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70-560: Bigyromonada is a recently described non- photosynthetic lineage of stramenopiles that at present contains two classes. Bigyromonads are characterized by biciliate cells that feed on bacteria through phagotrophy . They are marine organisms . Photosynthetic Photosynthesis ( / ˌ f oʊ t ə ˈ s ɪ n θ ə s ɪ s / FOH -tə- SINTH -ə-sis ) is a system of biological processes by which photosynthetic organisms , such as most plants, algae , and cyanobacteria , convert light energy , typically from sunlight, into

140-500: A biochemical pump that collects carbon from the organ interior (or from the soil ) and not from the atmosphere. Cyanobacteria possess carboxysomes , which increase the concentration of CO 2 around RuBisCO to increase the rate of photosynthesis. An enzyme, carbonic anhydrase , located within the carboxysome, releases CO 2 from dissolved hydrocarbonate ions (HCO 3 ). Before the CO 2 can diffuse out, RuBisCO concentrated within

210-409: A different leaf anatomy from C 3 plants, and fix the CO 2 at night, when their stomata are open. CAM plants store the CO 2 mostly in the form of malic acid via carboxylation of phosphoenolpyruvate to oxaloacetate , which is then reduced to malate. Decarboxylation of malate during the day releases CO 2 inside the leaves, thus allowing carbon fixation to 3-phosphoglycerate by RuBisCO. CAM

280-480: A phospholipid outer membrane, and an intermembrane space. Enclosed by the membrane is an aqueous fluid called the stroma. Embedded within the stroma are stacks of thylakoids (grana), which are the site of photosynthesis. The thylakoids appear as flattened disks. The thylakoid itself is enclosed by the thylakoid membrane, and within the enclosed volume is a lumen or thylakoid space. Embedded in the thylakoid membrane are integral and peripheral membrane protein complexes of

350-479: A photocomplex. When a photon is absorbed by a chromophore, it is converted into a quasiparticle referred to as an exciton , which jumps from chromophore to chromophore towards the reaction center of the photocomplex, a collection of molecules that traps its energy in a chemical form accessible to the cell's metabolism. The exciton's wave properties enable it to cover a wider area and try out several possible paths simultaneously, allowing it to instantaneously "choose"

420-405: A photon by the antenna complex loosens an electron by a process called photoinduced charge separation . The antenna system is at the core of the chlorophyll molecule of the photosystem II reaction center. That loosened electron is taken up by the primary electron-acceptor molecule, pheophytin . As the electrons are shuttled through an electron transport chain (the so-called Z-scheme shown in

490-422: A plant's photosynthetic response. Integrated chlorophyll fluorometer – gas exchange systems allow a more precise measure of photosynthetic response and mechanisms. While standard gas exchange photosynthesis systems can measure Ci, or substomatal CO 2 levels, the addition of integrated chlorophyll fluorescence measurements allows a more precise measurement of C C, the estimation of CO 2 concentration at

560-450: A redox-active tyrosine residue that is oxidized by the energy of P680 . This resets the ability of P680 to absorb another photon and release another photo-dissociated electron. The oxidation of water is catalyzed in photosystem II by a redox-active structure that contains four manganese ions and a calcium ion ; this oxygen-evolving complex binds two water molecules and contains the four oxidizing equivalents that are used to drive

630-399: A simpler method that employs a pigment similar to those used for vision in animals. The bacteriorhodopsin changes its configuration in response to sunlight, acting as a proton pump. This produces a proton gradient more directly, which is then converted to chemical energy. The process does not involve carbon dioxide fixation and does not release oxygen, and seems to have evolved separately from

700-481: A source of carbon atoms to carry out photosynthesis; photoheterotrophs use organic compounds, rather than carbon dioxide, as a source of carbon. In plants, algae, and cyanobacteria, photosynthesis releases oxygen. This oxygenic photosynthesis is by far the most common type of photosynthesis used by living organisms. Some shade-loving plants (sciophytes) produce such low levels of oxygen during photosynthesis that they use all of it themselves instead of releasing it to

770-534: A subsequent sequence of light-independent reactions called the Calvin cycle . In this process, atmospheric carbon dioxide is incorporated into already existing organic compounds, such as ribulose bisphosphate (RuBP). Using the ATP and NADPH produced by the light-dependent reactions, the resulting compounds are then reduced and removed to form further carbohydrates, such as glucose . In other bacteria, different mechanisms like

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840-405: A type of non- carbon-fixing anoxygenic photosynthesis, where the simpler photopigment retinal and its microbial rhodopsin derivatives are used to absorb green light and power proton pumps to directly synthesize adenosine triphosphate (ATP), the "energy currency" of cells. Such archaeal photosynthesis might have been the earliest form of photosynthesis that evolved on Earth, as far back as

910-591: A wide variety of colors. These pigments are embedded in plants and algae in complexes called antenna proteins. In such proteins, the pigments are arranged to work together. Such a combination of proteins is also called a light-harvesting complex . Although all cells in the green parts of a plant have chloroplasts, the majority of those are found in specially adapted structures called leaves . Certain species adapted to conditions of strong sunlight and aridity , such as many Euphorbia and cactus species, have their main photosynthetic organs in their stems. The cells in

980-490: Is an endothermic redox reaction. In general outline, photosynthesis is the opposite of cellular respiration : while photosynthesis is a process of reduction of carbon dioxide to carbohydrates, cellular respiration is the oxidation of carbohydrates or other nutrients to carbon dioxide. Nutrients used in cellular respiration include carbohydrates, amino acids and fatty acids. These nutrients are oxidized to produce carbon dioxide and water, and to release chemical energy to drive

1050-424: Is blocked. The functionality of rubisco activase involves removing RuBP and other inhibitory bonded molecules to re-enable carbamylation on the active site. Rubisco also catalyzes RuBP with oxygen ( O 2 ) in an interaction called photorespiration , a process that is more prevalent at high temperatures. During photorespiration RuBP combines with O 2 to become 3-PGA and phosphoglycolic acid. Like

1120-498: Is both an evolutionary precursor to C 4 and a useful carbon-concentrating mechanism in its own right. Xerophytes , such as cacti and most succulents , also use PEP carboxylase to capture carbon dioxide in a process called Crassulacean acid metabolism (CAM). In contrast to C 4 metabolism, which spatially separates the CO 2 fixation to PEP from the Calvin cycle, CAM temporally separates these two processes. CAM plants have

1190-474: Is commonly measured in μmols /( m / s ), parts per million, or volume per million; and H 2 O is commonly measured in mmols /(m /s) or in mbars . By measuring CO 2 assimilation , ΔH 2 O, leaf temperature, barometric pressure , leaf area, and photosynthetically active radiation (PAR), it becomes possible to estimate, "A" or carbon assimilation, "E" or transpiration , "gs" or stomatal conductance , and "Ci" or intracellular CO 2 . However, it

1260-474: Is converted into glucose. In the Calvin-Benson cycle , RuBP is a product of the phosphorylation of ribulose-5-phosphate (produced by glyceraldehyde 3-phosphate ) by ATP . RuBP acts as an enzyme inhibitor for the enzyme rubisco, which regulates the net activity of carbon fixation. When RuBP is bound to an active site of rubisco, the ability to activate via carbamylation with CO 2 and Mg

1330-430: Is converted to CO 2 by an oxalate oxidase enzyme, and the produced CO 2 can support the Calvin cycle reactions. Reactive hydrogen peroxide (H 2 O 2 ), the byproduct of oxalate oxidase reaction, can be neutralized by catalase . Alarm photosynthesis represents a photosynthetic variant to be added to the well-known C4 and CAM pathways. However, alarm photosynthesis, in contrast to these pathways, operates as

1400-419: Is freed from its locked position through a classic "hop". The movement of the electron towards the photo center is therefore covered in a series of conventional hops and quantum walks. Fossils of what are thought to be filamentous photosynthetic organisms have been dated at 3.4 billion years old. More recent studies also suggest that photosynthesis may have begun about 3.4 billion years ago, though

1470-412: Is further excited by the light absorbed by that photosystem . The electron is then passed along a chain of electron acceptors to which it transfers some of its energy . The energy delivered to the electron acceptors is used to move hydrogen ions across the thylakoid membrane into the lumen . The electron is eventually used to reduce the coenzyme NADP with an H to NADPH (which has functions in

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1540-478: Is limited in CAM photosynthesis due to kinetic delays in enzyme activation, again stemming from the ratio of carbon dioxide to oxygen. RuBP can be measured isotopically via the conversion of CO 2 and RuBP into glyceraldehyde 3-phosphate . G3P can then be measured using an enzymatic optical assay . Given the abundance of RuBP in biological samples, an added difficulty is distinguishing particular reservoirs of

1610-451: Is more common to use chlorophyll fluorescence for plant stress measurement , where appropriate, because the most commonly used parameters FV/FM and Y(II) or F/FM' can be measured in a few seconds, allowing the investigation of larger plant populations. Gas exchange systems that offer control of CO 2 levels, above and below ambient , allow the common practice of measurement of A/Ci curves, at different CO 2 levels, to characterize

1680-406: Is passed through a food chain . The fixation or reduction of carbon dioxide is a process in which carbon dioxide combines with a five-carbon sugar , ribulose 1,5-bisphosphate , to yield two molecules of a three-carbon compound, glycerate 3-phosphate , also known as 3-phosphoglycerate. Glycerate 3-phosphate, in the presence of ATP and NADPH produced during the light-dependent stages,

1750-623: Is reduced to glyceraldehyde 3-phosphate . This product is also referred to as 3-phosphoglyceraldehyde (PGAL) or, more generically, as triose phosphate. Most (five out of six molecules) of the glyceraldehyde 3-phosphate produced are used to regenerate ribulose 1,5-bisphosphate so the process can continue. The triose phosphates not thus "recycled" often condense to form hexose phosphates, which ultimately yield sucrose , starch , and cellulose , as well as glucose and fructose . The sugars produced during carbon metabolism yield carbon skeletons that can be used for other metabolic reactions like

1820-413: Is the highly unstable six-carbon intermediate known as 3-keto-2-carboxyarabinitol 1,5-bisphosphate, or 2'-carboxy-3-keto-D-arabinitol 1,5-bisphosphate (CKABP). This six-carbon β-ketoacid intermediate hydrates into another six-carbon intermediate in the form of a gem-diol . This intermediate then cleaves into two molecules of 3-phosphoglycerate (3-PGA) which is used in a number of metabolic pathways and

1890-429: Is then translocated to specialized bundle sheath cells where the enzyme RuBisCO and other Calvin cycle enzymes are located, and where CO 2 released by decarboxylation of the four-carbon acids is then fixed by RuBisCO activity to the three-carbon 3-phosphoglyceric acids . The physical separation of RuBisCO from the oxygen-generating light reactions reduces photorespiration and increases CO 2 fixation and, thus,

1960-404: Is then converted into the final carbohydrate products. The simple carbon sugars photosynthesis produces are then used to form other organic compounds , such as the building material cellulose , the precursors for lipid and amino acid biosynthesis, or as a fuel in cellular respiration . The latter occurs not only in plants but also in animals when the carbon and energy from plants

2030-499: Is used by 16,000 species of plants. Calcium-oxalate -accumulating plants, such as Amaranthus hybridus and Colobanthus quitensis , show a variation of photosynthesis where calcium oxalate crystals function as dynamic carbon pools , supplying carbon dioxide (CO 2 ) to photosynthetic cells when stomata are partially or totally closed. This process was named alarm photosynthesis . Under stress conditions (e.g., water deficit ), oxalate released from calcium oxalate crystals

2100-474: Is vital for climate processes, as it captures carbon dioxide from the air and binds it into plants, harvested produce and soil. Cereals alone are estimated to bind 3,825 Tg or 3.825 Pg of carbon dioxide every year, i.e. 3.825 billion metric tons. Most photosynthetic organisms are photoautotrophs , which means that they are able to synthesize food directly from carbon dioxide and water using energy from light. However, not all organisms use carbon dioxide as

2170-572: The Paleoarchean , preceding that of cyanobacteria (see Purple Earth hypothesis ). While the details may differ between species , the process always begins when light energy is absorbed by the reaction centers , proteins that contain photosynthetic pigments or chromophores . In plants, these proteins are chlorophylls (a porphyrin derivative that absorbs the red and blue spectrums of light, thus reflecting green) held inside chloroplasts , abundant in leaf cells. In bacteria, they are embedded in

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2240-408: The chemical energy necessary to fuel their metabolism . Photosynthesis usually refers to oxygenic photosynthesis , a process that produces oxygen. Photosynthetic organisms store the chemical energy so produced within intracellular organic compounds (compounds containing carbon) like sugars, glycogen , cellulose and starches . To use this stored chemical energy, an organism's cells metabolize

2310-637: The light reaction of photosynthesis by using chlorophyll fluorometers . Actual plants' photosynthetic efficiency varies with the frequency of the light being converted, light intensity , temperature , and proportion of carbon dioxide in the atmosphere , and can vary from 0.1% to 8%. By comparison, solar panels convert light into electric energy at an efficiency of approximately 6–20% for mass-produced panels, and above 40% in laboratory devices. Scientists are studying photosynthesis in hopes of developing plants with increased yield . The efficiency of both light and dark reactions can be measured, but

2380-434: The light-independent (or "dark") reactions, the enzyme RuBisCO captures CO 2 from the atmosphere and, in a process called the Calvin cycle , uses the newly formed NADPH and releases three-carbon sugars , which are later combined to form sucrose and starch . The overall equation for the light-independent reactions in green plants is Carbon fixation produces the three-carbon sugar intermediate , which

2450-417: The palisade mesophyll cells where most of the photosynthesis takes place. In the light-dependent reactions , one molecule of the pigment chlorophyll absorbs one photon and loses one electron . This electron is taken up by a modified form of chlorophyll called pheophytin , which passes the electron to a quinone molecule, starting the flow of electrons down an electron transport chain that leads to

2520-417: The photosynthetic capacity of the leaf . C 4 plants can produce more sugar than C 3 plants in conditions of high light and temperature . Many important crop plants are C 4 plants, including maize , sorghum , sugarcane , and millet . Plants that do not use PEP-carboxylase in carbon fixation are called C 3 plants because the primary carboxylation reaction , catalyzed by RuBisCO, produces

2590-462: The photosystems , quantum efficiency and the CO 2 assimilation rates. With some instruments, even wavelength dependency of the photosynthetic efficiency can be analyzed . A phenomenon known as quantum walk increases the efficiency of the energy transport of light significantly. In the photosynthetic cell of an alga , bacterium , or plant, there are light-sensitive molecules called chromophores arranged in an antenna-shaped structure called

2660-437: The plasma membrane . In these light-dependent reactions, some energy is used to strip electrons from suitable substances, such as water, producing oxygen gas. The hydrogen freed by the splitting of water is used in the creation of two important molecules that participate in energetic processes: reduced nicotinamide adenine dinucleotide phosphate (NADPH) and ATP. In plants, algae, and cyanobacteria, sugars are synthesized by

2730-450: The reverse Krebs cycle are used to achieve the same end. The first photosynthetic organisms probably evolved early in the evolutionary history of life using reducing agents such as hydrogen or hydrogen sulfide, rather than water, as sources of electrons. Cyanobacteria appeared later; the excess oxygen they produced contributed directly to the oxygenation of the Earth , which rendered

2800-433: The CO 2 concentration in the leaves under these conditions. Plants that use the C 4 carbon fixation process chemically fix carbon dioxide in the cells of the mesophyll by adding it to the three-carbon molecule phosphoenolpyruvate (PEP), a reaction catalyzed by an enzyme called PEP carboxylase , creating the four-carbon organic acid oxaloacetic acid . Oxaloacetic acid or malate synthesized by this process

2870-499: The Calvin-Benson Cycle, the photorespiratory pathway has been noted for its enzymatic inefficiency although this characterization of the enzymatic kinetics of rubisco have been contested. Due to enhanced RuBP carboxylation and decreased rubisco oxygenation stemming from the increased concentration of CO 2 in the bundle sheath , rates of photorespiration are decreased in C 4 plants . Similarly, photorespiration

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2940-419: The action spectrum is blue-green light, which allows these algae to use the blue end of the spectrum to grow in the deeper waters that filter out the longer wavelengths (red light) used by above-ground green plants. The non-absorbed part of the light spectrum is what gives photosynthetic organisms their color (e.g., green plants, red algae, purple bacteria ) and is the least effective for photosynthesis in

3010-521: The atmosphere. Although there are some differences between oxygenic photosynthesis in plants , algae , and cyanobacteria , the overall process is quite similar in these organisms. There are also many varieties of anoxygenic photosynthesis , used mostly by bacteria, which consume carbon dioxide but do not release oxygen. Carbon dioxide is converted into sugars in a process called carbon fixation ; photosynthesis captures energy from sunlight to convert carbon dioxide into carbohydrates . Carbon fixation

3080-480: The carboxysome quickly sponges it up. HCO 3 ions are made from CO 2 outside the cell by another carbonic anhydrase and are actively pumped into the cell by a membrane protein. They cannot cross the membrane as they are charged, and within the cytosol they turn back into CO 2 very slowly without the help of carbonic anhydrase. This causes the HCO 3 ions to accumulate within the cell from where they diffuse into

3150-491: The carboxysomes. Pyrenoids in algae and hornworts also act to concentrate CO 2 around RuBisCO. The overall process of photosynthesis takes place in four stages: Plants usually convert light into chemical energy with a photosynthetic efficiency of 3–6%. Absorbed light that is unconverted is dissipated primarily as heat , with a small fraction (1–2%) reemitted as chlorophyll fluorescence at longer (redder) wavelengths . This fact allows measurement of

3220-409: The conditions of non-cyclic electron flow in green plants is: Not all wavelengths of light can support photosynthesis. The photosynthetic action spectrum depends on the type of accessory pigments present. For example, in green plants , the action spectrum resembles the absorption spectrum for chlorophylls and carotenoids with absorption peaks in violet-blue and red light. In red algae ,

3290-503: The diagram), a chemiosmotic potential is generated by pumping proton cations ( H ) across the membrane and into the thylakoid space . An ATP synthase enzyme uses that chemiosmotic potential to make ATP during photophosphorylation , whereas NADPH is a product of the terminal redox reaction in the Z-scheme . The electron enters a chlorophyll molecule in Photosystem I . There it

3360-505: The equation for this process is: This equation emphasizes that water is both a reactant in the light-dependent reaction and a product of the light-independent reaction , but canceling n water molecules from each side gives the net equation: Other processes substitute other compounds (such as arsenite ) for water in the electron-supply role; for example some microbes use sunlight to oxidize arsenite to arsenate : The equation for this reaction is: Photosynthesis occurs in two stages. In

3430-518: The evolution of complex life possible. The average rate of energy captured by global photosynthesis is approximately 130 terawatts , which is about eight times the total power consumption of human civilization . Photosynthetic organisms also convert around 100–115 billion tons (91–104 Pg petagrams , or billions of metric tons), of carbon into biomass per year. Photosynthesis was discovered in 1779 by Jan Ingenhousz . He showed that plants need light, not just air, soil, and water. Photosynthesis

3500-400: The first direct evidence of photosynthesis comes from thylakoid membranes preserved in 1.75-billion-year-old cherts . Ribulose bisphosphate RuBP was originally discovered by Andrew Benson in 1951 while working in the lab of Melvin Calvin at UC Berkeley. Calvin, who had been away from the lab at the time of discovery and was not listed as a co-author, controversially removed

3570-582: The first stage, light-dependent reactions or light reactions capture the energy of light and use it to make the hydrogen carrier NADPH and the energy-storage molecule ATP . During the second stage, the light-independent reactions use these products to capture and reduce carbon dioxide. Most organisms that use oxygenic photosynthesis use visible light for the light-dependent reactions, although at least three use shortwave infrared or, more specifically, far-red radiation. Some organisms employ even more radical variants of photosynthesis. Some archaea use

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3640-441: The first step of the Z-scheme , requires an external source of electrons to reduce its oxidized chlorophyll a reaction center. The source of electrons for photosynthesis in green plants and cyanobacteria is water. Two water molecules are oxidized by the energy of four successive charge-separation reactions of photosystem II to yield a molecule of diatomic oxygen and four hydrogen ions. The electrons yielded are transferred to

3710-422: The full molecule name from the title of the initial paper, identifying it solely as "ribulose". At the time, the molecule was known as ribulose diphosphate (RDP or RuDP) but the prefix di- was changed to bis- to emphasize the nonadjacency of the two phosphate groups. The enzyme ribulose-1,5-bisphosphate carboxylase-oxygenase ( rubisco ) catalyzes the reaction between RuBP and carbon dioxide . The product

3780-399: The interior of a cell, giving the membrane a very large surface area and therefore increasing the amount of light that the bacteria can absorb. In plants and algae, photosynthesis takes place in organelles called chloroplasts . A typical plant cell contains about 10 to 100 chloroplasts. The chloroplast is enclosed by a membrane. This membrane is composed of a phospholipid inner membrane,

3850-433: The interior tissues of a leaf, called the mesophyll , can contain between 450,000 and 800,000 chloroplasts for every square millimeter of leaf. The surface of the leaf is coated with a water-resistant waxy cuticle that protects the leaf from excessive evaporation of water and decreases the absorption of ultraviolet or blue light to minimize heating . The transparent epidermis layer allows light to pass through to

3920-434: The light reaction, and infrared gas analyzers can measure the dark reaction . An integrated chlorophyll fluorometer and gas exchange system can investigate both light and dark reactions when researchers use the two separate systems together. Infrared gas analyzers and some moisture sensors are sensitive enough to measure the photosynthetic assimilation of CO 2 and of Δ H 2 O using reliable methods . CO 2

3990-433: The light-independent reaction); at that point, the path of that electron ends. The cyclic reaction is similar to that of the non-cyclic but differs in that it generates only ATP, and no reduced NADP (NADPH) is created. The cyclic reaction takes place only at photosystem I. Once the electron is displaced from the photosystem, the electron is passed down the electron acceptor molecules and returns to photosystem I, from where it

4060-432: The more common types of photosynthesis. In photosynthetic bacteria, the proteins that gather light for photosynthesis are embedded in cell membranes . In its simplest form, this involves the membrane surrounding the cell itself. However, the membrane may be tightly folded into cylindrical sheets called thylakoids , or bunched up into round vesicles called intracytoplasmic membranes . These structures can fill most of

4130-439: The most efficient route, where it will have the highest probability of arriving at its destination in the minimum possible time. Because that quantum walking takes place at temperatures far higher than quantum phenomena usually occur, it is only possible over very short distances. Obstacles in the form of destructive interference cause the particle to lose its wave properties for an instant before it regains them once again after it

4200-498: The organic compounds through cellular respiration . Photosynthesis plays a critical role in producing and maintaining the oxygen content of the Earth's atmosphere, and it supplies most of the biological energy necessary for complex life on Earth. Some bacteria also perform anoxygenic photosynthesis , which uses bacteriochlorophyll to split hydrogen sulfide as a reductant instead of water, producing sulfur instead of oxygen. Archaea such as Halobacterium also perform

4270-410: The organism's metabolism . Photosynthesis and cellular respiration are distinct processes, as they take place through different sequences of chemical reactions and in different cellular compartments (cellular respiration in mitochondria ). The general equation for photosynthesis as first proposed by Cornelis van Niel is: Since water is used as the electron donor in oxygenic photosynthesis,

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4340-556: The photosynthetic system. Plants absorb light primarily using the pigment chlorophyll . The green part of the light spectrum is not absorbed but is reflected, which is the reason that most plants have a green color. Besides chlorophyll, plants also use pigments such as carotenes and xanthophylls . Algae also use chlorophyll, but various other pigments are present, such as phycocyanin , carotenes , and xanthophylls in green algae , phycoerythrin in red algae (rhodophytes) and fucoxanthin in brown algae and diatoms resulting in

4410-476: The production of amino acids and lipids . In hot and dry conditions , plants close their stomata to prevent water loss. Under these conditions, CO 2 will decrease and oxygen gas , produced by the light reactions of photosynthesis, will increase, causing an increase of photorespiration by the oxygenase activity of ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) and decrease in carbon fixation. Some plants have evolved mechanisms to increase

4480-486: The relationship between the two can be complex. For example, the light reaction creates ATP and NADPH energy molecules , which C 3 plants can use for carbon fixation or photorespiration . Electrons may also flow to other electron sinks. For this reason, it is not uncommon for authors to differentiate between work done under non-photorespiratory conditions and under photorespiratory conditions . Chlorophyll fluorescence of photosystem II can measure

4550-462: The respective organisms . In plants , light-dependent reactions occur in the thylakoid membranes of the chloroplasts where they drive the synthesis of ATP and NADPH . The light-dependent reactions are of two forms: cyclic and non-cyclic . In the non-cyclic reaction, the photons are captured in the light-harvesting antenna complexes of photosystem II by chlorophyll and other accessory pigments (see diagram at right). The absorption of

4620-479: The site of carboxylation in the chloroplast, to replace Ci. CO 2 concentration in the chloroplast becomes possible to estimate with the measurement of mesophyll conductance or g m using an integrated system. Photosynthesis measurement systems are not designed to directly measure the amount of light the leaf absorbs, but analysis of chlorophyll fluorescence , P700 - and P515-absorbance, and gas exchange measurements reveal detailed information about, e.g.,

4690-406: The three-carbon 3-phosphoglyceric acids directly in the Calvin-Benson cycle . Over 90% of plants use C 3 carbon fixation, compared to 3% that use C 4 carbon fixation; however, the evolution of C 4 in over sixty plant lineages makes it a striking example of convergent evolution . C 2 photosynthesis , which involves carbon-concentration by selective breakdown of photorespiratory glycine,

4760-424: The ultimate reduction of NADP to NADPH . In addition, this creates a proton gradient (energy gradient) across the chloroplast membrane , which is used by ATP synthase in the synthesis of ATP . The chlorophyll molecule ultimately regains the electron it lost when a water molecule is split in a process called photolysis , which releases oxygen . The overall equation for the light-dependent reactions under

4830-408: The water-oxidizing reaction (Kok's S-state diagrams). The hydrogen ions are released in the thylakoid lumen and therefore contribute to the transmembrane chemiosmotic potential that leads to ATP synthesis . Oxygen is a waste product of light-dependent reactions, but the majority of organisms on Earth use oxygen and its energy for cellular respiration , including photosynthetic organisms . In

4900-462: Was emitted, hence the name cyclic reaction . Linear electron transport through a photosystem will leave the reaction center of that photosystem oxidized . Elevating another electron will first require re-reduction of the reaction center. The excited electrons lost from the reaction center ( P700 ) of photosystem I are replaced by transfer from plastocyanin , whose electrons come from electron transport through photosystem II . Photosystem II, as

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