three national home video market charts in its first week. It premiered No. 1 on Home Media Magazine's Rental Chart, the Nielsen VideoScan Blu-ray chart, and the Nielsen VideoScan First Alert Sales Chart, where it outdistanced its nearest competitor in sales by a 3 to 1 margin.[23]


document_title: Brushed DC electric motor

question_text: factors on which the speed of dc motor depends

text: Brushed DC electric motor

text: Simple two-pole DC motor[edit]

text: The following graphics illustrate a simple, two-pole, brushed, DC motor.

text: When a current passes through the coil wound around a soft iron core, the side of the positive pole is acted upon by an upwards force, while the other side is acted upon by a downward force. According to Fleming's left hand rule, the forces cause a turning effect on the coil, making it rotate. To make the motor rotate in a constant direction, "direct current" commutators make the current reverse in direction every half a cycle (in a two-pole motor) thus causing the motor to continue to rotate in the same direction.

text: A problem with the motor shown above is that when the plane of the coil is parallel to the magnetic field—i.e. when the rotor poles are 90 degrees from the stator poles—the torque is zero. In the pictures above, this occurs when the core of the coil is horizontal—the position it is just about to reach in the last picture on the right. The motor would not be able to start in this position. However, once it was started, it would continue to rotate through this position by momentum.

text: There is a second problem with this simple pole design. At the zero-torque position, both commutator brushes are touching (bridging) both commutator plates, resulting in a short-circuit. The power leads are shorted together through the commutator plates, and the coil is also short-circuited through both brushes (the coil is shorted twice, once through each brush independently). Note that this problem is independent of the non-starting problem above; even if there were a high current in the coil at this position, there would still be zero torque. The problem here is that this short uselessly consumes power without producing any motion (nor even any coil current.) In a low-current battery-powered demonstration this short-circuiting is generally not considered harmful. However, if a two-pole motor were designed to do actual work with several hundred watts of power output, this shorting could result in severe commutator overheating, brush damage, and potential welding of the brushes—if they were metallic—to the commutator. Carbon brushes, which are often used, would not weld. In any case, a short like this is very wasteful, drains batteries rapidly and, at a minimum, requires power supply components to be designed to much higher standards than would be needed just to run the motor without the shorting.

text: One simple solution is to put a gap between the commutator plates which is wider than the ends of the brushes. This increases the zero-torque range of angular positions but eliminates the shorting problem; if the motor is started spinning by an outside force it will continue spinning. With this modification, it can also be effectively turned off simply by stalling (stopping) it in a position in the zero-torque (i.e. commutator non-contacting) angle range. This design is sometimes seen in homebuilt hobby motors, e.g. for science fairs and such designs can be found in some published science project books. A clear downside of this simple solution is that the motor now coasts through a substantial arc of rotation twice per revolution and the torque is pulsed. This may work for electric fans or to keep a flywheel spinning but there are many applications, even where starting and stopping are not necessary, for which it is completely inadequate, such as driving the capstan of a tape transport, or any instance where to speed up and slow down often and quickly is a requirement. Another disadvantage is that, since the coils have a measure of self inductance, current flowing in them cannot suddenly stop. The current attempts to jump the opening gap between the commutator segment and the brush, causing arcing.

text: Even for fans and flywheels, the clear weaknesses remaining in this design—especially that it is not self-starting from all positions—make it impractical for working use, especially considering the better alternatives that exist. Unlike the demonstration motor above, DC motors are commonly designed with more than two poles, are able to start from any position, and do not have any position where current can flow without producing electromotive power by passing through some coil. Many common small brushed DC motors used in toys and small consumer appliances, the simplest mass-produced DC motors to be found, have three-pole armatures. The brushes can now bridge two adjacent commutator segments without causing a short circuit. These three-pole armatures also have the advantage that current from the brushes either flows through two coils in series or through just one coil. Starting with the current in an individual coil at half its nominal value (as a result of flowing through two coils in series), it rises to its nominal value and then falls to half this value. The sequence then continues with current in the reverse direction. This results in a closer step-wise approximation to the ideal sinusoidal coil current, producing a more even torque than the two-pole motor where the current in each coil is closer to a square wave. Since current changes are half those of a comparable two-pole motor, arcing at the brushes is consequently less.

text: If the shaft of a DC motor is turned by an external force, the motor will act like a generator and produce an Electromotive force (EMF). During normal operation, the spinning of the motor produces a voltage, known as the counter-EMF (CEMF) or back EMF, because it opposes the applied voltage on the motor. The back EMF is the reason that the motor when free-running does not appear to have the same low electrical resistance as the wire contained in its winding. This is the same EMF that is produced when the motor is used as a generator (for example when an electrical load, such as a light bulb, is placed across the terminals of the motor and the motor shaft is driven with an external torque). Therefore, the total voltage drop across a motor consists of the CEMF voltage drop, and the parasitic voltage drop resulting from the internal resistance of the armature's windings. The current through a motor is given by the following equation:

text: The mechanical power produced by the motor is given by:

text: As an unloaded DC motor spins, it generates a backwards-flowing electromotive force that resists the current being applied to the motor. The current through the motor drops as the rotational speed increases, and a free-spinning motor has very little current. It is only when a load is applied to the motor that slows the rotor that the current draw through the motor increases.

text: The commutating plane[edit]

text: In a dynamo, a plane through the centers of the contact areas where a pair of brushes touch the commutator and parallel to the axis of rotation of the armature is referred to as the commutating plane. In this diagram the commutating plane is shown for just one of the brushes, assuming the other brush made contact on the other side of the commutator with radial symmetry, 180 degrees from the brush shown.

text: Compensation for stator field distortion[edit]

text: In a real dynamo, the field is never perfectly uniform. Instead, as the rotor spins it induces field effects which drag and distort the magnetic lines of the outer non-rotating stator.

text:
Exaggerated example of how the field is distorted by the rotor.

Iron filings show the distorted field across the rotor.

text: The faster the rotor spins, the further the degree of field distortion. Because the dynamo operates most efficiently with the rotor field at right angles to the stator field, it is necessary to either retard or advance the brush position to put the rotor's field into the correct position to be at a right angle to the distorted field.

text:
Centered position of the commutating plane if there were no field distortion effects.

Actual position of the commutating plane to compensate for field distortion.

text: These field effects are reversed when the direction of spin is reversed. It is therefore difficult to build an efficient reversible commutated dynamo, since for highest field strength it is necessary to move the brushes to the opposite side of the normal neutral plane.

text: The effect can be considered to be somewhat similar to timing advance in an internal combustion engine. Generally a dynamo that has been designed to run at a certain fixed speed will have its brushes permanently fixed to align the field for highest efficiency at that speed.[1]

text: Motor design variations[edit]

text: DC motors[edit]

text: Brushed DC motors are constructed with wound rotors and either wound or permanent-magnet stators.

text: Wound stators[edit]

text:
A, Shunt
B, Series
C, Compound
f, Field coil

text: The field coils have traditionally existed in four basic formats: separately excited (sepex), series-wound, shunt-wound, and a combination of the latter two; compound-wound.

text: In a series wound motor, the field coils are connected electrically in series with the armature coils (via the brushes). In a shunt wound motor, the field coils are connected in parallel, or "shunted" to the armature coils. In a separately excited (sepex) motor the field coils are supplied from an independent source, such as a motor-generator and the field current is unaffected by changes in the armature current. The sepex system was sometimes used in DC traction motors to facilitate control of wheelslip.

text: Permanent-magnet motors[edit]

text: Permanent-magnet types have some performance advantages over direct-current, excited, synchronous types, and have become predominant in fractional horsepower applications. They are smaller, lighter, more efficient and reliable than other singly-fed electric machines.[2]

text: Originally all large industrial DC motors used wound field or rotor magnets. Permanent magnets have traditionally only been useful on small motors because it was difficult to find a material capable of retaining a high-strength field. Only recently have advances in materials technology allowed the creation of high-intensity permanent magnets, such as neodymium magnets, allowing the development of compact, high-power motors without the extra real-estate of field coils and excitation means. But as these high performance permanent magnets become more applied in electric motor or generator systems, other problems are realized (see Permanent magnet synchronous generator).

text: Axial field motors[edit]

text: Traditionally, the field has been applied radially—in and away from the rotation axis of the motor. However some designs have the field flowing along the axis of the motor, with the rotor cutting the field lines as it rotates. This allows for much stronger magnetic fields, particularly if halbach arrays are employed. This, in turn, gives power to the motor at lower speeds. However, the focused flux density cannot rise about the limited residual flux density of the permanent magnet despite high coercivity and like all electric machines, the flux density of magnetic core saturation is the design constraint.

text: Speed control[edit]

text: Generally, the rotational speed of a DC motor is proportional to the EMF in its coil (= the voltage applied to it minus voltage lost on its resistance), and the torque is proportional to the current. Speed control can be achieved by variable battery tappings, variable supply voltage, resistors or electronic controls. The direction of a wound field DC motor can be changed by reversing either the field or armature connections but not both. This is commonly done with a special set of contactors (direction contactors). The effective voltage can be varied by inserting a series resistor or by an electronically controlled switching device made of thyristors, transistors, or, formerly, mercury arc rectifiers.[3]

text: Series-parallel[edit]

text: Series-parallel control was the standard method of controlling railway traction motors before the advent of power electronics. An electric locomotive or train would typically have four motors which could be grouped in three different ways:

text:
All four in series (each motor receives one quarter of the line voltage)
Two parallel groups of two in series (each motor receives half the line voltage)
All four in parallel (each motor receives the full line voltage)

text: This provided three running speeds with minimal resistance losses. For starting and acceleration, additional control was provided by resistances. This system has been superseded by electronic control systems.

text: Field weakening[edit]

text: The speed of a DC motor can be increased by field weakening. Reducing the field strength is done by inserting resistance in series with a shunt field, or inserting resistances around a series-connected field winding, to reduce current in the field winding. When the field is weakened, the back-emf reduces, so a larger current flows through the armature winding and this increases the speed. Field weakening is not used on its own but in combination with other methods, such as series-parallel control.

text: Chopper[edit]

text: In a circuit known as a chopper, the average voltage applied to the motor is varied by switching the supply voltage very rapidly. As the "on" to "off" ratio is varied to alter the average applied voltage, the speed of the motor varies. The percentage "on" time multiplied by the supply voltage gives the average voltage applied to the motor. Therefore, with a 100 V supply and a 25% "on" time, the average voltage at the motor will be 25 V. During the "off" time, the armature's inductance causes the current to continue through a diode called a "flyback diode", in parallel with the motor. At this point in the cycle, the supply current will be zero, and therefore the average motor current will always be higher than the supply current unless the percentage "on" time is 100%. At 100% "on" time, the supply and motor current are equal. The rapid switching wastes less energy than series resistors. This method is also called pulse-width modulation (PWM) and is often controlled by a microprocessor. An output filter is sometimes installed to smooth the average voltage applied to the motor and reduce motor noise.

text: Since the series-wound DC motor develops its highest torque at low speed, it is often used in traction applications such as electric locomotives, and trams. Another application is starter motors for petrol and small diesel engines. Series motors must never be used in applications where the drive can fail (such as belt drives). As the motor accelerates, the armature (and hence field) current reduces. The reduction in field causes the motor to speed up until it destroys itself. This can also be a problem with railway motors in the event of a loss of adhesion since, unless quickly brought under control, the motors can reach speeds far higher than they would do under normal circumstances. This can not only cause problems for the motors themselves and the gears, but due to the differential speed between the rails and the wheels it can also cause serious damage to the rails and wheel treads as they heat and cool rapidly. Field weakening is used in some electronic controls to increase the top speed of an electric vehicle. The simplest form uses a contactor and field-weakening resistor; the electronic control monitors the motor current and switches the field weakening resistor into circuit when the motor current reduces below a preset value (this will be when the motor is at its full design speed). Once the resistor is in circuit, the motor will increase speed above its normal speed at its rated voltage. When motor current increases, the control will disconnect the resistor and low speed torque is made available.

text: Ward Leonard[edit]

text: A Ward Leonard control is usually used for controlling a shunt or compound wound DC motor, and developed as a method of providing a speed-controlled motor from an AC supply, though it is not without its advantages in DC schemes. The AC supply is used to drive an AC motor, usually an induction motor that drives a DC generator or dynamo. The DC output from the armature is directly connected to the armature of the DC motor (sometimes but not always of identical construction). The shunt field windings of both DC machines are independently excited through variable resistors. Extremely good speed control from standstill to full speed, and consistent torque, can be obtained by varying the generator and/or motor field current. This method of control was the de facto method from its development until it was superseded by solid state thyristor systems. It found service in almost any environment where good speed control was required, from passenger lifts through to large mine pit head winding gear and even industrial process machinery and electric cranes. Its principal disadvantage was that three machines were required to implement a scheme (five in very large installations, as the DC machines were often duplicated and controlled by a tandem variable resistor). In many applications, the motor-generator set was often left permanently running, to avoid the delays that would otherwise be caused by starting it up as required. Although electronic (thyristor) controllers have replaced most small to medium Ward-Leonard systems, some very large ones (thousands of horsepower) remain in service. The field currents are much lower than the armature currents, allowing a moderate sized thyristor unit to control a much larger motor than it could control directly. For example, in one installation, a 300 amp thyristor unit controls the field of the generator. The generator output current is in excess of 15,000 amperes, which would be prohibitively expensive (and inefficient) to control directly with thyristors.

text: Torque and speed of a DC motor[edit]

text: A DC motor's speed and torque characteristics vary according to three different magnetization sources, separately excited field, self-excited field or permanent-field, which are used selectively to control the motor over the mechanical load's range. Self-excited field motors can be series, shunt, or compound wound connected to the armature.

text: Basic properties[edit]

text: Define

text:
Eb, induced or counter EMF (V)
Ia, armature current (A)
kb, counter EMF equation constant
kn, speed equation constant
kT, torque equation constant
n, armature frequency (rpm)
Rm, motor resistance (Ω)
T, motor torque (Nm)
Vm, motor input voltage (V)
Φ, machine's total flux (Wb)

text: Counter EMF equation[edit]

text: The DC motor's counter emf is proportional to the product of the machine's total flux strength and armature speed:

text: Voltage balance equation[edit]

text: The DC motor's input voltage must overcome the counter emf as well as the voltage drop created by the armature current across the motor resistance, that is, the combined resistance across the brushes, armature winding and series field winding, if any:

text: Torque equation[edit]

text: The DC motor's torque is proportional to the product of the armature current and the machine's total flux strength:[7][8][9]

text: where

text: Speed equation[edit]

text: Since

text: we have[10][11][12]

text: where

text: Torque and speed characteristics[edit]

text: Shunt wound motor[edit]

text: With the shunt wound motor's high-resistance field winding connected in parallel with the armature, Vm, Rm and Ø are constant such that the no load to full load speed regulation is seldom more than 5%.[13] Speed control is achieved three ways:[14]

text:
Varying the field voltage
Field weakening
Variable resistance in the field circuit.

text: Series wound motor[edit]

text: The series motor responds to increased load by slowing down; the current increases and the torque rises in proportional to the square of the current since the same current flows in both the armature and the field windings. If the motor is stalled, the current is limited only by the total resistance of the windings and the torque can be very high, but there is a danger of the windings becoming overheated. Series wound motors were widely used as traction motors in rail transport[15] of every kind, but are being phased out in favour of power inverter-fed AC induction motors. The counter emf aids the armature resistance to limit the current through the armature. When power is first applied to a motor, the armature does not rotate, the counter emf is zero and the only factor limiting the armature current is the armature resistance.[16] As the prospective current through the armature is very large, the need arises for an additional resistance in series with the armature to limit the current until the motor rotation can build up the counter emf. As the motor rotation builds up, the resistance is gradually cut out.

text: The series wound DC motor's most notable characteristic is that it is almost entirely dependent on the torque required to drive the load. This suits large inertial loads as motor accelerates from maximum torque, torque reducing gradually as load increases.

text: As the series motor's speed can be dangerously high, series motors are often geared or direct-connected to the load.[17]

text: Permanent magnet motor[edit]

text: A permanent magnet DC motor is characterized by a linear relationship between stall torque when the torque is maximum with the shaft at standstill and no load speed with no applied shaft torque and maximum output speed . There is a quadratic power relationship between these two speed-axis points.[18]

text: Protection[edit]

text: To extend a DC motor’s service life, protective devices[19] and motor controllers are used to protect it from mechanical damage, excessive moisture, high dielectric stress and high temperature or thermal overloading.[20] These protective devices sense motor fault conditions[21] and either annunciate an alarm to notify the operator or automatically de-energize the motor when a faulty condition occurs. For overloaded conditions, motors are protected with thermal overload relays. Bi-metal thermal overload protectors are embedded in the motor's windings and made from two dissimilar metals. They are designed such that the bimetallic strips will bend in opposite directions when a temperature set point is reached to open the control circuit and de-energize the motor. Heaters are external thermal overload protectors connected in series with the motor’s windings and mounted in the motor contactor. Solder pot heaters melt in an overload condition, which cause the motor control circuit to de-energize the motor. Bimetallic heaters function the same way as embedded bimetallic protectors. Fuses and circuit breakers are overcurrent or short circuit protectors. Ground fault relays also provide overcurrent protection. They monitor the electric current between the motor’s windings and earth system ground. In motor-generators, reverse current relays prevent the battery from discharging and motorizing the generator. Since D.C. motor field loss can cause a hazardous runaway or overspeed condition, loss of field relays[22] are connected in parallel with the motor’s field to sense field current. When the field current decreases below a set point, the relay will deenergize the motor’s armature. A locked rotor condition prevents a motor from accelerating after its starting sequence has been initiated. Distance relays protect motors from locked-rotor faults. Undervoltage motor protection is typically incorporated into motor controllers or starters. In addition, motors can be protected from overvoltages or surges with isolation transformers, power conditioning equipment, MOVs, arresters and harmonic filters. Environmental conditions, such as dust, explosive vapors, water, and high ambient temperatures, can adversely affect the operation of a DC motor. To protect a motor from these environmental conditions, the National Electrical Manufacturers Association (NEMA) and the International Electrotechnical Commission (IEC) have standardized motor enclosure[23] designs based upon the environmental protection they provide from contaminants. Modern software can also be used in the design stage, such as Motor-CAD, to help increase the thermal efficiency of a motor.

text: DC motor starters[edit]

text: The counter-emf aids the armature resistance to limit the current through the armature. When power is first applied to a motor, the armature does not rotate. At that instant the counter-emf is zero and the only factor limiting the armature current is the armature resistance and inductance. Usually the armature resistance of a motor is less than 1 Ω; therefore the current through the armature would be very large when the power is applied. This current can make an excessive voltage drop affecting other equipment in the circuit and even trip overload protective devices.

text: Therefore, the need arises for an additional resistance in series with the armature to limit the current until the motor rotation can build up the counter-emf. As the motor rotation builds up, the resistance is gradually cut out.

text: Manual-starting rheostat[edit]

text: When electrical and DC motor technology was first developed, much of the equipment was constantly tended by an operator trained in the management of motor systems. The very first motor management systems were almost completely manual, with an attendant starting and stopping the motors, cleaning the equipment, repairing any mechanical failures, and so forth.

text: The first DC motor-starters were also completely manual, as shown in this image. Normally it took the operator about ten seconds to slowly advance the rheostat across the contacts to gradually increase input power up to operating speed. There were two different classes of these rheostats, one used for starting only, and one for starting and speed regulation. The starting rheostat was less expensive, but had smaller resistance elements that would burn out if required to run a motor at a constant reduced speed.

text: This starter includes a no-voltage magnetic holding feature, which causes the rheostat to spring to the off position if power is lost, so that the motor does not later attempt to restart in the full-voltage position. It also has overcurrent protection that trips the lever to the off position if excessive current over a set amount is detected.[24]

text: Three-point starter[edit]

text: The incoming power is indicated as L1 and L2.[where?] The components within the broken lines form the three-point starter. As the name implies there are only three connections to the starter. The connections to the armature are indicated as A1 and A2. The ends of the field (excitement) coil are indicated as F1 and F2. In order to control the speed, a field rheostat is connected in series with the shunt field. One side of the line is connected to the arm of the starter (represented by an arrow in the diagram). The arm is spring-loaded so, it will return to the "Off" position when not held at any other position.

text:
On the first step of the arm, full line voltage is applied across the shunt field. Since the field rheostat is normally set to minimum resistance, the speed of the motor will not be excessive; additionally, the motor will develop a large starting torque.
The starter also connects an electromagnet in series with the shunt field. It will hold the arm in position when the arm makes contact with the magnet.
Meanwhile, that voltage is applied to the shunt field, and the starting resistance limits the current to the armature.
As the motor picks up speed counter-emf is built up; the arm is moved slowly to short.

text: Four-point starter[edit]

text: The four-point starter eliminates the drawback of the three-point starter. In addition to the same three points that were in use with the three-point starter, the other side of the line, L1, is the fourth point brought to the starter when the arm is moved from the "Off" position. The coil of the holding magnet is connected across the line. The holding magnet and starting resistors function identical as in the three-point starter.

text: The possibility of accidentally opening the field circuit is quite remote. The four-point starter provides the no-voltage protection to the motor. If the power fails, the motor is disconnected from the line.