Photosynthesis 1

Photosynthesis

Composite image showing the global distribution of photosynthesis, including bothoceanic phytoplankton and land vegetation.

Overall equation for the type of photosynthesis that occurs in plants.

Photosynthesis (from the Greekφώτο- [photo-], "light," and σύνθεσις[synthesis], "putting together","composition") is a process thatconverts carbon dioxide into organiccompounds, especially sugars, usingthe energy from sunlight.[1]

Photosynthesis occurs in plants, algae,and many species of bacteria, but notin archaea. Photosynthetic organismsare called photoautotrophs, since theycan create their own food. In plants,algae, and cyanobacteria,photosynthesis uses carbon dioxideand water, releasing oxygen as a wasteproduct. Photosynthesis is vital for allaerobic life on Earth. As well asmaintaining the normal level of oxygenin the atmosphere, nearly all life eitherdepends on it directly as a source ofenergy, or indirectly as the ultimatesource of the energy in their food[2]

(the exceptions are chemoautotrophs that live in rocks or around deep sea hydrothermal vents). The rate of energycapture by photosynthesis is immense, approximately 100 terawatts,[3] which is about six times larger than the powerconsumption of human civilization.[4] As well as energy, photosynthesis is also the source of the carbon in all theorganic compounds within organisms' bodies. In all, photosynthetic organisms convert around 100–115  teragramsof carbon into biomass per year.[5] [6]

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

Photosynthesis 2

Overview of cycle between autotrophs and heterotrophs. Photosynthesis is themain means by which plants, algae and many bacteria produce organic compounds

and oxygen from carbon dioxide and water (green arrow).

The first photosynthetic organisms probablyevolved about 3500 [7] million years ago,early in the evolutionary history of life,when all forms of life on Earth weremicroorganisms and the atmosphere hadmuch more carbon dioxide. They mostlikely used hydrogen or hydrogen sulfide assources of electrons, rather than water.[8]

Cyanobacteria appeared later, around 3000[9] million years ago, and drasticallychanged the Earth when they began tooxygenate the atmosphere, beginning about2400 [10] million years ago.[11] This newatmosphere allowed the evolution ofcomplex life such as protists. Eventually, nolater than a billion years ago, one of theseprotists formed a symbiotic relationshipwith a cyanobacterium, producing theancestor of many plants and algae.[12] Thechloroplasts in modern plants are thedescendants of these ancient symbioticcyanobacteria.[13]

Overview

Photosynthesis changes the energy from the suninto chemical energy and splits water to liberate

O2 and fixes CO2 into sugar

Photosynthetic organisms are photoautotrophs, which means that theyare able to synthesize food directly from carbon dioxide using energyfrom light. However, not all organisms that use light as a source ofenergy carry out photosynthesis, since photoheterotrophs use organiccompounds, rather than carbon dioxide, as a source of carbon.[2] Inplants, algae and cyanobacteria, photosynthesis releases oxygen. Thisis called oxygenic photosynthesis. Although there are some differencesbetween oxygenic photosynthesis in plants, algae and cyanobacteria,the overall process is quite similar in these organisms. However, thereare some types of bacteria that carry out anoxygenic photosynthesis,which consumes carbon dioxide but does not release oxygen.

Carbon dioxide is converted into sugars in a process called carbonfixation. Carbon fixation is a redox reaction, so photosynthesis needsto supply both a source of energy to drive this process, and theelectrons needed to convert carbon dioxide into carbohydrate, which isa reduction reaction. In general outline, photosynthesis is the opposite

Photosynthesis 3

of cellular respiration, where glucose and other compounds are oxidized to produce carbon dioxide, water, andrelease chemical energy. However, the two processes take place through a different sequence of chemical reactionsand in different cellular compartments.The general equation for photosynthesis is therefore:

2n CO2 + 2n H2O + photons → 2(CH2O)n + n O2 + 2n ACarbon dioxide + electron donor + light energy → carbohydrate + oxygen + oxidized electron donorSince water is used as the electron donor in oxygenic photosynthesis, the equation for this process is:

2n CO2 + 2n H2O + photons → 2(CH2O)n + 2n O2carbon dioxide + water + light energy → carbohydrate + oxygen

Other processes substitute other compounds (such as arsenite) for water in the electron-supply role; the microbes usesunlight to oxidize arsenite to arsenate:[14] The equation for this reaction is:

(AsO33–) + CO2 + photons → CO + (AsO4

3–)[15]

carbon dioxide + arsenite + light energy → arsenate + carbon monoxide (used to build other compounds insubsequent reactions)

Photosynthesis occurs in two stages. In the first stage, light-dependent reactions or light reactions capture the energyof light and use it to make the energy-storage molecules ATP and NADPH. During the second stage, thelight-independent reactions use these products to capture and reduce carbon dioxide.Most organisms that utilize photosynthesis to produce oxygen use visible light to do so, although at least three useinfrared radiation.[16]

Photosynthetic membranes and organelles

Chloroplast ultrastructure: 1. outer membrane 2. intermembrane space3.inner membrane (1+2+3: envelope) 4. stroma (aqueous fluid) 5. thylakoid

lumen (inside of thylakoid) 6. thylakoid membrane 7. granum (stack ofthylakoids) 8. thylakoid (lamella) 9. starch 10. ribosome 11. plastidial DNA

12. plastoglobule (drop of lipids)

The proteins that gather light for photosynthesisare embedded within cell membranes. Thesimplest way these are arranged is inphotosynthetic bacteria, where these proteins areheld within the plasma membrane.[17] However,this membrane may be tightly folded intocylindrical sheets called thylakoids,[18] orbunched up into round vesicles calledintracytoplasmic membranes.[19] These structurescan fill most of the interior of a cell, giving themembrane a very large surface area and thereforeincreasing the amount of light that the bacteriacan absorb.[18]

In plants and algae, photosynthesis takes place inorganelles called chloroplasts. A typical plant cell contains about 10 to 100 chloroplasts. The chloroplast is enclosedby a membrane. This membrane is composed of a phospholipid inner membrane, a phospholipid outer membrane,and an intermembrane space between them. Within the membrane is an aqueous fluid called the stroma. The stromacontains stacks (grana) of thylakoids, which are the site of photosynthesis. The thylakoids are flattened disks,bounded by a membrane with a lumen or thylakoid space within it. The site of photosynthesis is the thylakoidmembrane, which contains integral and peripheral membrane protein complexes, including the pigments that absorblight energy, which form the photosystems.

Photosynthesis 4

Plants absorb light primarily using the pigment chlorophyll, which is the reason that most plants have a green color.Besides chlorophyll, plants also use pigments such as carotenes and xanthophylls.[20] Algae also use chlorophyll, butvarious other pigments are present as phycocyanin, carotenes, and xanthophylls in green algae, phycoerythrin in redalgae (rhodophytes) and fucoxanthin in brown algae and diatoms resulting in a wide variety of colors.These pigments are embedded in plants and algae in special antenna-proteins. In such proteins all the pigments areordered to work well together. Such a protein is also called a light-harvesting complex.Although all cells in the green parts of a plant have chloroplasts, most of the energy is captured in the leaves. Thecells in the interior tissues of a leaf, called the mesophyll, can contain between 450,000 and 800,000 chloroplasts forevery square millimeter of leaf. The surface of the leaf is uniformly coated with a water-resistant waxy cuticle thatprotects the leaf from excessive evaporation of water and decreases the absorption of ultraviolet or blue light toreduce heating. The transparent epidermis layer allows light to pass through to the palisade mesophyll cells wheremost of the photosynthesis takes place.

Light reactions

Light-dependent reactions of photosynthesis at the thylakoid membrane

In the light reactions, one molecule ofthe pigment chlorophyll absorbs onephoton and loses one electron. Thiselectron is passed to a modified formof chlorophyll called pheophytin,which passes the electron to a quinonemolecule, allowing the start of a flowof electrons down an electron transportchain that leads to the ultimatereduction of NADP to NADPH. Inaddition, this creates a proton gradientacross the chloroplast membrane; itsdissipation is used by ATP synthasefor the concomitant synthesis of ATP.The chlorophyll molecule regains the lost electron from a water molecule through a process called photolysis, whichreleases a dioxygen (O2) molecule. The overall equation for the light-dependent reactions under the conditions ofnon-cyclic electron flow in green plants is:[21]

2 H2O + 2 NADP+ + 3 ADP + 3 Pi + light → 2 NADPH + 2 H+ + 3 ATP + O2Not all wavelengths of light can support photosynthesis. The photosynthetic action spectrum depends on the type ofaccessory pigments present. For example, in green plants, the action spectrum resembles the absorption spectrum forchlorophylls and carotenoids with peaks for violet-blue and red light. In red algae, the action spectrum overlaps withthe absorption spectrum of phycobilins for blue-green light, which allows these algae to grow in deeper waters thatfilter out the longer wavelengths used by green plants. The non-absorbed part of the light spectrum is what givesphotosynthetic organisms their color (e.g., green plants, red algae, purple bacteria) and is the least effective forphotosynthesis in the respective organisms.

Photosynthesis 5

Z scheme

The "Z scheme"

In plants, light-dependent reactions occur in the thylakoid membranes of the chloroplasts and use light energy tosynthesize ATP and NADPH. The light-dependent reaction has two forms: cyclic and non-cyclic. In the non-cyclicreaction, the photons are captured in the light-harvesting antenna complexes of photosystem II by chlorophyll andother accessory pigments (see diagram at right). When a chlorophyll molecule at the core of the photosystem IIreaction center obtains sufficient excitation energy from the adjacent antenna pigments, an electron is transferred tothe primary electron-acceptor molecule, pheophytin, through a process called photoinduced charge separation. Theseelectrons are shuttled through an electron transport chain, the so called Z-scheme shown in the diagram, that initiallyfunctions to generate a chemiosmotic potential across the membrane. An ATP synthase enzyme uses thechemiosmotic potential to make ATP during photophosphorylation, whereas NADPH is a product of the terminalredox reaction in the Z-scheme. The electron enters a chlorophyll molecule in Photosystem I. The electron is exciteddue to the light absorbed by the photosystem. A second electron carrier accepts the electron, which again is passeddown lowering energies of electron acceptors. The energy created by the electron acceptors is used to movehydrogen 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, butdiffers in the form that it generates only ATP, and no reduced NADP (NADPH) is created. The cyclic reaction takesplace only at photosystem I. Once the electron is displaced from the photosystem, the electron is passed down theelectron acceptor molecules and returns back to photosystem I, from where it was emitted, hence the name cyclicreaction.

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

Photosynthesis 6

ATP synthesis. Oxygen is a waste product of light-dependent reactions, but the majority of organisms on Earth useoxygen for cellular respiration, including photosynthetic organisms.[22] [23]

Light-independent reactions

The Calvin CycleIn the Light-independent or dark reactions the enzyme RuBisCO captures CO2 from the atmosphere and in a processthat requires the newly formed NADPH, called the Calvin-Benson Cycle, releases three-carbon sugars, which arelater combined to form sucrose and starch. The overall equation for the light-independent reactions in green plantsis:[21]

3 CO2 + 9 ATP + 6 NADPH + 6 H+ → C3H6O3-phosphate + 9 ADP + 8 Pi + 6 NADP+ + 3 H2O

Overview of the Calvin cycle and carbon fixation

To be more specific, carbon fixationproduces an intermediate product,which is then converted to the finalcarbohydrate products. The carbonskeletons produced by photosynthesisare then variously used to form otherorganic compounds, such as thebuilding material cellulose, asprecursors for lipid and amino acidbiosynthesis, or as a fuel in cellularrespiration. The latter occurs not onlyin plants but also in animals when theenergy from plants gets passed througha food chain.

The fixation or reduction of carbondioxide is a process in which carbondioxide combines with a five-carbonsugar, ribulose 1,5-bisphosphate(RuBP), to yield two molecules of athree-carbon compound, glycerate3-phosphate (GP), also known as 3-phosphoglycerate (PGA). GP, in the presence of ATP and NADPH from thelight-dependent stages, is reduced to glyceraldehyde 3-phosphate (G3P). This product is also referred to as3-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 (seeCalvin-Benson cycle). The 1 out of 6 molecules of the triose phosphates not "recycled" often condense to formhexose phosphates, which ultimately yield sucrose, starch and cellulose. The sugars produced during carbonmetabolism yield carbon skeletons that can be used for other metabolic reactions like the production of amino acidsand lipids.

Photosynthesis 7

Carbon concentrating mechanisms

On land

Overview of C4 carbon fixation

In hot and dry conditions, plants close their stomatato prevent the loss of water. Under these conditions,CO2 will decrease, and oxygen gas, produced bythe light reactions of photosynthesis, will decreasein the stem, not leaves, causing an increase ofphotorespiration by the oxygenase activity ofribulose-1,5-bisphosphate carboxylase/oxygenaseand decrease in carbon fixation. Some plants haveevolved mechanisms to increase the CO2concentration in the leaves under these conditions.

C4 plants chemically fix carbon dioxide in the cellsof the mesophyll by adding it to the three-carbonmolecule phosphoenolpyruvate (PEP), a reactioncatalyzed by an enzyme called PEP carboxylaseand which creates the four-carbon organic acid,oxaloacetic acid. Oxaloacetic acid or malatesynthesized by this process is then translocated tospecialized bundle sheath cells where the enzyme,rubisco, and other Calvin cycle enzymes arelocated, and where CO2 released bydecarboxylation of the four-carbon acids is thenfixed by rubisco activity to the three-carbon sugar3-phosphoglyceric acids. The physical separation ofrubisco from the oxygen-generating light reactionsreduces photorespiration and increases CO2 fixation and thus photosynthetic capacity of the leaf.[24] C4 plants canproduce more sugar than C3 plants in conditions of high light and temperature. Many important crop plants are C4plants, including maize, sorghum, sugarcane, and millet. Plants that do not use PEP-carboxylase in carbon fixationare called C3 plants because the primary carboxylation reaction, catalyzed by rubisco, produces the three-carbonsugar 3-phosphoglyceric acids directly in the Calvin-Benson cycle. Over 90% of plants use C3 carbon fixation,compared to 3% that use C4 carbon fixation.[25]

Xerophytes, such as cacti and most succulents, also use PEP carboxylase to capture carbon dioxide in a processcalled Crassulacean acid metabolism (CAM). In contrast to C4 metabolism, which physically separates the CO2fixation to PEP from the Calvin cycle, CAM temporally separates these two processes. CAM plants have a differentleaf anatomy from C3 plants, and fix the CO2 at night, when their stomata are open. CAM plants store the CO2mostly in the form of malic acid via carboxylation of phosphoenolpyruvate to oxaloacetate, which is then reduced tomalate. Decarboxylation of malate during the day releases CO2 inside the leaves, thus allowing carbon fixation to3-phosphoglycerate by rubisco. 16,000 species of plants use CAM.[26]

Photosynthesis 8

In water

Cyanobacteria possess carboxysomes which increase the concentration of CO2 around rubisco to increase the rate ofphotosynthesis. This operates by carbonic anhydrase producing hydrocarbonate ions (HCO3

-) which are thenpumped into the carboxysome, before being processed by a different carbonic anhydrase to produce CO2.[27]

Pyrenoids in algae and hornworts also act to concentrate CO2 around rubisco.[28]

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

EfficiencyPlants usually convert light into chemical energy with a photosynthetic efficiency of 3–6%.[29] Actual plants'photosynthetic efficiency varies with the frequency of the light being converted, light intensity, temperature andproportion of carbon dioxide in the atmosphere, and can vary from 0.1% to 8%.[30] By comparison, solar panelsconvert light into electric energy at an efficiency of approximately 6–20% for mass-produced panels, and up to 41%in a research laboratory.[31]

Evolution

Plant cells with visible chloroplasts (from a moss, Plagiomnium affine).

Early photosynthetic systems, such as thosefrom green and purple sulfur and green andpurple nonsulfur bacteria, are thought tohave been anoxygenic, using variousmolecules as electron donors. Green andpurple sulfur bacteria are thought to haveused hydrogen and sulfur as an electrondonor. Green nonsulfur bacteria usedvarious amino and other organic acids.Purple nonsulfur bacteria used a variety ofnonspecific organic molecules. The use ofthese molecules is consistent with thegeological evidence that the atmosphere washighly reduced at that time.

Fossils of what are thought to befilamentous photosynthetic organisms havebeen dated at 3.4 billion years old.[32] [33]

The main source of oxygen in the atmosphere is oxygenic photosynthesis, and its first appearance is sometimesreferred 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,

Photosynthesis 9

which is oxidized to molecular oxygen (O2) in the photosynthetic reaction center.

Symbiosis and the origin of chloroplastsSeveral groups of animals have formed symbiotic relationships with photosynthetic algae. These are most commonin corals, sponges and sea anemones, possibly due to these animals having particularly simple body plans and largesurface areas compared to their volumes.[34] In addition, a few marine mollusks Elysia viridis and Elysia chloroticaalso maintain a symbiotic relationship with chloroplasts they capture from the algae in their diet and then store intheir bodies. This allows the molluscs to survive solely by photosynthesis for several months at a time.[35] [36] Someof the genes from the plant cell nucleus have even been transferred to the slugs, so that the chloroplasts can besupplied with proteins that they need to survive.[37]

An even closer form of symbiosis may explain the origin of chloroplasts. Chloroplasts have many similarities withphotosynthetic bacteria, including a circular chromosome, prokaryotic-type ribosomes, and similar proteins in thephotosynthetic reaction center.[38] [39] The endosymbiotic theory suggests that photosynthetic bacteria were acquired(by endocytosis) by early eukaryotic cells to form the first plant cells. Therefore, chloroplasts may be photosyntheticbacteria that adapted to life inside plant cells. Like mitochondria, chloroplasts still possess their own DNA, separatefrom the nuclear DNA of their plant host cells and the genes in this chloroplast DNA resemble those incyanobacteria.[40] DNA in chloroplasts codes for redox proteins such as photosynthetic reaction centers. The CoRRHypothesis proposes that this Co-location is required for Redox Regulation.

Cyanobacteria and the evolution of photosynthesisThe biochemical capacity to use water as the source for electrons in photosynthesis evolved once, in a commonancestor of extant cyanobacteria. The geological record indicates that this transforming event took place early inEarth's history, at least 2450–2320 million years ago (Ma), and possibly much earlier.[41] Available evidence fromgeobiological studies of Archean (>2500 Ma) sedimentary rocks indicates that life existed 3500 Ma, but the questionof when oxygenic photosynthesis evolved is still unanswered. A clear paleontological window on cyanobacterialevolution opened about 2000 Ma, revealing an already-diverse biota of blue-greens. Cyanobacteria remainedprincipal primary producers throughout the Proterozoic Eon (2500–543 Ma), in part because the redox structure ofthe oceans favored photoautotrophs capable of nitrogen fixation. Green algae joined blue-greens as major primaryproducers on continental shelves near the end of the Proterozoic, but only with the Mesozoic (251–65 Ma) radiationsof 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 biologicalnitrogen fixation, and, in modified form, as the plastids of marine algae.[42]

DiscoveryAlthough some of the steps in photosynthesis are still not completely understood, the overall photosynthetic equationhas been known since the 19th century.Jan van Helmont began the research of the process in the mid-17th century when he carefully measured the mass ofthe soil used by a plant and the mass of the plant as it grew. After noticing that the soil mass changed very little, hehypothesized that the mass of the growing plant must come from the water, the only substance he added to the pottedplant. His hypothesis was partially accurate—much of the gained mass also comes from carbon dioxide as well aswater. However, this was a signaling point to the idea that the bulk of a plant's biomass comes from the inputs ofphotosynthesis, not the soil itself.Joseph Priestley, a chemist and minister, discovered that when he isolated a volume of air under an inverted jar, andburned a candle in it, the candle would burn out very quickly, much before it ran out of wax. He further discoveredthat a mouse could similarly "injure" air. He then showed that the air that had been "injured" by the candle and themouse could be restored by a plant.

Photosynthesis 10

In 1778, Jan Ingenhousz, court physician to the Austrian Empress, repeated Priestley's experiments. He discoveredthat it was the influence of sunlight on the plant that could cause it to revive a mouse in a matter of hours.In 1796, Jean Senebier, a Swiss pastor, botanist, and naturalist, demonstrated that green plants consume carbondioxide and release oxygen under the influence of light. Soon afterwards, Nicolas-Théodore de Saussure showed thatthe increase in mass of the plant as it grows could not be due only to uptake of CO2, but also to the incorporation ofwater. Thus the basic reaction by which photosynthesis is used to produce food (such as glucose) was outlined.Cornelis Van Niel made key discoveries explaining the chemistry of photosynthesis. By studying purple sulfurbacteria and green bacteria he was the first scientist to demonstrate that photosynthesis is a light-dependent redoxreaction, in which hydrogen reduces carbon dioxide.Robert Emerson discovered two light reactions by testing plant productivity using different wavelengths of light.With the red alone, the light reactions were suppressed. When blue and red were combined, the output was muchmore substantial. Thus, there were two photosystems, one aborbing up to 600 nm wavelengths, the other up to 700.The former is known as PSII, the latter is PSI. PSI contains only chlorophyll a, PSII contains primarily chlorophyll awith most of the available chlorophyll b, among other pigments.[43]

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 thepresence of unnatural reducing agents like iron oxalate, ferricyanide or benzoquinone after exposure to light. TheHill reaction is as follows:

2 H2O + 2 A + (light, chloroplasts) → 2 AH2 + O2where A is the electron acceptor. Therefore, in light the electron acceptor is reduced and oxygen is evolved. Cyt b6,now known as a plastoquinone, is one electron acceptor.Samuel Ruben and Martin Kamen used radioactive isotopes to determine that the oxygen liberated in photosynthesiscame from the water.Melvin Calvin and Andrew Benson, along with James Bassham, elucidated the path of carbon assimilation (thephotosynthetic carbon reduction cycle) in plants. The carbon reduction cycle is known as the Calvin cycle, whichinappropriately ignores the contribution of Bassham and Benson. Many scientists refer to the cycle as theCalvin-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 theelectron transport chain.

Factors

The leaf is the primary site of photosynthesis inplants.

There are three main factors affecting photosynthesis and severalcorollary factors. The three main are:• Light irradiance and wavelength• Carbon dioxide concentration• Temperature.

Light intensity (irradiance), wavelength andtemperature

In the early 20th century Frederick Frost Blackman along with AlbertEinstein investigated the effects of light intensity (irradiance) andtemperature on the rate of carbon assimilation.

• At constant temperature, the rate of carbon assimilation varies with irradiance, initially increasing as theirradiance increases. However at higher irradiance this relationship no longer holds and the rate of carbon

Photosynthesis 11

assimilation reaches a plateau.• At constant irradiance, the rate of carbon assimilation increases as the temperature is increased over a limited

range. This effect is only seen at high irradiance levels. At low irradiance, increasing the temperature has littleinfluence on the rate of carbon assimilation.

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

Carbon dioxide levels and photorespirationAs carbon dioxide concentrations rise, the rate at which sugars are made by the light-independent reactions increasesuntil limited by other factors. RuBisCO, the enzyme that captures carbon dioxide in the light-independent reactions,has a binding affinity for both carbon dioxide and oxygen. When the concentration of carbon dioxide is high,RuBisCO will fix carbon dioxide. However, if the carbon dioxide concentration is low, RuBisCO will bind oxygeninstead of carbon dioxide. This process, called photorespiration, uses energy, but does not produce sugars.RuBisCO oxygenase activity is disadvantageous to plants for several reasons:1. 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. Ahigh oxygenase activity, therefore, drains the sugars that are required to recycle ribulose 5-bisphosphate and forthe continuation of the Calvin-Benson cycle.

2. Phosphoglycolate is quickly metabolized to glycolate that is toxic to a plant at a high concentration; it inhibitsphotosynthesis.

3. Salvaging glycolate is an energetically expensive process that uses the glycolate pathway and only 75% of thecarbon is returned to the Calvin-Benson cycle as 3-phosphoglycerate. The reactions also produce ammonia (NH3)which is able to diffuse out of the plant leading to a loss of nitrogen.

A highly simplified summary is:2 glycolate + ATP → 3-phosphoglycerate + carbon dioxide + ADP +NH3

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

See also• Jan Anderson (scientist)• Artificial photosynthesis• Calvin-Benson cycle• Carbon fixation• Cellular respiration• Chemosynthesis• Light-dependent reaction• Photobiology• Photoinhibition• Photosynthetic reaction center

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• Photosynthetically active radiation• Quantum biology• Red edge

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Further reading• Asimov, Isaac (1968). Photosynthesis. New York, London: Basic Books, Inc.. ISBN 0-465-05703-9.• Bidlack JE; Stern KR, Jansky S (2003). Introductory plant biology. New York: McGraw-Hill.

ISBN 0-07-290941-2.• Blankenship RE (2008). Molecular Mechanisms of Photosynthesis (2nd ed.). John Wiley & Sons Inc.

ISBN 0-470-71451-4.• Govindjee (1975). Bioenergetics of photosynthesis. Boston: Academic Press. ISBN 0-12-294350-3.• Govindjee Beatty JT,Gest H, Allen JF (2006). Discoveries in Photosynthesis. Advances in Photosynthesis and

Respiration. 20. Berlin: Springer. ISBN 1-4020-3323-0.• Gregory RL (1971). Biochemistry of photosynthesis. New York: Wiley-Interscience. ISBN 0-471-32675-5.• Rabinowitch E, Govindjee (1969). Photosynthesis. London: J. Wiley. ISBN 0-471-70424-5.• Reece, J, Campbell, N (2005). Biology. San Francisco: Pearson, Benjamin Cummings. ISBN 0-8053-7146-X.

Photosynthesis 14

External links• A collection of photosynthesis pages for all levels from a renowned expert (Govindjee) (http:/ / www. life. uiuc.

edu/ govindjee/ linksPSed. htm)• In depth, advanced treatment of photosynthesis, also from Govindjee (http:/ / www. life. uiuc. edu/ govindjee/

paper/ gov. html)• Science Aid: Photosynthesis (http:/ / scienceaid. co. uk/ biology/ biochemistry/ photosynthesis. html) Article

appropriate for high school science• Liverpool John Moores University, Dr.David Wilkinson (http:/ / www. ljmu. ac. uk/ NewsCentre/ 63012. htm)• Metabolism, Cellular Respiration and Photosynthesis – The Virtual Library of Biochemistry and Cell Biology

(http:/ / www. biochemweb. org/ metabolism. shtml)• Overall examination of Photosynthesis at an intermediate level (http:/ / www. chemsoc. org/ networks/ learnnet/

cfb/ Photosynthesis. htm)• Overall Energetics of Photosynthesis (http:/ / www. life. uiuc. edu/ govindjee/ photosynBook. html)• Photosynthesis Discovery Milestones (http:/ / www. juliantrubin. com/ bigten/ photosynthesisexperiments. html)

– experiments and background• The source of oxygen produced by photosynthesis (http:/ / bcs. whfreeman. com/ thelifewire/ content/ chp08/

0802001. html) Interactive animation, a textbook tutorial

Article Sources and Contributors 15

Article Sources and ContributorsPhotosynthesis  Source: http://en.wikipedia.org/w/index.php?oldid=393175949  Contributors: (jarbarf), *drew, -- April, 1exec1, 678910, @pple, AJRM, ASKingquestions, ATMarsden,Abductive, Abu-Fool Danyal ibn Amir al-Makhiri, Academic Challenger, Accuruss, Adashiel, Adenosine, Agathman, Ahoerstemeier, Aitias, Alan Liefting, Alansohn, Alexius08, Ali, Ali salter,Alksub, Amercer09, AmiDaniel, Amorymeltzer, Andris, AndyZ, Angam, Angela, Angr, AnonMoos, Anonymi, Ans, Antandrus, AnthonyQBachler, Antony2, Apokryltaros, Arcadian, Arjun01,Art LaPella, Atrian, Aut1221, Avicennasis, AxelBoldt, B820, BGFMSM, Bailey654, BanyanTree, Barras, Bateman1234, Bayerischermann, Bbatsell, Beetstra, Ben-Zin, Bencherlite, Bendzh,Bento00, Bergsten, Bertiethecat, Bfahome, Bhadani, Billare, Blahed768, Bluedeed, Bobo192, Boghog, Boing! said Zebedee, Bongwarrior, Bookandcoffee, Bowlhover, Brainmachine, BryanDerksen, Burnhamd, CERminator, CQJ, CWY2190, Cacycle, Call me Bubba, Caltas, CalumH93, Camw, Can't sleep, clown will eat me, CanadianLinuxUser, Canterbury Tail, CardinalDan,Cassius987, Causa sui, Cayte, Cbrown1023, Cdorman2, Celarnor, CerealKiller, Ceyockey, Chamal N, Chao, Chasingsol, Cheesy Yeast, Chepry, Chirpy7, Chlor, Chodorkovskiy,ChrisHodgesUK, Chrislk02, Chun-hian, Cimex, Ciphergoth, Ckatz, Clicketyclack, ClockworkSoul, ClockworkTroll, Closedmouth, Clubjuggle, Clyde10race, Cman12345657578, Cmdrjameson,Coching, Cohesion, Collegedegree, Color probe, ComCat, Commander, CommonsDelinker, Condem, Consequencefree, Conversion script, Corrigen, Courcelles, Crazychinchilla, Crazytales,Creidieki, Crittens, Crystallina, CurtisSwain, Cyde, Cyrius, DARTH SIDIOUS 2, DVD R W, Da monster under your bed, DaGizza, Dacrycarpus, Daharde, Dandv, Daniel233, Daniel5127,DanielCD, Danny B-), Dar-Ape, Darekun, Darklilac, Dasyornis, Davewild, David D., David Wahler, DavidDidwin, DavidWBrooks, Daycd, Db099221, Dbenbenn, Dbrett480, Debresser,Decltype, Delfigolo, Delta G, Demi s96, Demiurge, Deor, DerHexer, Dfrg.msc, Dienamight, Dina, Dinsdalea, Dirty MF Harry, Discospinster, Dj Capricorn, Domminico, Doniago, Dori,DotComCairney, Dr Dec, Dr. Mandellez, Drdaveng, Dreadstar, Dreg743, Drphilharmonic, Dsinghs, Dullhunk, Dungodung, Dweller, Dwmyers, Długosz, EScribe, ESkog, Eaglestrike117,EarthPerson, EconoPhysicist, Ed, EdTrist, Eeekster, Eik Corell, Eleassar, Elmer Clark, Epbr123, Epipelagic, EqualMusic, Eric Kvaalen, Etacar11, Etxrge, Eumeng.chong, Evercat, Everyking,Evil Monkey, Excirial, FF2010, Fahadsadah, Faithlessthewonderboy, Feezo, Feniouk, Fenteany, Fermion, Fgbhrgthnrtgnb, FocalPoint, Fox6453, Fram, Freakofnurture, Fredrik, FreplySpang,Frymaster, Fullmetal123321, Funnybunny, Funper, Fvw, GHe, GTBacchus, Gacha1, Gadfium, Gaff, Gaius Cornelius, Gary King, GeeJo, GeeOh, Gekedo, Gene Nygaard, Germen, Gggh,Ghewgill, Giftlite, Gigemag76, Gilliam, Ginkgo100, Glenn, Goatman12, Govindjee, GregAsche, GregMinton, Gregturn, Grondemar, Gruzd, Guettarda, Gwernol, HBNayr, HLHJ, Hadal, Hahamhanuka, HalfShadow, Hao2lian, Hardyplants, Hawaiian717, Hdt83, Hello32020, Henrik, Herbertxu, Heron, Hgrenbor, Hi112233, Hmains, Hometown, Hordaland, Hpfreak26, Hu12, Hunrizzo8,Hydrogen Iodide, II MusLiM HyBRiD II, ITOD, Iam angryrawr, Ian Pitchford, Iapetus, Icseaturtles, Ihope127, Imanoob69, Imi2012, Imogen89, InNuce, Incredio, Indon, Init, Intelligentsium,Irishguy, Iron Dragon91, Ixfd64, J.delanoy, JDnCoke, JForget, JWSchmidt, JYolkowski, Jackgeddes1234, Jaknouse, JamesR777, Jamesscottbrown, Jamib0y, Jan1nad, Jasonphollinger, Jauhienij,Jaxl, Jay-Sebastos, Jazzy211, Jecar, Jeffq, Jennavecia, Jfg284, Jklin, Jlittlet, Jni, Joanjoc, JoaoRicardo, Johann Wolfgang, John, John254, Johna100, Jondel, Josh Grosse, Joshjoshjosh1, JoshuaZ,Jovianeye, Jpoelma13, Jrbouldin, Jrtayloriv, Jtdirl, Jugger90, Juhwade, Jusjih, Jwoodger, K-UNIT, KFP, Kaabi, Kablammo, Kamal Chid, Kasper90, Kazvorpal, Kbh3rd, Kcordina, Keilana,Kevyn, Kim Bruning, King Toadsworth, King of Hearts, Kingpin13, Kopid03, Korg, Kungfuadam, Kungming2, Kupirijo, Kuru, Kww, L'Aquatique, LOL, La goutte de pluie, Lanoitarus,Larryorr, Laur2ro, Lawrenceuniversity1, Lemchesvej, Li-sung, Lightmouse, Lights, Likeabird, Lily15, Logan, Looxix, Lord 1284, Lord.lucan, Lotje, Luna Santin, Lupin, MC10, MER-C,MNAdam, MPF, Magister Mathematicae, Magnus Manske, Makemi, Malcolm, Malcolm Farmer, Malke 2010, Malkinann, Malljaja, Malo, Malomaboy06, Mani1, ManuelGR, Marj Tiefert,Martin451, Marysunshine, Master Jay, Mat-C, Materialscientist, Math Champion, Mattbr, Matthuxtable, MattieTK, Mav, Maxim, Mboverload, McSly, Mdavies 965, Me big fishy, Melchoir,Meltzerju, Mhking, Michael C Price, Michael Hardy, Michael Ward, Mikael Häggström, Miketam, Mikimickz, Minna Sora no Shita, Mion, Mj455972007, Mlessard, Modster, Modulatum,Molybdenumblue, Momo san, Mooinglemur, Morning277, Mrg024, Mrmonsterman, Mrrhum, Mrwarriorman, Munita Prasad, Munyanah, Mwarren us, Mygerardromance, Myrryam, NMChico24,NMFA Northern Eagle, 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