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Carbon Dioxide

Carbon dioxide

Carbon dioxide is an atmospheric gas comprised of one carbon and two oxygen atoms. A very widely known chemical compound, it is frequently called by its formula CO₂. In its solid state, it is commonly known as dry ice. Carbon dioxide derives from multiple sources including volcanic outgassing, the combustion of organic matter and respiration processes of living aerobic organisms. It is also produced by various microorganisms from fermentation and cellular respiration. Plants utilize carbon dioxide during photosynthesis, using both the carbon and the oxygen to construct carbohydrates. In addition, plants also release oxygen to the atmosphere, which is subsequently used for respiration by heterotrophic organisms, forming a cycle. It is present in the Earth's atmosphere at a low concentration and acts as a greenhouse gas. It is a major component of the carbon cycle.

Chemical and physical properties

Carbon dioxide is a colorless gas which, when inhaled at high concentrations (a dangerous activity because of the associated asphyxiation risk), produces a sour taste in the mouth and a stinging sensation in the nose and throat. These effects result from the gas dissolving in the mucous membranes and saliva, forming a weak solution of carbonic acid. Its density at 25 °C is 1.98 kg m⁻³, about 1.5 times that of air. The carbon dioxide molecule (O=C=O) contains two double bonds and has a linear shape. It has no electrical dipole. As it is fully oxidized, it is not very reactive and, in particular, not flammable. At temperatures below −78 °C, carbon dioxide condenses into a white solid called dry ice. Liquid carbon dioxide forms only at pressures above 5.1 atm; at atmospheric pressure, it passes directly between the gaseous and solid phases in a process called sublimation. Water will absorb its own volume of carbon dioxide, and more than this under pressure. About 1% of the dissolved carbon dioxide turns into carbonic acid. The carbonic acid in turn dissociates partly to form bicarbonate and carbonate ions. Test For Carbon Dioxide. When a lighted splint is inserted into a test tube containing carbon dioxide, the flame is immediately extinguished, as carbon dioxide does not support combustion. (Certain fire extinguishers contain carbon dioxide to extinguish the flame). To further confirm that the gas is carbon dioxide, the gas may be bubbled into calcium hydroxide solution. The calcium hydroxide turns milky because of the formation of calcium carbonate.

Uses

Liquid and solid carbon dioxide are important refrigerants, especially in the food industry, where they are employed during the transportation and storage of ice cream and other frozen foods. Carbon dioxide is used to produce carbonated soft drinks and soda water. Traditionally, the carbonation in beer and sparkling wine comes about through natural fermentation, but some manufacturers carbonate these beverages artificially. The leavening agents used in baking produce carbon dioxide to cause dough to rise. Baker's yeast produces carbon dioxide by fermentation within the dough, while chemical leaveners such as baking powder and baking soda release carbon dioxide when heated or exposed to acids. Carbon dioxide is often used as an inexpensive, nonflammable pressurized gas. Life jackets often contain canisters of pressured carbon dioxide for quick inflation. Steel capsules are also sold as supplies of compressed gas for airguns, paintball markers, for inflating bicycle tires, and for making seltzer. Rapid vaporization of liquid CO₂ is used for blasting in coal mines. Carbon dioxide extinguishes flames, and some fire extinguishers, especially those designed for electrical fires, contain liquid carbon dioxide under pressure. Carbon dioxide also finds use as an atmosphere for welding, although in the welding arc, it reacts to oxidize most metals. Use in the automotive industry is common despite significant evidence that welds made in carbon dioxide are brittler than those made in more inert atmospheres, and that such weld joints deteriorate over time because of the formation of carbonic acid. It is used as a welding gas primarily because it is much less expensive than more inert gases such as argon or helium. Liquid carbon dioxide is a good solvent for many organic compounds, and is used to remove caffeine from coffee. It has begun to attract attention in the pharmaceutical and other chemical processing industries as a less toxic alternative to more traditional solvents such as organochlorides. (See green chemistry.) Plants require carbon dioxide to conduct photosynthesis, and greenhouses may enrich their atmospheres with additional CO₂ to boost plant growth. It has been proposed that carbon dioxide from power generation be bubbled into ponds to grow algae that could then be converted into biodiesel fuel. High levels of carbon dioxide in the atmosphere effectively exterminate many pests. Greenhouses will raise the level of CO₂ to 10,000 ppm (1%) for several hours to eliminate pests such as whitefly, spider mites, and others. In medicine, up to 5% carbon dioxide is added to pure oxygen for stimulation of breathing after apnea and to stabilize the O₂/CO₂ balance in blood. A common type of industrial gas laser, the carbon dioxide laser, uses carbon dioxide as a medium. Carbon dioxide is commonly injected into or adjacent to producing oil wells. It will act as both a pressurizing agent and, when dissolved into the underground crude oil, will significantly reduce its viscosity, enabling the oil to flow more rapidly through the earth to the removal well. In mature oil fields, extensive pipe networks are used to carry the carbon dioxide to the injection points.

Dry Ice

Dry ice is a genericized trademark for solid ("frozen") carbon dioxide. The term was coined in 1925 by Prest Air Devices, a company formed in Long Island City, New York in 1923. Dry ice at normal pressures does not melt into liquid carbon dioxide but rather sublimates directly into carbon dioxide gas at −78.5 °C (−109.3 °F). Hence it is called "dry ice" as opposed to normal "wet" ice (frozen water). Dry ice is produced by compressing carbon dioxide gas to a liquid form, removing the heat produced by the compression (see Charles' law), and then letting the liquid carbon dioxide expand quickly. This expansion causes a drop in temperature so that some of the CO₂ freezes into "snow", which is then compressed into pellets or blocks.

Uses

temperature, New York, USA)]]
- Cooling foodstuffs, biological samples, and other perishable items.
- Producing "dry ice fog" for special effects. When dry ice is put into contact with water, the frozen carbon dioxide sublimates into a mixture of cold carbon dioxide gas and cold humid air. This causes condensation and the formation of fog; see fog machine. The effect of fog by the mixture of dry ice with water, is best formed when the water is warm, rather than cold.
- Tiny pellets of dry ice (instead of sand) are shot at a surface to be cleaned. Dry ice is not as hard as sand, but it speeds processing by sublimating to nothing and does not produce nearly as much lung-damaging dust.
- Increasing precipitation from existing clouds or decreasing cloud thickness by cloud seeding.
- Producing carbon dioxide gas as needed in such systems as the fuel tank inerting system in the B-47 aircraft.
- Brass or other metallic bushings are buried in dry ice to shrink their size so they will fit inside a machined hole. When the bushing warms back up, it expands and makes an extremely tight fit.

Handling

Because of its particular characteristics, dry ice requires special precautions when handling. It is extremely cold and there should be no direct contact with skin (i.e., wear proper insulating gloves). It is constantly sublimating to carbon dioxide gas, so it cannot be stored in a sealed container as the pressure buildup will quickly cause the container to explode. The sublimated gas must be ventilated; otherwise, it may fill the enclosed space and create a suffocation hazard. Special care for ventilating vehicles is needed as well because of the small space. People who handle dry ice should also be aware that carbon dioxide is heavier than air and will sink to the floor. Some markets require those purchasing dry ice to be of 18 years of age or older.

Biology

Carbon dioxide is an end product in organisms that obtain energy from breaking down sugars or fats with oxygen as part of their metabolism, in a process known as cellular respiration. This includes all plants, animals, many fungi and some bacteria. In higher animals, the carbon dioxide travels in the blood from the body's tissues to the lungs where it is exhaled. Carbon dioxide content in fresh air is approximately 0.04%, and in exhaled air approximately 4.5%. When inhaled in high concentrations (about 5% by volume), it is toxic to humans and other animals. Hemoglobin, the main oxygen-carrying molecule in red blood cells, can carry both oxygen and carbon dioxide, although in quite different ways. The decreased binding to oxygen in the blood due to increased carbon dioxide levels is known as the Haldane Effect, and is important in the transport of carbon dioxide from the tissues to the lungs. Conversely, a rise in the partial pressure of CO₂ or a lower pH will cause offloading of oxygen from hemoglobin. This is known as the Bohr Effect. According to a study by the USDA [http://itest.slu.edu/articles/90s/hannan.html], an average person's respiration generates approximately 450 liters (roughly 900 grams) of carbon dioxide per day. CO₂ is carried in blood in three different ways. Most of it (about 80%–90%) is converted to bicarbonate ions HCO₃⁻ by the enzyme carbonic anhydrase in the red blood cells. 5%–10% is dissolved in the plasma and 5%–10% is bound to hemoglobin as carbamino compounds. The exact percentages vary depending whether it is arterial or venous blood. The CO₂ bound to hemoglobin does not bind to the same site as oxygen; rather it combines with the N-terminal groups on the four globin chains. However, because of allosteric effects on the hemoglobin molecule, the binding of CO₂ does decrease the amount of oxygen that is bound for a given partial pressure of oxygen. Carbon dioxide may be one of the mediators of local autoregulation of blood supply. If it is high, the capillaries expand to allow a greater blood flow to that tissue. Bicarbonate ions are crucial for regulating blood pH. As breathing rate influences the level of CO₂ in blood, too slow or shallow breathing causes respiratory acidosis, while too rapid breathing, hyperventilation, leads to respiratory alkalosis. It is interesting to note that although it is oxygen that the body requires for metabolism, it is not low oxygen levels that stimulate breathing, but is instead higher carbon dioxide levels. As a result, breathing low-pressure air or a gas mixture with no oxygen at all (e.g., pure nitrogen) leads to loss of consciousness without subjective breathing problems. This is especially perilous for high-altitude fighter pilots, and is also the reason why the instructions in commercial airplanes for case of loss of cabin pressure stress that one should apply the oxygen mask to oneself before helping others—otherwise one risks going unconscious without being aware of the imminent peril. Plants remove carbon dioxide from the atmosphere by photosynthesis, which uses light energy to produce organic plant materials by combining carbon dioxide and water. This releases free oxygen gas. Sometimes carbon dioxide gas is pumped into greenhouses to promote plant growth. Plants also emit CO₂ during respiration, but on balance they are net sinks of CO₂. OSHA limits carbon dioxide concentration in the workplace to 0.5% for prolonged periods, or to 3% for brief exposures (up to ten minutes). OSHA considers concentrations exceeding 4% as "immediately dangerous to life and health." People who breathe 5% carbon dioxide for more than half an hour show signs of acute hypercapnia, while breathing 7%–10% carbon dioxide can produce unconsciousness in only a few minutes. Carbon dioxide, either as a gas or as dry ice, should be handled only in well-ventilated areas. See also: Arterial blood gas.

Atmosphere

Arterial blood gas As of 2004, the earth's atmosphere is about 0.038% by volume (380 µL/L or ppmv) or 0.053% by weight CO₂. This represents about 2.7 × 10¹² tonnes of CO₂. Because of the greater land area, and therefore greater plant life, in the northern hemisphere as compared to the southern hemisphere, there is an annual fluctuation of about 5 µL/L, peaking in May and reaching a minimum in October at the end of the northern hemisphere growing season, when the quantity of biomass on the planet is greatest. Despite its small concentration, CO₂ is a very important component of Earth's atmosphere, because it absorbs infrared radiation and enhances the greenhouse effect. The initial carbon dioxide in the atmosphere of the young Earth was produced by volcanic activity; this was essential for a warm and stable climate conducive to life. Volcanic activity now releases about 130 to 230 teragrams (145 million to 255 million short tons) of carbon dioxide each year. Volcanic releases are about 1% of the amount which is released by human activities. short tons–2000.]] Since the start of the Industrial Revolution, the atmospheric CO₂ concentration has increased by approximately 110 µL/L or about 40%, most of it released since 1945. Monthly measurements taken at Mauna Loa [http://cdiac.esd.ornl.gov/trends/co2/sio-mlo.htm] since 1958 show an increase from 316 µL/L in that year to 376 µL/L in 2003, an overall increase of 60 µL/L during the 44-year history of the measurements. Burning fossil fuels such as coal and petroleum is the leading cause of increased man-made CO₂; deforestation is the second major cause. In 1997, Indonesian peat fires may have released 13%–40% as much carbon as fossil fuel burning does [http://en.wikipedia.org/wiki/Peat#Peat_fires]. Various techniques have been proposed for removing excess carbon dioxide from the atmosphere in carbon dioxide sinks. Not all the emitted CO₂ remains in the atmosphere; some is absorbed in the oceans or biosphere. The ratio of the emitted CO₂ to the increase is atmospheric CO₂ is known as the airborne fraction (Keeling et al., 1995); this varies for short-term averages but is typically 57% over longer (5 year) periods.

carbon dioxide sink The Global Warming Theory (GWT) predicts that increased amounts of CO₂ in the atmosphere tend to enhance the greenhouse effect and thus contribute to global warming. The effect of combustion-produced carbon dioxide on climate is called the Callendar effect.

Variation in the past

Callendar effect The most direct method for measuring atmospheric carbon dioxide concentrations for periods before direct sampling is to measure bubbles of air (fluid or gas inclusions) trapped in the Antarctic or Greenland ice caps. The most widely accepted of such studies come from a variety of Antarctic cores and indicate that atmospheric CO₂ levels were about 260–280µL/L immediately before industrial emissions began and did not vary much from this level during the preceding 10,000 years. The longest ice core record comes from East Antarctica, where ice has been sampled to an age of 650,000 years before the present. [http://pubs.acs.org/cen/news/83/i48/8348notw1.html] During this time, the atmospheric carbon dioxide concentration has varied between 180–210 µL/L during ice ages, increasing to 280–300 µL/L during warmer interglacials. Some studies have disputed the claim of stable CO₂ levels during the present interglacial (the last 10 kyr). Based on an analysis of fossil leaves, Wagner et al. argued that CO₂ levels during the period 7–10 kyr ago were significantly higher (~300 µL/L) and contained substantial variations that may be correlated to climate variations. Others have disputed such claims, suggesting they are more likely to reflect calibration problems than actual changes in CO₂. Relevant to this dispute is the observation that Greenland ice cores often report higher and more variable CO₂ values than similar measurements in Antarctica. However, the groups responsible for such measurements (e.g., Smith et al.) believe the variations in Greenland cores result from in situ decomposition of calcium carbonate dust found in the ice. When dust levels in Greenland cores are low, as they nearly always are in Antarctic cores, the researchers report good agreement between Antarctic and Greenland CO₂ measurements. calcium carbonate On longer timescales, various proxy measurements have been used to attempt to determine atmospheric carbon dioxide levels millions of years in the past. These include boron and carbon isotope ratios in certain types of marine sediments, and the number of stomata observed on fossil plant leaves. While these measurements give much less precise estimates of carbon dioxide concentration than ice cores, there is evidence for very high CO₂ concentrations (>3,000 µL/L) between 600 and 400 Myr BP and between 200 and 150 Myr BP.[http://www.grida.no/climate/ipcc_tar/wg1/fig3-2.htm] On long timescales, atmospheric CO₂ content is determined by the balance among geochemical processes including organic carbon burial in sediments, silicate rock weathering, and vulcanism. The net effect of slight imbalances in the carbon cycle over tens to hundreds of millions of years has been to reduce atmospheric CO₂. The rates of these processes are extremely slow; hence they are of limited relevance to the atmospheric CO₂ response to emissions over the next hundred years. In more recent times, atmospheric CO₂ concentration continued to fall after about 60 Myr BP, and there is geochemical evidence that concentrations were <300 µL/L by about 20 Myr BP. Low CO₂ concentrations may have been the stimulus that favored the evolution of C4 plants, which increased greatly in abundance between 7 and 5 Myr BP. Although contemporary CO₂ concentrations were exceeded during earlier geological epochs, present carbon dioxide levels are likely higher now than at any time during the past 20 million years [http://www.grida.no/climate/ipcc_tar/wg1/107.htm#331] and at the same time lower than at any time in history if we look at time scales longer than 50 million years.

Oceans

The Earth's oceans contain a huge amount of carbon dioxide in the form of bicarbonate and carbonate ions—much more than the amount in the atmosphere. The bicarbonate is produced in reactions between rock, water, and carbon dioxide. One example is the dissolution of calcium carbonate: CaCO₃ + CO₂ + H₂O ⇌ Ca²⁺ + 2 HCO₃^- Reactions like this tend to buffer changes in atmospheric CO₂. Reactions between carbon dioxide and non-carbonate rocks also add bicarbonate to the seas, which can later undergo the reverse of the above reaction to form carbonate rocks, releasing half of the bicarbonate as CO₂. Over hundreds of millions of years this has produced huge quantities of carbonate rocks. If all the carbonate rocks in the earth's crust were to be converted back into carbon dioxide, the resulting carbon dioxide would weigh 40 times as much as the rest of the atmosphere. The vast majority of CO₂ added to the atmosphere will eventually be absorbed by the oceans and become bicarbonate ion, but the process takes on the order of a hundred years because most seawater rarely comes near the surface.

History

Carbon dioxide was one of the first gases to be described as a substance distinct from air. In the 17th century, the Flemish chemist Jan Baptist van Helmont observed that when he burned charcoal in a closed vessel, the mass of the resulting ash was much less than that of the original charcoal. His interpretation was that the rest of the charcoal had been transmuted into an invisible substance he termed a "gas" or "wild spirit" (spiritus sylvestre). Carbon dioxide's properties were studied more thoroughly in the 1750s by the Scottish physician Joseph Black. He found that limestone (calcium carbonate) could be heated or treated with acids to yield a gas he termed "fixed air." He observed that the fixed air was denser than air and did not support either flame or animal life. He also found that it would, when bubbled through an aqueous solution of lime (calcium hydroxide), precipitate calcium carbonate, and used this phenomenon to illustrate that carbon dioxide is produced by animal respiration and microbial fermentation. In 1772, Joseph Priestley used carbon dioxide produced from the action of sulfuric acid on limestone to prepare soda water, the first known instance of an artificially carbonated beverage. Carbon dioxide was first liquefied (at elevated pressures) in 1823 by Humphrey Davy and Michael Faraday. The earliest description of solid carbon dioxide was given by Charles Thilorier, who in 1834 opened a pressurized container of liquid carbon dioxide, only to find that the cooling produced by the rapid evaporation of the liquid yielded a "snow" of solid CO₂.

References

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External links

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- [http://www.dryiceinfo.com/science.htm Dry Ice information]
- Bassam Z. Shakhashiri: [http://scifun.chem.wisc.edu/chemweek/CO2/CO2.html Chemical of the Week: Carbon Dioxide]
- Keeling, C.D. and T.P. Whorf: [http://cdiac.esd.ornl.gov/trends/co2/sio-mlo.htm Atmospheric carbon dioxide record from Mauna Loa], 2002
- [http://www.usatoday.com/weather/news/2004-03-21-co2-buildup_x.htm Mauna Loa 2004 update]
- [http://www.uigi.com/carbondioxide.html CO2 Carbon Dioxide Properties, Uses, Applications]
- [http://www.compchemwiki.org/index.php?title=Carbon_dioxide Computational Chemistry Wiki]
- [http://scifun.chem.wisc.edu/chemweek/CO2/CO2_phase_diagram.gif Pressure-Temperature phase diagram for carbon dioxide] Category:Inorganic carbon compounds Category:Oxides Category:Greenhouse gases Category:Propellants Category:Household chemicals Category:Solvents Category:Refrigerants Category:Fire suppression agents ko:이산화 탄소 ms:Karbon dioksida ja:二酸化炭素 simple:Carbon dioxide th:คาร์บอนไดออกไซด์

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Gas

:For other meanings see gas (disambiguation). ---- A gas is one of the four main phases of matter (after solid and liquid, and followed by plasma), that subsequently appear as a solid material is subjected to increasingly higher temperatures. Thus, as energy in the form of heat is added, a solid (e.g. ice) will first melt to become a liquid (e.g. water), which will then boil or evaporate to become a gas (e.g. water vapor). In some circumstances, a solid (e.g. "dry ice") can directly turn into a gas: this is called sublimation. If the gas is further heated, its atoms or molecules can become (wholly or partially) ionized, turning the gas into a plasma.

Properties of a gas

#All collisions are perfectly elastic #The gas fills the entire container #The molecules have negligible volume In the gas phase, the atoms or molecules constituting the matter basically move independently, with no forces keeping them together or pushing them apart. Their only interactions are rare and random collisions. The particles move in random directions, at high speeds, whose range is dependent on the temperature and defined by the Maxwell-Boltzmann distribution. Therefore, the gas phase is a completely disordered state. Following the second law of thermodynamics, gas particles will immediately diffuse to homogeneously fill any shape or volume of space that is made available to them. The thermodynamic state of a gas is characterized by its volume, its temperature, which is determined by the average velocity or kinetic energy of the molecules, and its pressure, which is determined by the average velocity and density or number of molecules. These variables are related by the fundamental gas laws, which state that the pressure in an ideal gas is proportional to its temperature and number of molecules, but inversely proportional to its volume. Like liquids and plasmas, gases are fluids: they have the ability to flow and do not tend to return to their former configuration after deformation, although they do have viscosity. Unlike liquids, however, unconstrained gases do not occupy a fixed volume, but expand to fill whatever space they occupy. The kinetic energy per molecule in a gas is the second greatest of the states of matter (after plasma). Because of this high kinetic energy, gas atoms and molecules tend to bounce off of any containing surface and off one another, the more powerfully as the kinetic energy is increased. A common misconception is that the collisions of the molecules with each other is essential to explain gas pressure, but in fact their random velocities are sufficient to define that quantity. Mutual collisions are important only for establishing the Maxwell-Boltzmann distribution. Gas particles are normally well separated, as opposed to liquid particles, which are in contact. A material particle (say a dust mote) in a gas moves in Brownian Motion. Since it is at the limit of (or beyond) current technology to observe individual gas particles (atoms or molecules), only theoretical calculations give suggestions as to how they move, but their motion is different from Brownian Motion. The reason is that Brownian Motion involves a smooth drag due to the frictional force of many gas molecules, punctuated by violent collisions of an individual (or several) gas molecule(s) with the particle. The particle (generally consisting of millions or billions of atoms) thus moves in a jagged course, yet not so jagged as we would expect to find if we could examine an individual gas molecule.

Etymology

The word "gas" was apparently coined in the early 17th century by the Belgian chemist Jan Baptist van Helmont, as a re-spelling of his pronunciation of the Greek word chaos.

Oxygen

Oxygen is a chemical element in the periodic table. It has the symbol O and atomic number 8. The element is very common, found not only on Earth but throughout the universe, usually covalently bonded with other elements. Unbound oxygen (usually called molecular oxygen, O₂, a diatomic molecule) first appeared on Earth during the Paleoproterozoic era (between 2500 million years ago and 1600 million years ago) and as a product of the metabolic action of early anaerobes (archaea and bacteria). The presence of free oxygen drove most of the organisms then living to extinction. The atmospheric abundance of free oxygen in later geological epochs and up to the present has been largely driven by photosynthetic organisms, roughly three quarters by phytoplankton and algae in the oceans and one quarter from terrestrial plants.

Characteristics

At standard temperature and pressure, oxygen is mostly found as a gas consisting of a diatomic molecule with the chemical formula O₂. O₂ has two energetic forms:
- The low-energy predominant single-bonded diradical triplet oxygen. This native diradical quality of oxygen contributes to its destructive chemical nature. This form is stabilized by the degeneracy effect.
- The high-energy double-bonded molecule singlet oxygen. Oxygen is a major component of air, produced by plants during photosynthesis, and is necessary for aerobic respiration in animals. The word oxygen derives from two words in Greek, οξυς (oxys) (acid, sharp) and γεινομαι (geinomai) (engender). The name "oxygen" was chosen because, at the time it was discovered in the late 18th century, it was believed that all acids contained oxygen. The definition of acid has since been revised to not require oxygen in the molecular structure. Liquid O₂ and solid O₂ have a light blue color and both are highly paramagnetic. Liquid O₂ is usually obtained by the fractional distillation of liquid air. Liquid and solid O₃ (ozone) have a deeper color of blue. A recently discovered allotrope of oxygen, tetraoxygen (O₄), is a deep red solid that is created by pressurizing O₂ to the order of 20 GPa. Its properties are being studied for use in rocket fuels and similar applications, as it is a much more powerful oxidizer than either O₂ or O₃.

Applications

Liquid oxygen finds use as an oxidizer in rocket propulsion. Oxygen is essential to respiration, so oxygen supplementation has found use in medicine (as oxygen therapy). People who climb mountains or fly in airplanes sometimes have supplemental oxygen supplies (as air). Oxygen is used in welding (such as the oxyacetylene torch), and in the making of steel and methanol. Oxygen presents two absorption bands centered in the wavelengths 687 and 760 nanometers. Some scientists have proposed to use the measurement of the radiance coming from vegetation canopies in those oxygen bands to characterize plant health status from a satellite platform. This is because in those bands, it is possible to discriminate the vegetation's reflectance from the vegetation's fluorescence, which is much weaker. The measurement presents several technical difficulties due to the low signal to noise ratio and due to the vegetation's architecture, but it has been proposed as possibility to monitor the carbon cycle from satellite, thus in a global scale. Oxygen, as a mild euphoric, has a history of recreational use that extends into modern times. Oxygen bars can be seen at parties to this day. In the 19th century, oxygen was often mixed with nitrous oxide to promote an analgesic effect; indeed, such a mixture (Entonox) is commonly used in medicine today.

History

Oxygen was first discovered by Michał Sędziwój, Polish alchemist and philosopher in late 16th century. Sędziwój assumed the existence of oxygen by warming nitre (saltpeter). He thought of the gas given off as "the elixir of life". Oxygen was again discovered by the Swedish pharmacist Carl Wilhelm Scheele sometime before 1773, but the discovery was not published until after the independent discovery by Joseph Priestley on August 1, 1774, who called the gas dephlogisticated air (see phlogiston theory). Priestley published his discoveries in 1775 and Scheele in 1777; consequently Priestley is usually given the credit. It was named by Antoine Laurent Lavoisier after Priestley's publication in 1775.

Occurrence

Oxygen is the second most common component of the earth's atmosphere (20.947% by volume).

Compounds

Due to its electronegativity, oxygen forms chemical bonds with almost all other elements (which is the origin of the original definition of oxidation). The only elements to escape the possibility of oxidation are a few of the noble gases. The most famous of these oxides is dihydrogen monoxide, or water (H₂O). Other well known examples include compounds of carbon and oxygen, such as carbon dioxide (CO₂), alcohols (R-OH), aldehydes, (R-CHO), and carboxylic acids (R-COOH). Oxygenated radicals such as chlorates (ClO₃⁻), perchlorates (ClO₄⁻), chromates (CrO₄²⁻), dichromates (Cr₂O₇²⁻), permanganates (MnO₄⁻), and nitrates (NO₃⁻) are strong oxidizing agents in and of themselves. Many metals such as iron bond with oxygen atoms, iron (III) oxide (Fe₂O₃). Ozone (O₃) is formed by electrostatic discharge in the presence of molecular oxygen. A double oxygen molecule (O₂)₂ is known and is found as a minor component of liquid oxygen. Epoxides are ethers in which the oxygen atom is part of a ring of three atoms.

Isotopes

Oxygen has fifteen known isotopes with atomic masses ranging from 12 to 26. Three of them are stable and twelve are radioactive. The radioisotopes all have half lives of less than three minutes. The stable isotopes have mass numbers of 16, 17 and 18, of which oxygen-16 is the most common (over 99%).

Precautions

Oxygen can be toxic at elevated partial pressures (i.e. high relative concentrations). This is important in some forms of scuba diving, such as with a rebreather. Certain derivatives of oxygen, such as ozone (O₃), singlet oxygen, hydrogen peroxide, hydroxyl radicals and superoxide, are also highly toxic. The body has developed mechanisms to protect against these toxic species. For instance, the naturally-occurring glutathione can act as an antioxidant, as can bilirubin which is normally a breakdown product of hemoglobin. Highly concentrated sources of oxygen promote rapid combustion and therefore are fire and explosion hazards in the presence of fuels. This is true as well of compounds of oxygen such as chlorates, perchlorates, dichromates, etc. Compounds with a high oxidative potential can often cause chemical burns. The fire that killed the Apollo 1 crew on a test launchpad spread so rapidly because the pure oxygen atmosphere was at normal atmospheric pressure instead of the one third pressure that would be used during an actual launch. (See partial pressure.) Oxygen derivatives are prone to form free radicals, especially in metabolic processes. Because they can cause severe damage to cells and their DNA, they are thought to be related to cancer and aging.

References

- [http://periodic.lanl.gov/elements/8.html Los Alamos National Laboratory – Oxygen]
- [http://physics.nist.gov/cgi-bin/AtData/main_asd Nist atomic spectra database]
- [http://chartofthenuclides.com/default.html Nuclides and Isotopes Fourteenth Edition]: Chart of the Nuclides, General Electric Company, 1989

External links

- [http://www.priestleysociety.net Priestley Society, Dedicated to Joseph Priestley the man who discovered oxygen]
- [http://www.best-home-remedies.com/minerals/oxygen.htm Oxygen - Benefits, Deficiency Symptoms And Food Sources]
- [http://www.josephpriestley.info Joseph Priestley Information Website, about the man who discovered oxygen]
- [http://periodic.lanl.gov/elements/8.html Los Alamos National Laboratory – Oxygen]
- [http://www.webelements.com/webelements/elements/text/O/index.html WebElements.com – Oxygen]
- [http://education.jlab.org/itselemental/ele008.html It's Elemental – Oxygen]
- [http://members.tripod.com/tjaartdb0/html/oxygen_toxicity.html Oxygen Toxicity]
- [http://www.uigi.com/oxygen.html Oxygen (O2) Properties, Uses, Applications]
- [http://www.compchemwiki.org/index.php?title=Oxygen Computational Chemistry Wiki]
- [http://koti.mbnet.fi/antitz/dime/en Tests with liquid oxygen :-)] Category:Nonmetals Category:Chalcogens als:Sauerstoff ko:산소 ms:Oksigen ja:酸素 simple:Oxygen th:ออกซิเจน

Solid

A solid is a phase of matter characterized by resistance to deformation and to changes of volume. At the microscopic scale, a solid has these properties:
- The atoms or molecules that comprise the solid are packed close together.
- These constituent elements have fixed positions in space relative to each other. This accounts for the solid's rigidity.
- If sufficient force is applied, either of these properties can be violated, causing permanent deformation.
- Because any solid has some thermal energy, its atoms vibrate. However, this movement is very small and very rapid, and cannot be observed under ordinary conditions. The branch of physics that deals with solids is called solid-state physics, and is a type of condensed matter physics. Materials science is primarily concerned with properties of solids such as strength and phase transformations. It overlaps strongly with solid state physics. Solid-state chemistry overlaps both of these fields, but is especially concerned with the synthesis of novel materials. The lightest known solid is man-made and is called aerogel. The lightest aerogel produced has a density of 1.9 mg per cm³ or 1.9 kg/m³ (526.3 times lighter than water).

Combustion

Combustion or burning is a chemical process, an exothermic reaction between a substance (the fuel) and a gas (the oxidizer), usually O₂, to release heat. In a complete combustion reaction, a compound reacts with an oxidizing element, and the products are compounds of each element in the fuel with the oxidizing element. For example: CH₂S + 6 F₂ → CF₄ + 2 HF + SF₆ + heat

Rapid combustion

Rapid combustion is a form of combustion in which large amounts of heat and light energy are released. This often occurs as a fire. This is used in forms of machinery, such as internal combustion engines, and in fuel-air explosives.

Slower combustion

Slow combustion is a form of combustion which takes place at low temperatures. Respiration is an example of slow combustion.

Complete combustion

In complete combustion, the reactant will burn in oxygen, producing a limited number of products. When a hydrocarbon burns in oxygen, the reaction will only yield carbon dioxide and water. When elements such as carbon, nitrogen, sulfur, and iron are burned, they will yield the most common oxides. Carbon will yield carbon dioxide. Nitrogen will yield nitrogen dioxide. Sulfur will yield sulfur dioxide. Iron will yield iron(III) oxide. Complete combustion is generally impossible to achieve unless the reaction occurs where conditions are carefully controlled (e.g. in a lab environment).

Incomplete combustion

In incomplete combustion there is an inadequate supply of oxygen for the combustion to occur completely. The reactant will burn in oxygen, but will produce numerous products. When a hydrocarbon burns in oxygen, the reaction will yield carbon dioxide, water, carbon monoxide, and various other compounds such as nitrogen oxides. Incomplete combustion is much more common and will produce large amounts of byproducts, and in the case of burning fuel in automobiles, these byproducts can be quite lethal and damaging to the environment

Chemical equation

Generally, the chemical equation for burning a hydrocarbon (such as octane) in oxygen is as follows: :C_xH_y + (x + y/4)O₂ → xCO₂ + (y/2)H₂O For example, the burning of propane is: :C₃H₈ + 5O₂ → 3CO₂ + 4H₂O The simple word equation for the combustion of a hydrocarbon is: :Fuel + Oxygen → Heat + Water + Carbon dioxide.

Combustion phases

The act of combustion consists of three relatively distinct but overlapping phases:
- Preheating phase, when the unburned fuel is heated up to its flash point and then fire point. Flammable gases start being evolved in a process similar to dry distillation.
- Distillation phase or Gaseous phase, when the mix of evolved flammable gases with oxygen is ignited. Energy is produced in the form of heat and light, flame is often visible.
- Charcoal phase or Solid phase, when the output of flammable gases from the material is too low for persistent presence of flame and the charred fuel does not burn rapidly anymore but just glows and later only smoulders.

Combustion temperatures

Assuming perfect combustion conditions, such as an adiabatic (no heat loss) and complete combustion, the adiabatic combustion temperature can be determined. The formula that yields this temperature is based on the first law of thermodynamics and takes note of the fact that the heat of combustion (calculated from the fuel's heating value) is used entirely for warming up fuel and gas (e.g. oxygen or air). In the case of fossil fuels burnt in air, the combustion temperature depends on
- the heating value
- the stoichiometric air ratio

- the heat capacity of fuel and air
- air and fuel inlet temperatures The adiabatic combustion temperature increases for higher heating values and inlet temperatures and stoiciometric ratios towards one. Typically, the adiabatic combustion temperatures for coals are around 1500 deg C (for inlet temperatures of room temperatures and

), around 2000 deg C for oil and 2200 deg C for natural gas.

Organic compound

An organic compound is any member of a large class of chemical compounds whose molecules contain carbon, with the exception of carbides, carbonates, carbon oxides and gases containing carbon.The study of organic compounds is termed organic chemistry. Many of these compounds, such as proteins, fats, and carbohydrates (sugars), are also of prime importance in biochemistry. The dividing line between organic and inorganic is contended and historically arbitrary; generally speaking, however, organic compounds are defined as those compounds which have carbon-hydrogen bonds, and inorganic compounds, those without. Thus carbonic acid is inorganic, whereas formic acid, the first fatty acid, is organic, although it could as well be called "carbonous acid" and its anhydride, carbon monoxide, is inorganic. The name "organic" is a historical name, dating back to 19th century, when it was believed that organic compounds could only be synthesised in living organisms through vis vitalis - the "life-force". The theory that organic compounds were fundamentally different than those that were "inorganic", that is, not synthesized through a life-force, was disproven with the synthesis of urea, an organic compound, from potassium cyanate and ammonium sulfate by Friedrich Wöhler. Most pure organic compounds are artificially produced; however, the term "organic" is also used to describe products produced without artificial chemicals (see organic production).

Microorganism

A microorganism or microbe is an organism that is so small that it is microscopic (invisible to the naked eye). Microorganisms are often illustrated using single-celled, or unicellular organisms; however, some unicellular protists are visible to the naked eye, and some multicellular species are microscopic. The study of microorganisms is called microbiology.

Micro-organisms and unicellular organisms

Micro-organisms may be found almost anywhere in the taxonomic structure. Unicellular organisms carry out all the functions of life. Bacteria and archaea are almost always microscopic. A number of eukaryotes are also microscopic, including most protists and a number of fungi. Unicellular species are those whose members consist of a single cell throughout their life cycle. This latter qualification is significant since most multicellular organisms consist of a single cell at the beginning of their life cycles. Usually, unicellular organisms contain only a single copy of their genome (except when undergoing cell division), although some organisms have multiple cell nuclei (see coenocyte).

Habitats and ecology

Microorganisms are found everywhere in nature. Even in hostile environments, like the poles, deserts, geysers, rocks, and the deep sea, some types of microorganisms have adapted to the extreme conditions and sustained colonies; these organisms are known as extremophiles. Some extremophiles have been known to survive for a prolonged time in a vacuum, and some can be unusually resistant to radiation. Microorganisms are used in brewing, baking, biotechnology, recycling of other organisms' remains and waste products, and many other processes. They can also be harmful as pathogens when, as parasites, they cause infections. Microorganisms have an important place in all ecosystems and in most higher-order multicellular organisms. For mankind they are important because they participate in the earth's element cycles (such as the Carbon cycle), and because of their use in the creation of certain types of food, medicines and biological weapons.

External links

- [http://www.genomenewsnetwork.org/categories/index/microbes.php Microbe News from Genome News Network]
- [http://news.bbc.co.uk/1/hi/sci/tech/1569264.stm BBC News, 28 September, 2001: The microbes that 'rule the world'] Citat: "... The Earth's climate may be dependent upon microbes that eat rock beneath the sea floor, according to new research....The number of the worm-like tracks in the rocks diminishes with depth; at 300 metres (985 feet) below the sea floor, they become much rarer..."
- [http://news.bbc.co.uk/1/hi/sci/tech/827063.stm BBC News, 10 July, 2000, Snow microbes found at South Pole] Citat: "...able to survive the large doses of ultraviolet radiation, extreme cold and darkness...The microbes have DNA sequences similar to a category of bacteria known as Deinococcus..."
- [http://news.bbc.co.uk/1/hi/sci/tech/1764716.stm BBCNews: 16 January, 2002, Tough bugs point to life on Mars] Citat: "...This research demonstrates that certain microbes can thrive in the absence of sunlight by using hydrogen gas..."
- [http://news.bbc.co.uk/1/hi/sci/tech/1765792.stm BBCNews: 17 January, 2002, Alien life could be like Antarctic bugs]
- [http://gsbs.utmb.edu/microbook/toc.htm Microbiology] ko:미생물 ja:微生物 th:จุลินทรีย์

Cellular respiration

Cellular respiration is the process in which the chemical bonds of energy-rich molecules such as glucose are converted into energy usable for life processes. Oxidation of organic material—in a bonfire, for example—is an exothermic reaction that releases a large amount of energy rather quickly. The equation for the oxidation of glucose is: :::C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy released In a fire there is a massive uncontrolled release of energy as light and heat. Cellular respiration is the same process but it occurs in gradual steps that result in the conversion of the energy stored in glucose to usable chemical energy in the form of ATP.

Aerobic respiration

exothermic reaction Aerobic respiration requires oxygen in order to generate energy. It is the preferred method of pyruvate breakdown from glycolysis and requires that pyruvate enter the mitochondrion to be fully oxidised by the Krebs cycle. The product of this process is energy in the form of ATP, by substrate-level phosphorylation, NADH and FADH2. The reducing potential of NADH and FADH2 is converted to more ATP via an electron transport chain with oxygen as the "terminal electron acceptor". Most of the ATP produced by cellular respiration is by oxidative phosphorylation, ATP molecules are made due to the chemiosmotic potential driving ATP synthase. Respiration is the process by which cells obtain energy when oxygen is present in the cell. Theoretically, 36 ATP molecules can be made per glucose during cellular respiration, however, such conditions are generally not realized due to such losses as the cost of moving pyruvate into mitochondria. Aerobic metabolism is rather more efficient than anaerobic metabolism. They share the initial pathway of glycolysis but aerobic metabolism continues with the Krebs cycle and oxydative phosphorylation. The post glycolytic reactions take place in the mitochondria in eukaryotic cells, and at the cell membrane in prokaryotic cells.

Glycolysis

:Main article: Glycolysis Glycolysis is a metabolic pathway that is found in all living organisms and does not require oxygen. The process converts one molecule of glucose into two molecules of pyruvate, and makes energy in the form of two net molecules of ATP. Four molecules of ATP per glucose are actually produced but two are consumed for the preparatory phase. The initial phosphorylation of glucose is required to destablise the molecule for cleavage into two triose sugars. During the pay-off phase of glycolysis four phosphate groups are transfered to ADP by substrate-level phosphorylation to make four ATP and two NADH are produced when the triose sugars are oxidised. Glycolysis takes place in the cytoplasm of the cell. The overall reaction can be expressed this way: :Glucose + 2 ATP + 2 NAD⁺ + 2 P_i + 4 ADP → 2 pyruvate + 2 ADP + 2 NADH + 4 ATP + 2 H₂O + 4 H⁺

Oxidative decarboxylation

:Main Article: Oxidative decarboxylation Produces acetyl-CoA from pyruvate. This oxidation reaction also releases carbon dioxide as a product.

Krebs cycle/Citric Acid cycle

:Main article: Citric acid cycle When oxygen is present, acetyl-CoA enters the citric acid cycle, and gets oxidised to CO2 while at the same time reducing NAD to NADH. NADH can be used by the electron tranport chain to create further ATP as part of oxidative phosphorylation.

Oxidative phosphorylation

In eukaryotes, oxidative phosphorylation ocurrs in the mitochondria. It comprises of the electron transport chain that establishes a proton gradient (chemiosmotic potential) across the inner membrane by oxidising the NADH produced from the Krebs cycle. ATP is synthesised by the ATP synthase enzyme when the chemiosmotic gradient is used to drive the phosphorylation of ADP.

Theoretical yields

The yields in the table below are for one glucose and molecule being fully oxidised to carbon dioxide. It is assumed that all the reduced coenzymes are oxidised by the electron transport chain and used for oxidative phosphorylation.

Anaerobic respiration

In the absence of oxygen pyruvate is not metabolized by cellular respiration, but undergoes fermentation.

External links

- [http://www.people.virginia.edu/~rjh9u/glycol.html A detailed diagram of glycolysis]
- [http://departments.oxy.edu/biology/bio130/lectures_2000/metabolic_products.htm Chart of Important Metabolic Products]
- [http://www.ufp.pt/~pedros/bq/respi.htm A detailed description of respiration vs. fermentation]
- [http://www.ufp.pt/~pedros/anim/2frame-iien.htm Interactive Molecular models of electron-transfer complexes] Category:Cellular respiration Category:Metabolism ja:呼吸

Plant

- Land plants (embryophytes)
- Non-vascular plants (bryophytes)
- Marchantiophyta - liverworts
- Anthocerotophyta - hornworts
- Bryophyta - mosses
- Vascular plants (tracheophytes)
- Lycopodiophyta - clubmosses
- Equisetophyta - horsetails
- Pteridophyta - "true" ferns
- Psilotophyta - whisk ferns
- Ophioglossophyta - adderstongues
- Seed plants (spermatophytes)
- †Pteridospermatophyta - seed ferns
- Pinophyta - conifers
- Cycadophyta - cycads
- Ginkgophyta - ginkgo
- Gnetophyta - gnetae
- Magnoliophyta - flowering plants Magnoliophyta Plants are a major group of living things (about 300,000 species), including familiar organisms such as trees, flowers, herbs, and ferns. Aristotle divided all living things between plants, which generally do not move or have sensory organs, and animals. In Linnaeus' system, these became the Kingdoms Vegetabilia (later Plantae) and Animalia. Since then, it has become clear that the Plantae as originally defined included several unrelated groups, and the fungi and several groups of algae were removed to new kingdoms. However, these are still often considered plants in many contexts. Indeed, any attempt to match "plant" with a single taxon is doomed to fail, because plant is a vaguely defined concept unrelated to the presumed phylogenic concepts on which modern taxonomy is based.

Embryophytes

:See main article at Embryophytes Most familiar are the multicellular land plants, called embryophytes. They include the vascular plants, plants with full systems of leaves, stems, and roots. They also include a few of their close relatives, often called bryophytes, of which mosses and liverworts are the most common. All of these plants have eukaryotic cells with cell walls composed of cellulose, and most obtain their energy through photosynthesis, using light and carbon dioxide to synthesize food. About three hundred plant species do not photosynthesize but are parasites on other species of photosynthetic plants. Plants are distinguished from green algae, from which they evolved, by having specialized reproductive organs protected by non-reproductive tissues. Bryophytes first appeared during the early Palaeozoic. They can only survive where moisture is available for significant periods, although some species are desiccation tolerant. Most species of bryophyte remain small throughout their life-cycle. This involves an alternation between two generations: a haploid stage, called the gametophyte, and a diploid stage, called the sporophyte. The sporophyte is short-lived and remains dependent on its parent gametophyte. Vascular plants first appeared during the Silurian period, and by the Devonian had diversified and spread into many different land environments. They have a number of adaptations that allowed them to overcome the limitations of the bryophytes. These include a cuticle resistant to desiccation, and vascular tissues which transport water throughout the organism. In most the sporophyte acts as a separate individual, while the gametophyte remains small. Devonians (Pteridophyta) more closely allied to seed plants than they are to clubmosses (Lycopodiophyta)]] The first primitive seed plants, Pteridosperms (seed ferns) and Cordaites, both groups now extinct, appeared in the late Devonian and diversified through the Carboniferous, with further evolution through the Permian and Triassic periods. In these the gametophyte stage is completely reduced, and the sporophyte begins life inside an enclosure called a seed, which develops while on the parent plant, and with fertilisation by means of pollen grains. Whereas other vascular plants, such as ferns, reproduce by means of spores and so need moisture to develop, some seed plants can survive and reproduce in extremely arid conditions. Early seed plants are referred to as gymnosperms (naked seeds), as the seed embryo is not enclosed in a protective structure at pollination, with the pollen landing directly on the embryo. Four surviving groups remain widespread now, particularly the conifers, which are dominant trees in several biomes. The angiosperms, comprising the flowering plants, were the last major group of plants to appear, emerging from within the gymnosperms during the Jurassic and diversifying rapidly during the Cretaceous. These differ in that the seed embryo is enclosed, so the pollen has to grow a tube to penetrate the protective seed coat; they are the predominant group of flora in most biomes today.

Algae and Fungi

The algae comprise several different groups of organisms that produce energy through photosynthesis. However, they are not classified within the kingdom plantae but in the kingdom protista instead. The most conspicuous are the seaweeds, multicellular algae that often closely resemble terrestrial plants, but as stated above are not plants, found among the green, red, and brown algae. These and other algal groups also include various single-celled creatures and forms that are simple collections of cells, without differentiated tissues. Many can move about, and some have even lost their ability to photosynthesize; when first discovered, these were considered as both plants and animals. Now they are considered neither, but protists. The embryophytes developed from green algae; the two are collectively referred to as the green plants or Viridiplantae. The kingdom Plantae is now usually taken to mean this monophyletic group, as shown above. With a few exceptions among the green algae, all such forms have cell walls containing cellulose and chloroplasts containing chlorophylls a and b, and store food in the form of starch. They undergo closed mitosis without centrioles, and typically have mitochondria with flat cristae. The chloroplasts of green plants are surrounded by two membranes, suggesting they originated directly from endosymbiotic cyanobacteria. The same is true of the red algae, and the two groups are generally believed to have a common origin. In contrast, most other algae have chloroplasts with three or four membranes. They are not in general close relatives of the green plants, acquiring chloroplasts separately from ingested or symbiotic green and red algae. Unlike embryophytes and algae, fungi are not photosynthetic, but are saprophytes: they obtain their food by breaking down and absorbing surrounding materials. Most fungi are formed by microscopic tubes called hyphae, which may or may not be divided into cells but contain eukaryotic nuclei. Fruiting bodies, of which mushrooms are the most familiar, are actually only the reproductive structures of fungi. They are not related to any of the photosynthetic groups, but are close relatives of animals. Therefore, fungus has a kingdom of its own.

Importance

The photosynthesis and carbon fixation conducted by land plants and algae are the ultimate source of energy and organic material in nearly all habitats. These processes also radically changed the composition of the Earth's atmosphere, which as a result contains a large proportion of oxygen. Animals and most other organisms are aerobic, relying on oxygen; those that do not are confined to relatively few, anaerobic environments. Much of human nutrition depends on cereals. Other plants that are eaten include fruits, vegetables, herbs, and spices. Some vascular plants, referred to as trees and shrubs, produce woody stems and are an important source of building material. A number of plants are used decoratively, including a variety of flowers.

Growth

It is a common misconception that most of the solid material in a plant is taken from the soil, when in fact almost all of it is actually taken from the air. Through a process known as photosynthesis, plants use the energy in sunlight to convert carbon dioxide from the air into simple sugars. These sugars are then used as building blocks and form the main structural component of the plant. Plants rely on soil primarily for water (in quantitative terms), but also obtain nitrogen, phosphorus and other crucial nutrients. phosphorus Simple plants like algae may have short life spans as individuals, but their populations are commonly seasonal. Other plants may be organized according to their seasonal growth pattern:
- Annual: live and reproduce within one growing season.
- Biennial: live for two growing seasons; usually reproduce in second year.
- Perennial: live for many growing seasons; continue to reproduce once mature. Among the vascular plants, perennials include both evergreens that keep their leaves the entire year, and deciduous plants which lose their leaves for some part. In temperate and boreal climates, they generally lose their leaves during the winter; many tropical plants lose their leaves during the dry season. The growth rate of plants is extremely variable. Some mosses grow less than 0.001 mm/h, while most trees grow 0.025-0.250 mm/h. Some climbing species, such as kudzu, which do not need to produce thick supportive tissue, may grow up to 12.5 mm/h.

Fossils

Plant fossils include roots, wood, leaves, seeds, fruit, pollen, spores, phytoliths, and amber (the fossilized resin produced by some plants). Fossil land plants are recorded in terrestrial, lacustrine, fluvial and nearshore marine sediments. Pollen, spores and algae (dinoflagellates and acritarchs) are used for dating sedimentary rock sequences. The remains of fossil plants are not as common as fossil animals, although plant fossils are locally abundant in many regions worldwide. Early fossils of these ancient plants show the individual cells within the plant tissue. The Devonian period also saw the evolution of what many believe to be the first modern tree, Archaeopteris. This fern-like tree combined a woody trunk with the fronds of a fern, but produced no seeds. Archaeopteris The Coal Measures are a major source of Palaeozoic plant fossils, with many groups of plants in existence at this time. The spoil heaps of coal mines are the best places to collect; coal itself is the remains of fossilised plants, though structural detail of the plant fossils is rarely visible in coal. In the Fossil Forest at Victoria Park in Glasgow, Scotland, the stumps of Lepidodendron trees are found in their original growth positions. The fossilized remains of conifer and angiosperm roots, stems and branches may be locally abundant in lake and inshore sedimentary rocks from the Mesozoic and Caenozoic eras. Sequoia and its allies, magnolia, oak, and palms are often found. Petrified wood is common in some parts of the world, and is most frequently found in arid or desert areas were it is more readily exposed by erosion. Petrified wood is often heavily silicified (the organic material replaced by silicon dioxide), and the impregnated tissue is often preserved in fine detail. Such specimens may be cut and polished using lapidary equipment. Fossil forests of petrified wood have been found in all continents. Fossils of seed ferns such as Glossopteris are widely distributed throughout several continents of the southern hemisphere, a fact that gave support to Alfred Wegener's early ideas regarding Continental drift theory.

Distribution

References and further reading

- Kenrick, Paul & Crane, Peter R. (1997). The Origin and Early Diversification of Land Plants: A Cladistic Study. Washington, D. C.: Smithsonian Institution Press. ISBN 1-56098-730-8.
- Raven, Peter H., Evert, Ray F., & Eichhorn, Susan E. (2005). Biology of Plants (7th ed.). New York: W. H. Freeman and Company. ISBN 0-7167-1007-2.
- Taylor, Thomas N. & Taylor, Edith L. (1993). The Biology and Evolution of Fossil Plants. Englewood Cliffs, NJ: Prentice Hall. ISBN 0-13-651589-4.

External links

- [http://tolweb.org/tree?group=Green_plants&contgroup=Eukaryotes Tree of Life]
- Chaw, S.-M. et al. [http://mbe.library.arizona.edu/data/1997/1401/7chaw.pdf Molecular Phylogeny of Extant Gymnosperms and Seed Plant Evolution: Analysis of Nuclear 18s rRNA Sequences (pdf file)] Molec. Biol. Evol. 14 (1): 56-68. 1997.
- [http://florabase.calm.wa.gov.au/phylogeny/cronq88.html Interactive Cronquist classification]

Botanical and vegetation databases

- [http://www.efloras.org/index.aspx e-Floras (Flora of China, Flora of North America and others)]
- [http://plants.usda.gov/ United States of America]
- [http://rbg-web2.rbge.org.uk/FE/fe.html Flora Europaea]
- [http://www.anbg.gov.au/cpbr/databases/ Australia]
- [http://davesgarden.com/pdb/ 'Dave's Garden' horticultural plant database]
- [http://www.chilebosque.cl Chilean plants at Chilebosque] Category:Plants Category:Plant_taxonomy zh-min-nan:Si̍t-bu̍t ko:식물 ms:Tumbuhan ja:植物 simple:Plant th:พืช

Photosynthesis

Photosynthesis is an important biochemical process in which plants, algae, and some bacteria harness the energy of sunlight to produce food. Ultimately, nearly all living things depend on energy produced from photosynthesis for their nourishment, making it vital to life on Earth. It is also responsible for producing the oxygen that makes up a large portion of the Earth's atmosphere. Organisms that produce energy through photosynthesis are called photoautotrophs.

Plant photosynthesis

Plants are photoautotrophs, which means they are able to synthesize food directly from inorganic compounds using light energy, instead of eating other organisms or relying on material derived from them. This is distinct from chemoautotrophs that do not depend on light energy, but use energy from inorganic compounds. The energy for photosynthesis ultimately comes from absorbed photons and involves a reducing agent, which is water in the case of plants, releasing oxygen as a waste product. The light energy is converted to chemical energy, in the form of ATP and NADPH, using the light-dependent reactions and is then available for carbon fixation. Most notably plants use the chemical energy to fix carbon dioxide into carbohydrates and other organic compounds through light-independent reactions. The overall equation for photosynthesis in green plants is: :n CO₂ + 2n H₂O + light energy → (CH₂O)_n + n O₂ + n H₂O Where n is defined according to the structure of the resulting carbohydrate. However, hexose sugars and starch are the primary products, so the following generalised equation is often used to represent photosynthesis: :6 CO₂ + 12 H₂O + light energy → C₆H₁₂O₆ + 6 O₂ + 6 H₂O More specifically, photosynthetic reactions usually produces an intermediate product, which is then converted to the final hexose carbohydrate products. These carbohydrate products are then variously used to form other organic compounds, such as the building material cellulose, as precursors for lipid and amino acid biosynthesis or as a fuel in cellular respiration. The latter not only occurs in plants, but also in animals when the energy from plants get passed through a food chain. In general outline, cellular respiration is the opposite of photosynthesis: glucose and other compounds are oxidised to produce carbon dioxide, water, and chemical energy. However, both processes actually take place through a different sequence of reactions and in different cellular compartments. food chain Plants capture light primarily using the pigment chlorophyll, which is the reason that most plants have a green color. The function of chlorophyll is often supported by other accessory pigments such as carotenoids and xanthophylls. Both chlorophyll and accessory pigments are contained in organelles (compartments within the cell) called chloroplasts. Although all cells in the green parts of a plant have chloroplasts, most of the energy is captured in the leaves. The cells in the interior tissues of a leaf, called the mesophyll, contain about half a million chloroplasts for every square millimeter of leaf. The surface of the leaf is uniformly coated with a water-resistant, waxy cuticle, that protects the leaf from excessive evaporation of water as well as decreasing the absorption of ultraviolet or blue light to reduce heating. The transparent, colourless epidermis layer allows light to pass through to the palisade mesophyll cells where most of the photosynthesis takes place.

Photosynthesis in algae and bacteria

Algae range from multicellular forms like kelp to microscopic, single-celled organisms. Although they are not as complex as land plants, photosynthesis takes place biochemically the same way. Light is absorbed by chlorophyll, although various accessory pigments to give them a wide variety of colours, located inside chloroplasts. All algae produce oxygen, and many are autotrophic. However, some are heterotrophic, relying on materials produced by other organisms. Photosynthetic bacteria do not have chloroplasts. Instead, photosynthesis takes place directly within the cell. The cyanobacteria contain chlorophyll and oxygen, in the same way that chloroplasts do, in fact chloroplasts are now considered to have evolved from cyanobacteria by endosymbiosis. The other photosynthetic bacteria have a variety of different pigments, called bacteriochlorophylls, and do not produce oxygen.

Molecular production

Light-dependent reaction

bacteriochlorophyll bacteriochlorophyll The products of the light dependent reactions are ATP from photophosphorylation and NADPH from photoreduction. Both are then utilized as an energy source for the light-independent reactions.

Z scheme

In plants, the light-dependent reactions occur in the thylakoid membranes of the chloroplasts and use light energy to sythesize ATP and NADPH. The photons are captured in the antenna complexes of photosystem I and II by chlorophyll and accessory pigments (see diagram at right). When a chorophyll a molecule at a photosystems reaction center absorbs energy, an electron is excited and transferred to an electron-acceptor molecule through a process called Photoinduced charge separation. These electrons are shuttled through an electron transport chain that initially functions to generate a chemiosmotic potential across the membrane, the so called Z-scheme shown in the diagram. An ATP synthase enzyme uses the chemiosmotic potential to make ATP during photophosphorylation while NADPH is a product of the terminal redox reaction in the Z-scheme.

Water photolysis

The NADPH is the main reducing agent in chloroplasts, providing a source of energetic electrons to other reactions. Its production leaves chlorophyll with a deficit of electrons (oxidized), which must be obtained from some other reducing agent. The excited electrons lost from chlorophyll in photosystem I are replaced from the electron transport chain by plastocyanin. However, since photosystem II includes the first steps of the Z-scheme, an external source of electrons is required to reduce its oxidized chlorophyll a molecules. This role is played by water during a reaction known as photolysis and results in water being split to give electrons, oxygen and hydrogen ions. Photosystem II is the only known biological enzyme that carries out this oxidation of water. Initially, the hydrogen ions from photolysis contribute to the chemiosmotic potential but eventually they combine with the hydrogen carrier molecule NADP⁺ to form NADPH. Oxygen is a waste product of photosynthesis but it has a vital role for all organisms that use it for cellular respiration.

Oxygen and photosynthesis

With respect to oxygen and photosynthesis, there are two important concepts.
- Plant and algal cells also use oxygen for cellular respiration, although they have a net output of oxygen since much more is produced during photosynthesis.
- Oxygen is a product of the photolysis reaction not the fixation of carbon dioxide during the light-independent reactions. Consequently, the source of oxygen during photosynthesis is water, NOT carbon dioxide.

Bacterial variations

The second concept was first proposed by Cornelis Bernadus van Neil in the 1930s, who studied photosynthetic bacteria. Aside from the cyanobacteria, bacteria only have one photosystem and use reducing agents other than water. They get electrons from a variety of different inorganic chemicals including sulfide or hydrogen, so for most of these bacteria oxygen is not produced. Others, such as the halophiles (an Archeae) produced so called purple membranes where the bacteriorhodopsin could harvest light and produce energy. The purple membranes was one of the first to be used to demonstrate the chemiosmotic theory: light hit the membranes and the pH of the solution that contained the purple membranes dropped as protons were pumping out of the membrane.

Light-independent reaction

The fixation of carbon dioxide is a light-independent process in which carbon dioxide combines with a five-carbon sugar, ribulose bisphosphate (RuBP), to give two molecules of a three-carbon compound, glycerate 3-phosphate (GP). This compound is also sometimes known as 3-phosphoglycerate (PGA). GP, in the presence of ATP and NADPH from the light-dependent stages, is reduced to glyceraldehye 3-phosphate (G3P). This product is also referred to as 3-phosphoglyceraldehyde (PGAL) or even as triose phosphate (a three-carbon sugar). This is the point at which carbohydrates are produced during photosynthesis. Some of the triose phosphates condense to form hexose phosphates, sucrose, starch and cellulose or are converted to acetylcoenzyme A to make amino acids and lipids. Others go on to regenerate RuBP so the process can continue (see Calvin Cycle).

Discovery

Although some of the steps in photosynthesis are still not completely understood, the overall photosynthetic equation has been known since the 1800s. Jan van Helmont began the research of the process in the mid-1600s when he carefully measured the mass of the soil used by a plant and the mass of the plant as it grew. After noticing that the soil mass changed very little, he hypothesized that the mass of the growing plant must come from the water, the only substance he added to the potted plant. This was a partially accurate hypothesis - much of the gained mass also comes from carbon dioxide as well as water. However, this was a signalling point to the idea that the bulk of a plant's biomass comes from the inputs of photosynthesis, not the soil itself. Joseph Priestley, a chemist and minister, discovered that when he isolated a volume of air under an inverted jar, and burned a candle in it, the candle would burn out very quickly, much before it ran out of wax. He further discovered that a mouse could similarly "injure" air. He then showed that the air that had been "injured" by the candle and the mouse could be restored by a plant. In 1778, Jan Ingenhousz, court physician to the Austrian Empress, repeated Priestley's experiments. He discovered that it was the influence of sun and light on the plant that could cause it to rescue a mouse in a matter of hours. In 1796, Jean Senebier, a French pastor, showed that CO₂ was the "fixed" or "injured" air and that it was taken up by plants in photosynthesis. Soon afterwards, Nicolas-Théodore de Saussure showed that the increase in mass of the plant as it grows could not be due only to uptake of CO₂, but also to the incorporation of water. Thus the basic reaction by which photosynthesis is used to produce food (such as glucose) was outlined. Modern scientists built on the foundation of knowledge from those scientists centuries ago and were able to discover many things. Cornelius Van Niel made key discoveries explaining the chemistry of photosynthesis. By studying purple sulfur bacteria and green bacteria he was the first scientist to demonstrate that photosynthesis is a light-dependent redox reaction, in which hydrogen reduces carbon dioxide. Further experiments to prove that the oxygen developed during the photosynthesis of green plants came from water, were performed by Robert Hill in 1937 and 1939. He showed that isolated chloroplasts give off oxygen in the presence of unnatural reducing agents like iron oxalate, ferricyanide or benzoquinone after exposure to light. The Hill reaction is as follows: :2 H₂O + 2 A + (light, chloroplasts) → 2 AH₂