Shopping on line can be easy, simple and save you lots of money. It can also take a lot of your time, frustrate you, and result in unwanted purchases. Now the same can be said for regular high street shopping, but with the vast opportunity presented by the Internet it will pay you to spend a few minutes reading this and understanding how to better optimize your Photosynthesis shopping experience:

1. Compare - without doubt the biggest advantage that the Photosynthesis offers shoppers today is the ability to compare thousands of Photosynthesis at a time. This is a great thing, but not necessarily all the time! Too much can be daunting at times so take advantage of the great comparison sites and where possible let them do the hard work for you.

2. Research - if it has been said it will be on the internet. Ignorance is no longer a justifiable reason for buying the wrong thing. Take the time to research in detail everything that you could possible want to know about

3. Testimonials - don't know anybody that has bought a Photosynthesis? Wrong! If the Photosynthesis is good the internet will let you know. Use the Internet as a friend and get testimonials before you buy.

4. Questions - Got a question about Photosynthesis then search the Forums, FAQ's, Blogs etc. Don't be afraid to ask .....

5. Reputation - Never heard of the company selling Photosynthesis? Don't worry, no reason why you should know every company in the world, but you know someone that does! Use the internet to find out what people are saying about Photosynthesis and build up a picture of their reputation for sales, returns, customer service, delivery etc.

6. Returns - still worried that even after all of the above your Photosynthesis wont be what you want? Check out the returns policy. There is so much competition now that someone, somewhere is bound to offer the terms that you are comfortable with.

7. Feedback - happy with your Photosynthesis then let people know, after all you are depending on others people input in your buying decision, so why not give a little back.

8. Security - check for the yellow padlock on the Photosynthesis site before you buy, and the s after http:/ /i.e. https:// = a secure site

9. Contact - got a question about Photosynthesis, or want to leave a comment then check out the sites contact page. Reputable companies have them and respond.

10. Payment - ready to pay for your Photosynthesis, then use your credit card or PayPal! Be aware of companies that don't accept them, there may be genuine reasons but given the huge amount of choice you have when buying online there is no reason at all not to buy via credit card or PayPal.

is the primary site of photosynthesis in plants.Photosynthesis is the conversion of light energy into chemical energy by living organisms. The raw materials are carbon dioxide and water, the energy source is sunlight, and the end-products include glucose and oxygen. It is arguably the most important biochemical pathway,

Photosynthetic bacteria do not have chloroplasts (or any membrane-bound organelles). Instead, photosynthesis takes place directly within the cell. Cyanobacteria contain thylakoid membranes very similar to those in chloroplasts and are the only prokaryotes that perform oxygen-generating photosynthesis. In fact chloroplasts are now considered to have evolution from an endosymbiosis bacterium, which was also an ancestor of and later gave rise to cyanobacterium. The other photosynthetic bacteria have a variety of different pigments, called bacteriochlorophylls, and do not produce oxygen. Some bacteria, such as Chromatium, oxidize hydrogen sulfide instead of water for photosynthesis, producing sulfur as waste.

Evolution The ability to convert light energy to chemical energy confers a significant Natural selection to living organisms. Early photosynthetic systems, such as those from Green sulfur bacteria and Purple sulfur bacteria and Chloroflexi and purple bacteria, are thought to have been anoxygenic, using various molecules as electron donors. Green and purple sulfur bacteria are thought to have used hydrogen and sulfur as an electron donor. Green nonsulfur bacteria used various amino acid and other organic acids. Purple nonsulfur bacteria used a variety of non-specific organic molecules. The use of these molecules is consistent with the geological evidence that the atmosphere was highly Reducing environment at History of Earth#The Hadean eon.

Fossils of what are thought to be filamentous photosynthetic organisms have been dated at 3.4 billion years old. New Scientist, 19 Aug., 2006

Oxygen in the Earth's atmosphere exists due to the evolution of Oxygen evolution, 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 Redox into molecular oxygen by the absorption of a photon by the photosynthetic reaction centre.

Origin of chloroplasts In plants the process of photosynthesis occurs in organelles called chloroplasts. Chloroplasts have many similarities with cyanobacteria including a circular chromosome, prokaryotic-type ribosomes, and similar proteins in the photosynthetic reaction centre.

The endosymbiotic theory suggests that photosynthetic bacteria were acquired (by endocytosis or gene fusion) by early Eukaryote cells to form the first plant cells. In other words, chloroplasts may simply be primitive photosynthetic bacteria adapted to life inside plant cells, while plants themselves have not actually evolved photosynthetic processes on their own. Another example of this can be found in complex animals, including humans, whose cells depend upon mitochondria as their energy source; mitochondria are thought to have evolved from endosymbiotic bacteria, related to modern Rickettsia bacteria. Both chloroplasts and mitochondria actually have their own DNA, separate from the nuclear DNA of their animal or plant host cells.

This contention is supported by the finding that the marine Molluscas Elysia viridis and Elysia chlorotica seem to maintain a symbiosis relationship with chloroplasts from algae with similar RDA structures that they encounter. However, they do not transfer these chloroplasts to the next generations.

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

Molecular production Light to chemical energy The light energy is converted to chemical energy using the light-dependent reactions. This chemical energy production is more than 90% efficient with only 5-8% of the energy transferred thermally. The products of the light-dependent reactions are adenosine triphosphate from photophosphorylation and NADPH from photoreduction. Both are then utilized as an energy source for the light-independent reactions.

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

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

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

Quantum mechanical effects Through photosynthesis, sunlight energy is transferred to molecular reaction centers for conversion into chemical energy with nearly 100-percent efficiency. The transfer of the solar energy takes place almost instantaneously, so little energy is wasted as heat.

A study led by researchers with the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) and the University of California at Berkeley suggests that long-lived wavelike electronic quantum coherence plays an important part in this instantaneous transfer of energy by allowing the photosynthetic system to simultaneously try each potential energy pathway and choose the most efficient option. Results of the study are presented in the April 12, 2007 issue of the journal Nature.Lawrence Berkeley National Lab. "Quantum secrets of photosynthesis revealed", physorg.com, April 12, 2007. Accessed April 13, 2007.

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

Bacterial variation The concept that oxygen production is not directly associated with the fixation of carbon dioxide was first proposed by Cornelis Van Niel in the 1930s, who studied photosynthetic bacteria. Aside from the cyanobacteria, bacteria only have one photosystem and use reducing agents other than water. They get electrons from a variety of different inorganic chemicals including sulfide or hydrogen, so for most of these bacteria oxygen is not produced.

Others, such as the halophiles (an Archaea) produced so called purple membranes where the bacteriorhodopsin could harvest light and produce energy. The purple membranes was one of the first to be used to demonstrate the chemiosmosis 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.

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

C4, C3 and CAM In hot and dry conditions, plants will close their stomata to prevent loss of water. Under these conditions, oxygen gas, produced by the light reactions of photosynthesis, will concentrate in the leaves causing photorespiration to occur. Some plants have evolution mechanisms to increase the CO2 concentration in the leaves under these conditions.

C4 carbon fixation capture carbon dioxide using an enzyme called PEP Carboxylase that adds carbon dioxide to the three carbon molecule Phosphoenolpyruvate creating the 4 carbon molecule oxaloacetic acid. Plants without this enzyme are called C3 carbon fixation because the primary carboxylation reaction produces the three carbon sugar 3-phosphoglycerate directly in the Calvin-Benson Cycle. When oxygen levels rise in the leaf, C4 plants reverse the reaction to release carbon dioxide thus preventing photorespiration. By preventing photorespiration, C4 plants can produce more sugar than C3 plants in conditions of strong light and high temperature. Many important crop plants are C4 plants including maize, sorghum, sugarcane, and millet.

Xerophytes such as Cacti and most succulents also can use PEP Carboxylase to capture carbon dioxide in a process called CAM photosynthesis. They store the CO2 in different molecules than the C4 plants (mostly they store it in the form of malic acid via carboxylation of phosphoenolpyruvate to oxaloacetate which is then reduced to malate). Nevertheless, C4 plants capture the CO2 in one type of cell tissue (mesophyll) and then transfer it to another type of tissue (bundle sheath cells) so that carbon fixation may occur via the Calvin cycle. They also have a different leaf anatomy than C4 plants. They grab the CO2 at night when their stomata are open, and they release it into the leaves during the day to increase their photosynthetic rate. C4 metabolism physically separates CO2 fixation from the Calvin cycle, while CAM metabolism temporally separates CO2 fixation from the 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. 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 (ecology) comes from the inputs of photosynthesis, not the soil itself.

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

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

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

Modern scientists built on the foundation of knowledge from those scientists centuries ago and were able to discover many things.

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

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

2 H2O + 2 A + (light, chloroplasts) → 2 AH2 + O2

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

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

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

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

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

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

These two experiments illustrate vital points: firstly, from research it is known that photochemical reactions are not generally affected by temperature. However, these experiments clearly show that temperature affects the rate of carbon assimilation, so there must be two sets of reactions in the full process of carbon assimilation. These are of course the Light-dependent reaction stage and the Light-independent reaction stage. Secondly, Blackman's experiments illustrate the concept of limiting factors. Another limiting factor is the wavelength of light. Cyanobacteria which reside several meters underwater cannot receive the correct wavelengths required to cause photoinduced charge separation in conventional photosynthetic pigments. To combat this problem a series of proteins with different pigments surround the reaction center. This unit is called a phycobilisome.

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

RuBisCO oxygenase activity is disadvantageous to plants for several reasons:
  • One product of oxygenase activity is phosphoglycolate (2 carbon) instead of 3-phosphoglycerate (3 carbon). Phosphoglycolate cannot be metabolized by the Calvin-Benson cycle and represents carbon lost from the cycle. A high oxygenase activity, therefore, drains the sugars that are required to recycle ribulose 5-bisphosphate and for the continuation of the Calvin-Benson cycle.
  • Phosphoglycolate is quickly metabolized to glycolate that is toxic to a plant at a high concentration; it inhibits photosynthesis.
  • Salvaging glycolate is an energetically expensive process that uses the glycolate pathway and only 75% of the carbon is returned to the Calvin-Benson cycle as 3-phosphoglycerate.


    • :A highly simplified summary is:

    ::2 glycolate + ATP → 3-phophoglycerate + carbon dioxide + ADP +NH3

    The salvaging pathway for the products of RuBisCO oxygenase activity is more commonly known as photorespiration since it is characterized by light dependent oxygen consumption and the release of carbon dioxide.

    See also

    Notes

    References

    External links

    is the primary site of photosynthesis in plants.Photosynthesis is the conversion of light energy into chemical energy by living organisms. The raw materials are carbon dioxide and water, the energy source is sunlight, and the end-products include glucose and oxygen. It is arguably the most important biochemical pathway,

    Photosynthetic bacteria do not have chloroplasts (or any membrane-bound organelles). Instead, photosynthesis takes place directly within the cell. Cyanobacteria contain thylakoid membranes very similar to those in chloroplasts and are the only prokaryotes that perform oxygen-generating photosynthesis. In fact chloroplasts are now considered to have evolution from an endosymbiosis bacterium, which was also an ancestor of and later gave rise to cyanobacterium. The other photosynthetic bacteria have a variety of different pigments, called bacteriochlorophylls, and do not produce oxygen. Some bacteria, such as Chromatium, oxidize hydrogen sulfide instead of water for photosynthesis, producing sulfur as waste.

    Evolution The ability to convert light energy to chemical energy confers a significant Natural selection to living organisms. Early photosynthetic systems, such as those from Green sulfur bacteria and Purple sulfur bacteria and Chloroflexi and purple bacteria, are thought to have been anoxygenic, using various molecules as electron donors. Green and purple sulfur bacteria are thought to have used hydrogen and sulfur as an electron donor. Green nonsulfur bacteria used various amino acid and other organic acids. Purple nonsulfur bacteria used a variety of non-specific organic molecules. The use of these molecules is consistent with the geological evidence that the atmosphere was highly Reducing environment at History of Earth#The Hadean eon.

    Fossils of what are thought to be filamentous photosynthetic organisms have been dated at 3.4 billion years old. New Scientist, 19 Aug., 2006

    Oxygen in the Earth's atmosphere exists due to the evolution of Oxygen evolution, 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 Redox into molecular oxygen by the absorption of a photon by the photosynthetic reaction centre.

    Origin of chloroplasts In plants the process of photosynthesis occurs in organelles called chloroplasts. Chloroplasts have many similarities with cyanobacteria including a circular chromosome, prokaryotic-type ribosomes, and similar proteins in the photosynthetic reaction centre.

    The endosymbiotic theory suggests that photosynthetic bacteria were acquired (by endocytosis or gene fusion) by early Eukaryote cells to form the first plant cells. In other words, chloroplasts may simply be primitive photosynthetic bacteria adapted to life inside plant cells, while plants themselves have not actually evolved photosynthetic processes on their own. Another example of this can be found in complex animals, including humans, whose cells depend upon mitochondria as their energy source; mitochondria are thought to have evolved from endosymbiotic bacteria, related to modern Rickettsia bacteria. Both chloroplasts and mitochondria actually have their own DNA, separate from the nuclear DNA of their animal or plant host cells.

    This contention is supported by the finding that the marine Molluscas Elysia viridis and Elysia chlorotica seem to maintain a symbiosis relationship with chloroplasts from algae with similar RDA structures that they encounter. However, they do not transfer these chloroplasts to the next generations.

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

    Molecular production Light to chemical energy The light energy is converted to chemical energy using the light-dependent reactions. This chemical energy production is more than 90% efficient with only 5-8% of the energy transferred thermally. The products of the light-dependent reactions are adenosine triphosphate from photophosphorylation and NADPH from photoreduction. Both are then utilized as an energy source for the light-independent reactions.

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

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

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

    Quantum mechanical effects Through photosynthesis, sunlight energy is transferred to molecular reaction centers for conversion into chemical energy with nearly 100-percent efficiency. The transfer of the solar energy takes place almost instantaneously, so little energy is wasted as heat.

    A study led by researchers with the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) and the University of California at Berkeley suggests that long-lived wavelike electronic quantum coherence plays an important part in this instantaneous transfer of energy by allowing the photosynthetic system to simultaneously try each potential energy pathway and choose the most efficient option. Results of the study are presented in the April 12, 2007 issue of the journal Nature.Lawrence Berkeley National Lab. "Quantum secrets of photosynthesis revealed", physorg.com, April 12, 2007. Accessed April 13, 2007.

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

    Bacterial variation The concept that oxygen production is not directly associated with the fixation of carbon dioxide was first proposed by Cornelis Van Niel in the 1930s, who studied photosynthetic bacteria. Aside from the cyanobacteria, bacteria only have one photosystem and use reducing agents other than water. They get electrons from a variety of different inorganic chemicals including sulfide or hydrogen, so for most of these bacteria oxygen is not produced.

    Others, such as the halophiles (an Archaea) produced so called purple membranes where the bacteriorhodopsin could harvest light and produce energy. The purple membranes was one of the first to be used to demonstrate the chemiosmosis 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.

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

    C4, C3 and CAM In hot and dry conditions, plants will close their stomata to prevent loss of water. Under these conditions, oxygen gas, produced by the light reactions of photosynthesis, will concentrate in the leaves causing photorespiration to occur. Some plants have evolution mechanisms to increase the CO2 concentration in the leaves under these conditions.

    C4 carbon fixation capture carbon dioxide using an enzyme called PEP Carboxylase that adds carbon dioxide to the three carbon molecule Phosphoenolpyruvate creating the 4 carbon molecule oxaloacetic acid. Plants without this enzyme are called C3 carbon fixation because the primary carboxylation reaction produces the three carbon sugar 3-phosphoglycerate directly in the Calvin-Benson Cycle. When oxygen levels rise in the leaf, C4 plants reverse the reaction to release carbon dioxide thus preventing photorespiration. By preventing photorespiration, C4 plants can produce more sugar than C3 plants in conditions of strong light and high temperature. Many important crop plants are C4 plants including maize, sorghum, sugarcane, and millet.

    Xerophytes such as Cacti and most succulents also can use PEP Carboxylase to capture carbon dioxide in a process called CAM photosynthesis. They store the CO2 in different molecules than the C4 plants (mostly they store it in the form of malic acid via carboxylation of phosphoenolpyruvate to oxaloacetate which is then reduced to malate). Nevertheless, C4 plants capture the CO2 in one type of cell tissue (mesophyll) and then transfer it to another type of tissue (bundle sheath cells) so that carbon fixation may occur via the Calvin cycle. They also have a different leaf anatomy than C4 plants. They grab the CO2 at night when their stomata are open, and they release it into the leaves during the day to increase their photosynthetic rate. C4 metabolism physically separates CO2 fixation from the Calvin cycle, while CAM metabolism temporally separates CO2 fixation from the 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. 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 (ecology) comes from the inputs of photosynthesis, not the soil itself.

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

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

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

    Modern scientists built on the foundation of knowledge from those scientists centuries ago and were able to discover many things.

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

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

    2 H2O + 2 A + (light, chloroplasts) → 2 AH2 + O2

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

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

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

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

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

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

    These two experiments illustrate vital points: firstly, from research it is known that photochemical reactions are not generally affected by temperature. However, these experiments clearly show that temperature affects the rate of carbon assimilation, so there must be two sets of reactions in the full process of carbon assimilation. These are of course the Light-dependent reaction stage and the Light-independent reaction stage. Secondly, Blackman's experiments illustrate the concept of limiting factors. Another limiting factor is the wavelength of light. Cyanobacteria which reside several meters underwater cannot receive the correct wavelengths required to cause photoinduced charge separation in conventional photosynthetic pigments. To combat this problem a series of proteins with different pigments surround the reaction center. This unit is called a phycobilisome.

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

    RuBisCO oxygenase activity is disadvantageous to plants for several reasons:
  • One product of oxygenase activity is phosphoglycolate (2 carbon) instead of 3-phosphoglycerate (3 carbon). Phosphoglycolate cannot be metabolized by the Calvin-Benson cycle and represents carbon lost from the cycle. A high oxygenase activity, therefore, drains the sugars that are required to recycle ribulose 5-bisphosphate and for the continuation of the Calvin-Benson cycle.
  • Phosphoglycolate is quickly metabolized to glycolate that is toxic to a plant at a high concentration; it inhibits photosynthesis.
  • Salvaging glycolate is an energetically expensive process that uses the glycolate pathway and only 75% of the carbon is returned to the Calvin-Benson cycle as 3-phosphoglycerate.


    • :A highly simplified summary is:

    ::2 glycolate + ATP → 3-phophoglycerate + carbon dioxide + ADP +NH3

    The salvaging pathway for the products of RuBisCO oxygenase activity is more commonly known as photorespiration since it is characterized by light dependent oxygen consumption and the release of carbon dioxide.

    See also

    Notes

    References

    External links



    Photosynthesis
    Green plants are producers. This means that they can survive without animals! They can make lots of organic chemicals from a few simple inorganic chemicals.

    BBC - GCSE Bitesize - Biology | Green plants | Photosynthesis
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    Photosynthesis and transpiration (made easy)
    Childrens guide to how Photosynthesis and transpiration works. ... Photosynthesis and transpiration (made easy) Photosynthesis is the way a plant makes food for itself.

    Photosynthesis - Wikipedia, the free encyclopedia
    Photosynthesis is a series of enzyme-catalyzed steps for the conversion of light energy into chemical energy by living organisms. Its initial substrates are carbon dioxide and ...

    PHOTOSYNTHESIS
    It is important to get the process of photosynthesis into perspective, especially in relation to the processes covered in previous topics: respiration, and digestion in animals.

    BUBL LINK: Photosynthesis
    Titles: Descriptions: Biology Project: Online Interactive Resource for Learning Biology; Internet Chemistry: Oxidation / Reduction; Tree Physiology; What is Photosynthesis?

    photosynthesis
    animated outline of photosynthesis light phase. photosynthesis dark phase. INDEX. This animation is the copyrighted property of John Kyrk. It may not be displayed except on the web ...

    PhotoSynthesis
    PhotoSynthesis PhotoSynthesis is the process where light is turned into complex carbon compounds. Almost all life on earth is dependent on PhotoSynthesis as its primary ...

    Definition: photosynthesis from Online Medical Dictionary
    The Online Medical Dictionary is a searchable dictionary of definitions from medicine, science and technology.

     

    Photosynthesis



     
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