Biological process to convert light into chemical energySchematic of photosynthesis in plants. The carbohydrates produced are stored in or used by the plant. Overall equation for the type of photosynthesis that occurs in plants Composite image showing the global distribution of photosynthesis, including both oceanic and terrestrial . Dark red and blue-green indicate regions of high photosynthetic activity in the ocean and on land, respectively.
Photosynthesis is a process used by plants and other organisms to convert energy into that can later be to fuel the organisms' activities (). This chemical energy is stored in , such as , which are synthesized from and – hence the name photosynthesis, from the , phōs, "light", and , synthesis, "putting together". In most cases, is also released as a waste product. Most , most , and perform photosynthesis; such organisms are called . Photosynthesis is largely responsible for producing and maintaining the of the Earth's atmosphere, and supplies all of the organic compounds and most of the energy necessary for .
Although photosynthesis is performed differently by different species, the process always begins when energy from light is absorbed by called that contain green pigments. In plants, these proteins are held inside called , which are most abundant in leaf cells, while in bacteria they are embedded in the . In these light-dependent reactions, some energy is used to strip from suitable substances, such as water, producing oxygen gas. The hydrogen freed by the splitting of water is used in the creation of two further compounds that serve as short-term stores of energy, enabling its transfer to drive other reactions: these compounds are reduced (NADPH) and (ATP), the "energy currency" of cells.
In plants, algae and cyanobacteria, long-term energy storage in the form of sugars is produced by a subsequent sequence of light-independent reactions called the ; some bacteria use different mechanisms, such as the , to achieve the same end. In the Calvin cycle, atmospheric carbon dioxide is into already existing organic carbon compounds, such as (RuBP). Using the ATP and NADPH produced by the light-dependent reactions, the resulting compounds are then and removed to form further carbohydrates, such as .
The first photosynthetic organisms probably early in the and most likely used such as or , rather than water, as sources of electrons. Cyanobacteria appeared later; the excess oxygen they produced contributed directly to the , which rendered the possible. Today, the average rate of energy capture by photosynthesis globally is approximately 130 , which is about three times the current . Photosynthetic organisms also convert around 100–115 thousand million tonnes of carbon into per year.
OverviewPhotosynthesis changes sunlight into chemical energy, splits water to liberate O2, and fixes CO2 into sugar.
Photosynthetic organisms are , which means that they are able to food directly from carbon dioxide and water using energy from light. However, not all organisms that use light as a source of energy carry out photosynthesis; use organic compounds, rather than carbon dioxide, as a source of carbon. In plants, algae, and cyanobacteria, photosynthesis releases oxygen. This is called oxygenic photosynthesis and is by far the most common type of photosynthesis used by living organisms. Although there are some differences between oxygenic photosynthesis in , , and , the overall process is quite similar in these organisms. There are also many varieties of , used mostly by certain types of bacteria, which consume carbon dioxide but do not release oxygen.
Carbon dioxide is converted into sugars in a process called ; photosynthesis captures energy from sunlight to convert carbon dioxide into . Carbon fixation is an reaction. In general outline, photosynthesis is the opposite of : while photosyntesis is a process of reduction of carbon dioxide to carbohydrate, cellular respiration is the oxidation of carbohydrate or other 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 the organism's . Photosynthesis and cellular respiration are distinct processes, as they take place through different sequences of chemical reactions and in different cellular compartments.
The general for photosynthesis as first proposed by is therefore:CO2carbon
dioxide + 2H2Aelectron donor + light energy → [CH2O] + 2Aoxidized
donor + H2Owater
Since water is used as the electron donor in oxygenic photosynthesis, the equation for this process is:CO2carbon
dioxide + 2H2Owater + photonslight energy → [CH2O]carbohydrate + O2oxygen + H2Owater
This equation emphasizes that water is both a reactant in the and a product of the , but canceling n water molecules from each side gives the net equation:CO2carbon
dioxide + H2O water + photonslight energy → [CH2O]carbohydrate + O2 oxygen
Other processes substitute other compounds (such as ) for water in the electron-supply role; for example some microbes use sunlight to oxidize arsenite to : The equation for this reaction is:CO2carbon
dioxide + (AsO3−
arsenite + photonslight energy → (AsO3−
arsenate + COcarbon
monoxide(used to build other compounds in subsequent reactions)
Photosynthesis occurs in two stages. In the first stage, light-dependent reactions or light reactions capture the energy of light and use it to make the energy-storage molecules and . During the second stage, the light-independent reactions use these products to capture and reduce carbon dioxide.
Most organisms that utilize oxygenic photosynthesis use for the light-dependent reactions, although at least three use shortwave or, more specifically, far-red radiation.
Some organisms employ even more radical variants of photosynthesis. Some use a simpler method that employs a pigment similar to those used for vision in animals. The changes its configuration in response to sunlight, acting as a proton pump. This produces a proton gradient more directly, which is then converted to chemical energy. The process does not involve carbon dioxide fixation and does not release oxygen, and seems to have evolved separately from the more common types of photosynthesis.
Photosynthetic membranes and organellesChloroplast ultrastructure:
1. outer membrane
2. intermembrane space
3. inner membrane (1+2+3: envelope)
4. stroma (aqueous fluid)
5. thylakoid lumen (inside of thylakoid)
6. thylakoid membrane
7. granum (stack of thylakoids)
8. thylakoid (lamella)
11. plastidial DNA
12. plastoglobule (drop of lipids)
Main articles: and
In photosynthetic bacteria, the proteins that gather light for photosynthesis are embedded in . In its simplest form, this involves the membrane surrounding the cell itself. However, the membrane may be tightly folded into cylindrical sheets called , or bunched up into round called intracytoplasmic membranes. These structures can fill most of the interior of a cell, giving the membrane a very large surface area and therefore increasing the amount of light that the bacteria can absorb.
In plants and algae, photosynthesis takes place in called . A typical contains about 10 to 100 chloroplasts. The chloroplast is enclosed by a membrane. This membrane is composed of a phospholipid inner membrane, a phospholipid outer membrane, and an intermembrane space. Enclosed by the membrane is an aqueous fluid called the stroma. Embedded within the stroma are stacks of thylakoids (grana), which are the site of photosynthesis. The thylakoids appear as flattened disks. The thylakoid itself is enclosed by the thylakoid membrane, and within the enclosed volume is a lumen or thylakoid space. Embedded in the thylakoid membrane are integral and complexes of the photosynthetic system.
Plants absorb light primarily using the . 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 and . Algae also use chlorophyll, but various other pigments are present, such as , , and in , in (rhodophytes) and in and resulting in 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 .
Although all cells in the green parts of a plant have chloroplasts, the majority of those are found in specially adapted structures called . Certain species adapted to conditions of strong sunlight and , such as many and species, have their main photosynthetic organs in their stems. The cells in the interior tissues of a leaf, called the , 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 that protects the leaf from excessive of water and decreases the absorption of or to reduce . The transparent layer allows light to pass through to the mesophyll cells where most of the photosynthesis takes place.
Light-dependent reactionsLight-dependent reactions of photosynthesis at the thylakoid membrane
In the , one molecule of the absorbs one and loses one . This electron is passed to a modified form of chlorophyll called , which passes the electron to a molecule, starting the flow of electrons down an that leads to the ultimate reduction of to . In addition, this creates a (energy gradient) across the , which is used by in the synthesis of . The chlorophyll molecule ultimately regains the electron it lost when a water molecule is split in a process called , which releases a (O2) molecule as a waste product.
The overall equation for the light-dependent reactions under the conditions of non-cyclic electron flow in green plants is:2 H2O + 2 NADP+ + 3 ADP + 3 Pi + light → 2 NADPH + 2 H+ + 3 ATP + O2
Not all of light can support photosynthesis. The photosynthetic action spectrum depends on the type of present. For example, in green plants, the resembles the for and with absorption peaks in violet-blue and red light. In red algae, the action spectrum is blue-green light, which allows these algae to use the blue end of the spectrum to grow in the deeper waters that filter out the longer wavelengths (red light) used by above ground green plants. The non-absorbed part of the 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 schemeThe "Z scheme"
In plants, occur in the of the 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 are captured in the light-harvesting of by and other (see diagram at right). The absorption of a photon by the antenna complex frees an electron by a process called . The antenna system is at the core of the chlorophyll molecule of the photosystem II reaction center. That freed electron is transferred to the primary electron-acceptor molecule, pheophytin. As the electrons are shuttled through an (the so-called Z-scheme shown in the diagram), it initially functions to generate a by pumping proton cations (H+) across the membrane and into the thylakoid space. An enzyme uses that chemiosmotic potential to make ATP during , whereas is a product of the terminal reaction in the Z-scheme. The electron enters a chlorophyll molecule in . There it is further excited by the light absorbed by that . The electron is then passed along a chain of to which it transfers some of its energy. The energy delivered to the electron acceptors is used to move hydrogen ions across the thylakoid membrane into the lumen. The electron is eventually used to reduce the co-enzyme NADP with a H+ to NADPH (which has functions in the light-independent reaction); at that point, the path of that electron ends.
The cyclic reaction is similar to that of the non-cyclic, but differs in that it generates only ATP, and no reduced NADP (NADPH) is created. The cyclic reaction takes place only at photosystem I. Once the electron is displaced from the photosystem, the electron is passed down the electron acceptor molecules and returns to photosystem I, from where it was emitted, hence the name cyclic reaction.
Main articles: and
The NADPH is the main produced by chloroplasts, which then goes on to provide a source of energetic electrons in other cellular reactions. Its production leaves chlorophyll in photosystem I with a deficit of electrons (chlorophyll has been oxidized), which must be balanced by some other reducing agent that will supply the missing electron. The excited electrons lost from chlorophyll from are supplied from the electron transport chain by . However, since is the first step of the Z-scheme, an external source of electrons is required to reduce its oxidized chlorophyll a molecules. The source of electrons in green-plant and cyanobacterial photosynthesis is water. Two water molecules are oxidized by four successive charge-separation reactions by photosystem II to yield a molecule of diatomic and four ions; the electrons yielded are transferred to a redox-active residue that then reduces the oxidized chlorophyll a (called P680) that serves as the primary light-driven electron donor in the photosystem II reaction center. That photo receptor is in effect reset and is then able to repeat the absorption of another photon and the release of another photo-dissociated electron. The oxidation of water is in photosystem II by a redox-active structure that contains four ions and a calcium ion; this binds two water molecules and contains the four oxidizing equivalents that are used to drive the water-oxidizing reaction (Dolai's S-state diagrams). Photosystem II is the only known biological that carries out this oxidation of water. The hydrogen ions released contribute to the transmembrane chemiosmotic potential that leads to ATP synthesis. Oxygen is a waste product of light-dependent reactions, but the majority of organisms on Earth use oxygen for , including photosynthetic organisms.
Main articles: and
In the (or "dark") reactions, the captures from the and, in a process called the , it uses the newly formed NADPH and releases three-carbon sugars, which are later combined to form sucrose and starch. The overall equation for the light-independent reactions in green plants is:1283 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
Carbon fixation produces the intermediate three-carbon sugar product, which is then converted to the final carbohydrate products. The simple carbon sugars produced by photosynthesis are then used in the forming of other organic compounds, such as the building material , the precursors for and biosynthesis, or as a fuel in . The latter occurs not only in plants but also in when the energy from plants is passed through a .
The fixation or reduction of carbon dioxide is a process in which combines with a five-carbon sugar, , to yield two molecules of a three-carbon compound, , also known as 3-phosphoglycerate. Glycerate 3-phosphate, in the presence of and produced during the light-dependent stages, is reduced to . This product is also referred to as 3-phosphoglyceraldehyde () or, more generically, as phosphate. Most (5 out of 6 molecules) of the glyceraldehyde 3-phosphate produced is used to regenerate ribulose 1,5-bisphosphate so the process can continue. The triose phosphates not thus "recycled" often condense to form phosphates, which ultimately yield , and . The sugars produced during carbon yield carbon skeletons that can be used for other metabolic reactions like the production of and .
Carbon concentrating mechanisms
In hot and dry conditions, plants close their to prevent water loss. Under these conditions, CO2 will decrease and oxygen gas, produced by the light reactions of photosynthesis, will increase, causing an increase of by the activity of and decrease in carbon fixation. Some plants have mechanisms to increase the CO2 concentration in the leaves under these conditions.
Plants that use the carbon fixation process chemically fix carbon dioxide in the cells of the mesophyll by adding it to the three-carbon molecule , a reaction catalyzed by an enzyme called , creating the four-carbon organic acid . Oxaloacetic acid or synthesized by this process is then translocated to specialized cells where the enzyme and other Calvin cycle enzymes are located, and where CO2 released by of the four-carbon acids is then fixed by RuBisCO activity to the three-carbon . The physical separation of RuBisCO from the oxygen-generating light reactions reduces photorespiration and increases CO2 fixation and, thus, the of the leaf. C4 plants can produce more sugar than C3 plants in conditions of high light and temperature. Many important crop plants are C4 plants, including maize, sorghum, sugarcane, and millet. Plants that do not use PEP-carboxylase in carbon fixation are called because the primary carboxylation reaction, catalyzed by RuBisCO, produces the three-carbon 3-phosphoglyceric acids directly in the Calvin-Benson cycle. Over 90% of plants use C3 carbon fixation, compared to 3% that use C4 carbon fixation; however, the evolution of C4 in over 60 plant lineages makes it a striking example of .
, such as and most , also use PEP carboxylase to capture carbon dioxide in a process called (CAM). In contrast to C4 metabolism, which spatially separates the CO2 fixation to PEP from the Calvin cycle, CAM temporally separates these two processes. CAM plants have a different leaf anatomy from C3 plants, and fix the CO2 at night, when their stomata are open. CAM plants store the CO2 mostly in the form of via carboxylation of to oxaloacetate, which is then reduced to malate. Decarboxylation of malate during the day releases CO2 inside the leaves, thus allowing carbon fixation to 3-phosphoglycerate by RuBisCO. Sixteen thousand species of plants use CAM.
possess , which increase the concentration of CO2 around RuBisCO to increase the rate of photosynthesis. An enzyme, , located within the carboxysome releases CO2 from the dissolved hydrocarbonate ions (HCO−
3). Before the CO2 diffuses out it is quickly sponged up by RuBisCO, which is concentrated within the carboxysomes. HCO−
3 ions are made from CO2 outside the cell by another carbonic anhydrase and are actively pumped into the cell by a membrane protein. They cannot cross the membrane as they are charged, and within the cytosol they turn back into CO2 very slowly without the help of carbonic anhydrase. This causes the HCO−
3 ions to accumulate within the cell from where they diffuse into the carboxysomes. in and also act to concentrate CO2 around rubisco.
Order and kinetics
The overall process of photosynthesis takes place in four stages:Stage Description Time scale 1 Energy transfer in antenna chlorophyll (thylakoid membranes) to 2 Transfer of electrons in photochemical reactions (thylakoid membranes) to 3 Electron transport chain and ATP synthesis (thylakoid membranes) to 4 Carbon fixation and export of stable products to
usually convert light into with a of 3–6%. Absorbed light that is unconverted is dissipated primarily as heat, with a small fraction (1–2%) re-emitted as at longer (redder) wavelengths. This fact allows measurement of 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, convert light into at an efficiency of approximately 6–20% for mass-produced panels, and above 40% in laboratory devices.
The efficiency of both light and dark reactions can be measured but the relationship between the two can be complex. For example, the ATP and NADPH energy molecules, created by the light reaction, can be used for carbon fixation or for photorespiration in C3 plants. 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 the light reaction, and Infrared gas analyzers can measure the dark reaction. It is also possible to investigate both at the same time using an integrated chlorophyll fluorometer and gas exchange system, or by using two separate systems together. Infrared gas analyzers and some moisture sensors are sensitive enough to measure the photosynthetic assimilation of CO2, and of ΔH2O using reliable methods CO2 is commonly measured in μmols/m2/s−1, parts per million or volume per million and H20 is commonly measured in mmol/m2/s−1 or in mbars. By measuring CO2 assimilation, ΔH2O, leaf temperature, barometric pressure, leaf area, and photosynthetically active radiation or PAR, it becomes possible to estimate, “A” or carbon assimilation, “E” or transpiration, “gs” or stomatal conductance, and Ci or intracellular CO2. However, it is more common to used chlorophyll fluorescence for plant stress measurement, where appropriate, because the most commonly used measuring parameters FV/FM and Y(II) or F/FM’ can be made in a few seconds, allowing the measurement of larger plant populations.
Gas exchange systems that offer control of CO2 levels, above and below ambient, allow the common practice of measurement of A/Ci curves, at different CO2 levels, to characterize a plant’s photosynthetic response.
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 CO2 levels, the addition of integrated chlorophyll fluorescence measurements allows a more precise measurement of CC to replace Ci. The estimation of CO2 at the site of carboxylation in the chloroplast, or CC, becomes possible with the measurement of mesophyll conductance or gm using an integrated system.
Photosynthesis measurement systems are not designed to directly measure the amount of light absorbed by the leaf. But analysis of chlorophyll-fluorescence, P700- and P515-absorbance and gas exchange measurements reveal detailed information about e.g. the photosystems, quantum efficiency and the CO2 assimilation rates. With some instruments even wavelength-dependency of the photosynthetic efficiency can be analyzed.
A phenomenon known as increases the efficiency of the energy transport of light significantly. In the photosynthetic cell of an algae, bacterium, or plant, there are light-sensitive molecules called arranged in an antenna-shaped structure named a photocomplex. When a photon is absorbed by a chromophore, it is converted into a referred to as an , which jumps from chromophore to chromophore towards the reaction center of the photocomplex, a collection of molecules that traps its energy in a chemical form that makes it accessible for the cell's metabolism. The exciton's wave properties enable it to cover a wider area and try out several possible paths simultaneously, allowing it to instantaneously "choose" the most efficient route, where it will have the highest probability of arriving at its destination in the minimum possible time. Because that quantum walking takes place at temperatures far higher than quantum phenomena usually occur, it is only possible over very short distances, due to obstacles in the form of destructive interference that come into play. These obstacles cause the particle to lose its wave properties for an instant before it regains them once again after it is freed from its locked position through a classic "hop". The movement of the electron towards the photo center is therefore covered in a series of conventional hops and quantum walks.
Early photosynthetic systems, such as those in and and and , are thought to have been , and used various other molecules as rather than water. Green and purple sulfur bacteria are thought to have used and as electron donors. Green nonsulfur bacteria used various and other as an electron donor. Purple nonsulfur bacteria used a variety of nonspecific organic molecules. The use of these molecules is consistent with the geological evidence that Earth's early atmosphere was highly at .
Fossils of what are thought to be photosynthetic organisms have been dated at 3.4 billion years old. More recent studies, reported in March 2018, also suggest that photosynthesis may have begun about 3.4 billion years ago.
The main source of in the derives from , and its first appearance is sometimes referred to as the . Geological evidence suggests that oxygenic photosynthesis, such as that in , became important during the 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 to molecular oxygen (O
2) in the .
Symbiosis and the origin of chloroplasts
Several groups of animals have formed relationships with photosynthetic algae. These are most common in , and . It is presumed that this is due to the particularly simple and large surface areas of these animals compared to their volumes. In addition, a few marine and also maintain a symbiotic relationship with chloroplasts they capture from the algae in their diet and then store in their bodies. This allows the mollusks to survive solely by photosynthesis for several months at a time. Some of the genes from the plant have even been transferred to the slugs, so that the chloroplasts can be supplied with proteins that they need to survive.
An even closer form of symbiosis may explain the origin of chloroplasts. Chloroplasts have many similarities with , including a circular , prokaryotic-type , and similar proteins in the photosynthetic reaction center. The suggests that photosynthetic bacteria were acquired (by ) by early cells to form the first cells. Therefore, chloroplasts may be photosynthetic bacteria that adapted to life inside plant cells. Like , chloroplasts possess their own DNA, separate from the of their plant host cells and the genes in this chloroplast DNA resemble those found in . DNA in chloroplasts codes for proteins such as those found in the photosynthetic reaction centers. The proposes that this Co-location of genes with their gene products is required for Redox Regulation of gene expression, and accounts for the persistence of DNA in bioenergetic organelles.
Cyanobacteria and the evolution of photosynthesis
The biochemical capacity to use water as the source for electrons in photosynthesis evolved once, in a of extant . The geological record indicates that this transforming event took place early in Earth's history, at least 2450–2320 million years ago (Ma), and, it is speculated, much earlier. Because the Earth's atmosphere contained almost no oxygen during the estimated development of photosynthesis, it is believed that the first photosynthetic cyanobacteria did not generate oxygen. Available evidence from geobiological studies of (>2500 Ma) indicates that life existed 3500 Ma, but the question of when oxygenic photosynthesis evolved is still unanswered. A clear paleontological window on cyanobacterial opened about 2000 Ma, revealing an already-diverse biota of blue-green algae. remained the principal of oxygen throughout the (2500–543 Ma), in part because the redox structure of the oceans favored photoautotrophs capable of . joined blue-green algae as the major primary producers of oxygen on near the end of the , but it was only with the (251–66 Ma) radiations of dinoflagellates, coccolithophorids, and diatoms did the of oxygen in marine shelf waters take modern form. Cyanobacteria remain critical to as primary producers of oxygen in oceanic gyres, as agents of biological nitrogen fixation, and, in modified form, as the of marine algae.
Although some of the steps in photosynthesis are still not completely understood, the overall photosynthetic equation has been known since the 19th century.
began the research of the process in the mid-17th century when he carefully measured the 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 comes from the inputs of photosynthesis, not the soil itself.
, 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, , repeated Priestley's experiments. He discovered that it was the influence of sunlight on the plant that could cause it to revive a mouse in a matter of hours.
In 1796, , a Swiss pastor, botanist, and naturalist, demonstrated that green plants consume carbon dioxide and release oxygen under the influence of light. Soon afterward, 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.
made key discoveries explaining the chemistry of photosynthesis. By studying and green bacteria he was the first to demonstrate that photosynthesis is a light-dependent reaction, in which hydrogen reduces carbon dioxide.
discovered two light reactions by testing plant productivity using different wavelengths of light. With the red alone, the light reactions were suppressed. When blue and red were combined, the output was much more substantial. Thus, there were two photosystems, one absorbing up to 600 nm wavelengths, the other up to 700 nm. The former is known as PSII, the latter is PSI. PSI contains only chlorophyll "a", PSII contains primarily chlorophyll "a" with most of the available chlorophyll "b", among other pigment. These include phycobilins, which are the red and blue pigments of red and blue algae respectively, and fucoxanthol for brown algae and diatoms. The process is most productive when the absorption of quanta are equal in both the PSII and PSI, assuring that input energy from the antenna complex is divided between the PSI and PSII system, which in turn powers the photochemistry.works in his photosynthesis laboratory.
thought that a complex of reactions consisting of an intermediate to cytochrome b6 (now a plastoquinone), another is from cytochrome f to a step in the carbohydrate-generating mechanisms. These are linked by plastoquinone, which does require energy to reduce cytochrome f for it is a sufficient reductant. Further experiments to prove that the oxygen developed during the photosynthesis of green plants came from water, were performed by Hill in 1937 and 1939. He showed that isolated give off oxygen in the presence of unnatural reducing agents like , or 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.
and used to determine that the oxygen liberated in photosynthesis came from the water.
and , along with , elucidated the path of carbon assimilation (the photosynthetic carbon reduction cycle) in plants. The carbon reduction cycle is known as the , which ignores the contribution of Bassham and Benson. Many scientists refer to the cycle as the Calvin-Benson Cycle, Benson-Calvin, and some even call it the Calvin-Benson-Bassham (or CBB) Cycle.
-winning scientist was able to discover the function and significance of the electron transport chain.
and discovered the I-quantum photosynthesis reaction that splits the CO2, activated by the respiration.
In 1950, first experimental evidence for the existence of in vivo was presented by using intact cells and interpreting his findings as light-dependent formation. In 1954, et al. discovered photophosphorylation in vitro in isolated with the help of P32.
and discovered that chlorophyll a will absorb one light, oxidize cytochrome f, chlorophyll a (and other pigments) will absorb another light, but will reduce this same oxidized cytochrome, stating the two light reactions are in series.
Development of the concept
In 1893, proposed two terms, photosyntax and photosynthesis, for the biological process of synthesis of complex carbon compounds out of carbonic acid, in the presence of chlorophyll, under the influence of light. Over time, the term photosynthesis came into common usage as the term of choice. Later discovery of anoxygenic photosynthetic bacteria and photophosphorylation necessitated redefinition of the term.
C3 : C4 photosynthesis research
After WWII at late 1940 at the , the details of photosynthetic carbon metabolism were sorted out by the chemists , Andrew Benson, James Bassham and a score of students and researchers utilizing the carbon-14 isotope and paper chromatography techniques. The pathway of CO2 fixation by the algae Chlorella in a fraction of a second in light resulted in a 3 carbon molecule called phosphoglyceric acid (PGA). For that original and ground-breaking work, a was awarded to Melvin Calvin in 1961. In parallel, plant physiologists studied leaf gas exchanges using the new method of infrared gas analysis and a leaf chamber where the net photosynthetic rates ranged from 10 to 13 u mole CO2/square metere.sec., with the conclusion that all terrestrial plants having the same photosynthetic capacities that were light saturated at less than 50% of sunlight.
Later in 1958-1963 at , field grown was reported to have much greater leaf photosynthetic rates of 40 u mol CO2/square meter.sec and was not saturated at near full sunlight. This higher rate in maize was almost double those observed in other species such as wheat and soybean, indicating that large differences in photosynthesis exist among higher plants. At the University of Arizona, detailed gas exchange research on more than 15 species of monocot and dicot uncovered for the first time that differences in leaf anatomy are crucial factors in differentiating photosynthetic capacities among species. In tropical grasses, including maize, sorghum, sugarcane, Bermuda grass and in the dicot amaranthus, leaf photosynthetic rates were around 38−40 u mol CO2/square meter.sec., and the leaves have two types of green cells, i. e. outer layer of mesophyll cells surrounding a tightly packed cholorophyllous vascular bundle sheath cells. This type of anatomy was termed Kranz anatomy in the 19th century by the botanist while studying leaf anatomy of sugarcane. Plant species with the greatest photosynthetic rates and Kranz anatomy showed no apparent photorespiration, very low CO2 compensation point, high optimum temperature, high stomatal resistances and lower mesophyll resistances for gas diffusion and rates never saturated at full sun light. The research at Arizona was designated Citation Classic by the ISI 1986. These species was later termed C4 plants as the first stable compound of CO2 fixation in light has 4 carbon as malate and aspartate. Other species that lack Kranz anatomy were termed C3 type such as cotton and sunflower, as the first stable carbon compound is the 3-carbon PGA acid. At 1000 ppm CO2 in measuring air, both the C3 and C4 plants had similar leaf photosynthetic rates around 60 u mole CO2/square meter.sec. indicating the suppression of photorespiration in C3 plants.
FactorsThe is the primary site of photosynthesis in plants.
There are three main factors affecting photosynthesis and several corollary factors. The three main are:
Total photosynthesis is limited by a range of environmental factors. These include the amount of light available, the amount of area a plant has to capture light (shading by other plants is a major limitation of photosynthesis), rate at which carbon dioxide can be supplied to the to support photosynthesis, the availability of water, and the availability of suitable temperatures for carrying out photosynthesis.
Light intensity (irradiance), wavelength and temperature
See also:spectra of free chlorophyll a (green) and b (red) in a solvent. The action spectra of chlorophyll molecules are slightly modified in vivo depending on specific pigment-protein interactions.
The process of photosynthesis provides the main input of free energy into the biosphere, and is one of four main ways in which radiation is important for plant life.
The radiation climate within plant communities is extremely variable, with both time and space.
In the early 20th century, and investigated the effects of light intensity () and temperature on the rate of carbon assimilation.
- At constant temperature, the rate of carbon assimilation varies with irradiance, increasing as the irradiance increases, but reaching a plateau at higher irradiance.
- At low irradiance, increasing the temperature has little influence on the rate of carbon assimilation. At constant high irradiance, the rate of carbon assimilation increases as the temperature is increased.
These two experiments illustrate several important points: First, it is known that, in general, reactions are not affected by . 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 the temperature-independent stage, and the stage. Second, Blackman's experiments illustrate the concept of . 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 .
Carbon dioxide levels and photorespirationPhotorespiration
As carbon dioxide concentrations rise, the rate at which sugars are made by the increases until limited by other factors. , 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 . However, if the carbon dioxide concentration is low, RuBisCO will bind oxygen instead of carbon dioxide. This process, called , uses energy, but does not produce sugars.
RuBisCO oxygenase activity is disadvantageous to plants for several reasons:
- One product of oxygenase activity is phosphoglycolate (2 carbon) instead of (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 .
- Phosphoglycolate is quickly metabolized to glycolate that is toxic to a plant at a high concentration; it inhibits photosynthesis.
- Salvaging glycolate is an energetically expensive process that uses the glycolate pathway, and only 75% of the carbon is returned to the Calvin-Benson cycle as 3-phosphoglycerate. The reactions also produce (NH3), which is able to out of the plant, leading to a loss of nitrogen.
The salvaging pathway for the products of RuBisCO oxygenase activity is more commonly known as , since it is characterized by light-dependent oxygen consumption and the release of carbon dioxide.
- . .
- . ; ; at the
- . ; ; at the
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