Photosynthesis

Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that can later be released to fuel the organisms' activities. This chemical energy is stored in carbohydrate molecules, such as sugars, which are synthesized from carbon dioxide and water – hence the name photosynthesis, from the Greek φῶς, phōs, "light", and σύνθεσις, synthesis, "putting together".[1][2][3] In most cases, oxygen is also released as a waste product. Most plants, most algae, and cyanobacteria perform photosynthesis; such organisms are called photoautotrophs. Photosynthesis is largely responsible for producing and maintaining the oxygen content of the Earth's atmosphere, and supplies all of the organic compounds and most of the energy necessary for life on Earth.[4]

Although photosynthesis is performed differently by different species, the process always begins when energy from light is absorbed by proteins called reaction centres that contain green chlorophyll pigments. In plants, these proteins are held inside organelles called chloroplasts, which are most abundant in leaf cells, while in bacteria they are embedded in 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 further compounds that serve as short-term stores of energy, enabling its transfer to drive other reactions: these compounds are reduced nicotinamide adenine dinucleotide phosphate (NADPH) and adenosine triphosphate (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 Calvin cycle; some bacteria use different mechanisms, such as the reverse Krebs cycle, to achieve the same end. In the Calvin cycle, atmospheric carbon dioxide is incorporated into already existing organic carbon compounds, such as ribulose bisphosphate (RuBP).[5] 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.

The first photosynthetic organisms probably evolved early in the evolutionary history of life and most likely used reducing agents such as hydrogen or hydrogen sulfide, rather than water, as sources of electrons.[6] Cyanobacteria appeared later; the excess oxygen they produced contributed directly to the oxygenation of the Earth,[7] which rendered the evolution of complex life possible. Today, the average rate of energy capture by photosynthesis globally is approximately 130 terawatts,[8][9][10] which is about eight times the current power consumption of human civilization.[11] Photosynthetic organisms also convert around 100–115 billion tonnes (91-104 petagrams) of carbon into biomass per year.[12][13]

Photosynthesis en
Schematic of photosynthesis in plants. The carbohydrates produced are stored in or used by the plant.
Photosynthesis equation
Overall equation for the type of photosynthesis that occurs in plants
Seawifs global biosphere
Composite image showing the global distribution of photosynthesis, including both oceanic phytoplankton and terrestrial vegetation. Dark red and blue-green indicate regions of high photosynthetic activity in the ocean and on land, respectively.

Overview

Simple photosynthesis overview
Photosynthesis changes sunlight into chemical energy, splits water to liberate O2, and fixes CO2 into sugar.

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 a source of carbon atoms to carry out photosynthesis; photoheterotrophs use organic compounds, rather than carbon dioxide, as a source of carbon.[4] 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 plants, algae, and cyanobacteria, the overall process is quite similar in these organisms. There are also many varieties of anoxygenic photosynthesis, 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 carbon fixation; photosynthesis captures energy from sunlight to convert carbon dioxide into carbohydrate. Carbon fixation 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 carbohydrate, cellular respiration is the oxidation of carbohydrate 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 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.

The general equation for photosynthesis as first proposed by Cornelis van Niel is therefore:[14]

+ + + +

Since water is used as the electron donor in oxygenic photosynthesis, 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:[15] The equation for this reaction is:

+ + + (used to build other compounds in subsequent reactions)[16]

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 ATP and NADPH. During the second stage, the light-independent reactions use these products to capture and reduce carbon dioxide.

Most organisms that utilize oxygenic photosynthesis use visible light for the light-dependent reactions, although at least three use shortwave infrared or, more specifically, far-red radiation.[17]

Some organisms employ even more radical variants of photosynthesis. Some archaea use 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 the more common types of photosynthesis.[18][19]

Photosynthetic membranes and organelles

Chloroplast
Chloroplast 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)
9. starch
10. ribosome
11. plastidial DNA
12. plastoglobule (drop of lipids)

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.[20] However, the membrane may be tightly folded into cylindrical sheets called thylakoids,[21] or bunched up into round vesicles called intracytoplasmic membranes.[22] 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.[21]

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, 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 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.[23] 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 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.[24]

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 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 reduce heating. The transparent epidermis layer allows light to pass through to the palisade mesophyll cells where most of the photosynthesis takes place.

Light-dependent reactions

Thylakoid membrane 3
Light-dependent reactions of photosynthesis at the thylakoid membrane

In the light-dependent reactions, one molecule of the pigment chlorophyll absorbs one photon and loses one electron. This electron is passed to 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 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 a dioxygen (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:[25]

2 H2O + 2 NADP+ + 3 ADP + 3 Pi + light → 2 NADPH + 2 H+ + 3 ATP + O2

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, 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 the respective organisms.

Z scheme

Z-scheme
The "Z scheme"

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 a photon by the antenna complex frees 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 freed electron is transferred to the primary electron-acceptor molecule, pheophytin. As the electrons are shuttled through an electron transport chain (the so-called Z-scheme shown in the diagram), it initially functions to generate a chemiosmotic potential 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 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 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.

Water photolysis

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 the first step of the Z-scheme, requires an external source of electrons to reduce its oxidized chlorophyll a reaction center, called P680. The source of electrons for photosynthesis in green plants and cyanobacteria 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 electrons yielded are transferred to a redox-active tyrosine residue that then reduces the oxidized 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 the water-oxidizing reaction (Dolai's S-state diagrams).[26] Photosystem II is the only known biological enzyme that carries out this oxidation of water. The hydrogen ions are released in the thylakoid lumen andd 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 for cellular respiration, including photosynthetic organisms.[27][28]

Light-independent reactions

Calvin cycle

In the light-independent (or "dark") reactions, the enzyme RuBisCO captures CO2 from the atmosphere and, in a process called the Calvin cycle, 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[25]:128

3 CO2 + 9 ATP + 6 NADPH + 6 H+ → C3H6O3-phosphate + 9 ADP + 8 Pi + 6 NADP+ + 3 H2O
Calvin-cycle4
Overview of the Calvin cycle and carbon fixation

Carbon fixation produces the intermediate three-carbon sugar product, which is then converted into 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 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 energy from plants 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, is reduced to glyceraldehyde 3-phosphate. This product is also referred to as 3-phosphoglyceraldehyde (PGAL) or, more generically, as triose 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 hexose phosphates, which ultimately yield sucrose, starch and cellulose. The sugars produced during carbon metabolism yield carbon skeletons that can be used for other metabolic reactions like the production of amino acids and lipids.

Carbon concentrating mechanisms

On land

HatchSlackpathway2
Overview of C4 carbon fixation

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 and decrease in carbon fixation. Some plants have evolved mechanisms to increase the CO
2
concentration in the leaves under these conditions.[29]

Plants that use the C4 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 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, the photosynthetic capacity of the leaf.[30] 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 C3 plants 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;[31] however, the evolution of C4 in over 60 plant lineages makes it a striking example of convergent evolution.[29]

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 C4 metabolism, which spatially separates the CO
2
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 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. Sixteen thousand species of plants use CAM.[32]

In water

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 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.[33] Pyrenoids in algae and hornworts also act to concentrate CO
2
around rubisco.[34]

Order and kinetics

The overall process of photosynthesis takes place in four stages:[13]

Stage Description Time scale
1 Energy transfer in antenna chlorophyll (thylakoid membranes) femtosecond to picosecond
2 Transfer of electrons in photochemical reactions (thylakoid membranes) picosecond to nanosecond
3 Electron transport chain and ATP synthesis (thylakoid membranes) microsecond to millisecond
4 Carbon fixation and export of stable products millisecond to second

Efficiency

Plants usually convert light into chemical energy with a photosynthetic efficiency of 3–6%.[35] Absorbed light that is unconverted is dissipated primarily as heat, with a small fraction (1–2%)[36] re-emitted as chlorophyll fluorescence at longer (redder) wavelengths. This fact allows measurement of the light reaction of photosynthesis by using chlorophyll fluorometers.[37]

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%.[38] 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.

The efficiency of both light and dark reactions can be measured but the relationship between the two can be complex.[39] 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.[39] Electrons may also flow to other electron sinks.[40][41][42] For this reason, it is not uncommon for authors to differentiate between work done under non-photorespiratory conditions and under photorespiratory conditions.[43][44][45]

Chlorophyll fluorescence of photosystem II can measure the light reaction, and Infrared gas analyzers can measure the dark reaction.[46] 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.[47] Infrared gas analyzers and some moisture sensors are sensitive enough to measure the photosynthetic assimilation of CO2, and of ΔH2O using reliable methods[48] 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.[48] 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.[48] 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.[45]

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.[48]

Integrated chlorophyll fluorometer – gas exchange systems allow a more precise measure of photosynthetic response and mechanisms.[46][47] 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.[47][49] 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.[46][47][50]

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.[51]

A phenomenon known as quantum walk 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 chromophores arranged in an antenna-shaped structure named 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 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.[52][53][54]

Evolution

Early photosynthetic systems, such as those in green and purple sulfur and green and purple nonsulfur bacteria, are thought to have been anoxygenic, and used various other molecules as electron donors rather than water. Green and purple sulfur bacteria are thought to have used hydrogen and sulfur as electron donors. Green nonsulfur bacteria used various amino and other organic acids 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 reducing at that time.[55]

Fossils of what are thought to be filamentous photosynthetic organisms have been dated at 3.4 billion years old.[56][57] More recent studies, reported in March 2018, also suggest that photosynthesis may have begun about 3.4 billion years ago.[58][59]

The main source of oxygen in the Earth's atmosphere derives from oxygenic photosynthesis, and its first appearance is sometimes referred to as the oxygen catastrophe. Geological evidence suggests that oxygenic photosynthesis, such as that in cyanobacteria, became important during the Paleoproterozoic era around 2 billion years ago. Modern photosynthesis in plants and most photosynthetic prokaryotes is oxygenic. Oxygenic photosynthesis uses water as an electron donor, which is oxidized to molecular oxygen (O
2
) in the photosynthetic reaction center.

Symbiosis and the origin of chloroplasts

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

Several groups of animals have formed symbiotic relationships with photosynthetic algae. These are most common in corals, sponges and sea anemones. It is presumed that this is due to the particularly simple body plans and large surface areas of these animals compared to their volumes.[60] In addition, a few marine mollusks Elysia viridis and Elysia chlorotica also maintain a symbiotic relationship with chloroplasts they capture from the algae in their diet and then store in their bodies (see Kleptoplasty). This allows the mollusks to survive solely by photosynthesis for several months at a time.[61][62] Some of the genes from the plant cell nucleus have even been transferred to the slugs, so that the chloroplasts can be supplied with proteins that they need to survive.[63]

An even closer form of symbiosis may explain the origin of chloroplasts. Chloroplasts have many similarities with photosynthetic bacteria, including a circular chromosome, prokaryotic-type ribosome, and similar proteins in the photosynthetic reaction center.[64][65] 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 photosynthetic bacteria that adapted to life inside plant cells. Like mitochondria, chloroplasts possess their own DNA, separate from the nuclear DNA of their plant host cells and the genes in this chloroplast DNA resemble those found in cyanobacteria.[66] DNA in chloroplasts codes for redox proteins such as those found in the photosynthetic reaction centers. The CoRR Hypothesis 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.[67]

Cyanobacteria and the evolution of photosynthesis

The biochemical capacity to use water as the source for electrons in photosynthesis evolved once, in a common ancestor of extant cyanobacteria (formerly called blue-green algae). 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.[68][69] 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.[70] Available evidence from geobiological studies of Archean (>2500 Ma) sedimentary rocks indicates that life existed 3500 Ma, but the question of when oxygenic photosynthesis evolved is still unanswered. A clear paleontological window on cyanobacterial evolution opened about 2000 Ma, revealing an already-diverse biota of Cyanobacteria. Cyanobacteria remained the principal primary producers of oxygen throughout the Proterozoic Eon (2500–543 Ma), in part because the redox structure of the oceans favored photoautotrophs capable of nitrogen fixation. Green algae joined blue-green algae as the major primary producers of oxygen on continental shelves near the end of the Proterozoic, but it was only with the Mesozoic (251–66 Ma) radiations of dinoflagellates, coccolithophorids, and diatoms did the primary production of oxygen in marine shelf waters take modern form. Cyanobacteria remain critical to marine ecosystems as primary producers of oxygen in oceanic gyres, as agents of biological nitrogen fixation, and, in modified form, as the plastids of marine algae.[71]

Discovery

Although some of the steps in photosynthesis are still not completely understood, the overall photosynthetic equation has 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 of the soil used by a plant and the mass of the plant as it grew. After noticing that the soil mass changed very little, he hypothesized that the mass of the growing plant must come from the water, the only substance he added to the potted plant. His hypothesis was partially accurate – much of the gained mass also comes from carbon dioxide as well as water. However, this was a signaling point to the idea that the bulk of a plant's biomass comes from the inputs of photosynthesis, not the soil itself.

Joseph Priestley, a chemist and minister, discovered that, when he isolated a volume of air under an inverted jar, and burned a candle in it (which gave off CO2), the candle would burn out very quickly, much before it ran out of wax. He further discovered that a mouse could similarly "injure" air. He then showed that the air that had been "injured" by the candle and the mouse could be restored by a plant.

In 1778, Jan Ingenhousz, 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, Jean Senebier, a Swiss pastor, botanist, and naturalist, demonstrated that green plants consume carbon dioxide and release oxygen under the influence of light. Soon afterward, Nicolas-Théodore de Saussure showed that the increase in mass of the plant as it grows could not be due only to uptake of CO2 but also to the incorporation of water. Thus, the basic reaction by which photosynthesis is used to produce food (such as glucose) was outlined.

Cornelis Van Niel made key discoveries explaining the chemistry of photosynthesis. By studying purple sulfur bacteria and green bacteria he was the first to demonstrate that photosynthesis is a light-dependent redox reaction, in which hydrogen reduces (donates its electron to) 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 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.[13]

Melvin Calvin
Melvin Calvin works in his photosynthesis laboratory.

Robert Hill 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 chloroplasts give off oxygen in the presence of unnatural reducing agents like iron oxalate, ferricyanide or benzoquinone after exposure to light. The Hill reaction[72] 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.

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

Melvin Calvin and Andrew Benson, along with James Bassham, elucidated the path of carbon assimilation (the photosynthetic carbon reduction cycle) in plants. The carbon reduction cycle is known as the Calvin cycle, which 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.

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

Otto Heinrich Warburg and Dean Burk discovered the I-quantum photosynthesis reaction that splits the CO2, activated by the respiration.[73]

In 1950, first experimental evidence for the existence of photophosphorylation in vivo was presented by Otto Kandler using intact Chlorella cells and interpreting his findings as light-dependent ATP formation.[74] In 1954, Daniel I. Arnon et al. discovered photophosphorylation in vitro in isolated chloroplasts with the help of P32.[75][76]

Louis N.M. Duysens and Jan Amesz 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, Charles Reid Barnes 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.[77]

C3 : C4 photosynthesis research

After WWII at late 1940 at the University of California, Berkeley, the details of photosynthetic carbon metabolism were sorted out by the chemists Melvin Calvin, Andrew Benson, James Bassham and a score of students and researchers utilizing the carbon-14 isotope and paper chromatography techniques.[78] 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 Nobel Prize in Chemistry 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 μmol CO2·m-2·s-1, with the conclusion that all terrestrial plants having the same photosynthetic capacities that were light saturated at less than 50% of sunlight.[79][80]

Later in 1958-1963 at Cornell University, field grown maize was reported to have much greater leaf photosynthetic rates of 40 μmol CO2·m-2·s-1 and was not saturated at near full sunlight.[81][82] 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.[83][84] In tropical grasses, including maize, sorghum, sugarcane, Bermuda grass and in the dicot amaranthus, leaf photosynthetic rates were around 38−40 μmol CO2·m-2·s-1, 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 Gottlieb Haberlandt while studying leaf anatomy of sugarcane.[85] 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.[86] The research at Arizona was designated Citation Classic by the ISI 1986.[84] These species was later termed C4 plants as the first stable compound of CO2 fixation in light has 4 carbon as malate and aspartate.[87][88][89] 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 μmol CO2·m-2·s-1 indicating the suppression of photorespiration in C3 plants.[83][84]

Factors

Leaf 1 web
The leaf 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 leaf 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 chloroplasts to support photosynthesis, the availability of water, and the availability of suitable temperatures for carrying out photosynthesis.[90]

Light intensity (irradiance), wavelength and temperature

Chlorophyll ab spectra-en
Absorbance spectra of free chlorophyll a (blue) 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.[91]

The radiation climate within plant communities is extremely variable, with both time and space.

In the early 20th century, Frederick Blackman and Gabrielle Matthaei investigated the effects of light intensity (irradiance) 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, photochemical reactions are not affected by temperature. However, these experiments clearly show that temperature affects the rate of carbon assimilation, so there must be two sets of reactions in the full process of carbon assimilation. These are the light-dependent 'photochemical' temperature-independent 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 cause photoinduced charge separation in conventional photosynthetic pigments. To combat this problem, a series of proteins with different pigments surround the reaction center. This unit is called a phycobilisome.

Carbon dioxide levels and photorespiration

Photorespiration
Photorespiration

As carbon dioxide concentrations rise, the rate at which sugars are made by the light-independent reactions increases until limited by other factors. RuBisCO, the enzyme that captures carbon dioxide in the light-independent reactions, has a binding affinity for both carbon dioxide and oxygen. When the concentration of carbon dioxide is high, RuBisCO will fix carbon dioxide. However, if the carbon dioxide concentration is low, RuBisCO will bind oxygen instead 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. A high oxygenase activity, therefore, drains the sugars that are required to recycle ribulose 5-bisphosphate and for the continuation of the Calvin-Benson cycle.
  2. Phosphoglycolate is quickly metabolized to glycolate that is toxic to a plant at a high concentration; it inhibits photosynthesis.
  3. 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 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 as photorespiration, since it is characterized by light-dependent oxygen consumption and the release of carbon dioxide.

See also

References

  1. ^ "photosynthesis". Online Etymology Dictionary.
  2. ^ φῶς. Liddell, Henry George; Scott, Robert; A Greek–English Lexicon at the Perseus Project
  3. ^ σύνθεσις. Liddell, Henry George; Scott, Robert; A Greek–English Lexicon at the Perseus Project
  4. ^ a b Bryant DA, Frigaard NU (Nov 2006). "Prokaryotic photosynthesis and phototrophy illuminated". Trends in Microbiology. 14 (11): 488–496. doi:10.1016/j.tim.2006.09.001. PMID 16997562.
  5. ^ Reece J, Urry L, Cain M, Wasserman S, Minorsky P, Jackson R (2011). Biology (International ed.). Upper Saddle River, NJ: Pearson Education. pp. 235, 244. ISBN 978-0-321-73975-9. This initial incorporation of carbon into organic compounds is known as carbon fixation.
  6. ^ Olson JM (May 2006). "Photosynthesis in the Archean era". Photosynthesis Research. 88 (2): 109–117. doi:10.1007/s11120-006-9040-5. PMID 16453059.
  7. ^ Buick R (Aug 2008). "When did oxygenic photosynthesis evolve?". Philosophical Transactions of the Royal Society of London, Series B. 363 (1504): 2731–2743. doi:10.1098/rstb.2008.0041. PMC 2606769. PMID 18468984.
  8. ^ Nealson KH, Conrad PG (Dec 1999). "Life: past, present and future". Philosophical Transactions of the Royal Society of London, Series B. 354 (1392): 1923–1939. doi:10.1098/rstb.1999.0532. PMC 1692713. PMID 10670014.
  9. ^ Whitmarsh J, Govindjee (1999). "The photosynthetic process". In Singhal GS, Renger G, Sopory SK, Irrgang KD, Govindjee (eds.). Concepts in photobiology: photosynthesis and photomorphogenesis. Boston: Kluwer Academic Publishers. pp. 11–51. ISBN 978-0-7923-5519-9. 100×1015 grams of carbon/year fixed by photosynthetic organisms, which is equivalent to 4×1018 kJ/yr = 4×1021 J/yr of free energy stored as reduced carbon.
  10. ^ Steger U, Achterberg W, Blok K, Bode H, Frenz W, Gather C, Hanekamp G, Imboden D, Jahnke M, Kost M, Kurz R, Nutzinger HG, Ziesemer T (2005). Sustainable development and innovation in the energy sector. Berlin: Springer. p. 32. ISBN 978-3-540-23103-5. The average global rate of photosynthesis is 130 TW.
  11. ^ "World Consumption of Primary Energy by Energy Type and Selected Country Groups, 1980–2004". Energy Information Administration. July 31, 2006. Archived from the original (XLS) on November 9, 2006. Retrieved 2007-01-20.
  12. ^ Field CB, Behrenfeld MJ, Randerson JT, Falkowski P (Jul 1998). "Primary production of the biosphere: integrating terrestrial and oceanic components". Science. 281 (5374): 237–240. Bibcode:1998Sci...281..237F. doi:10.1126/science.281.5374.237. PMID 9657713.
  13. ^ a b c "Photosynthesis". McGraw-Hill Encyclopedia of Science & Technology. 13. New York: McGraw-Hill. 2007. ISBN 978-0-07-144143-8.
  14. ^ Whitmarsh J, Govindjee (1999). [hhttps://books.google.com/books?id=dqSuoOtDM1cC&pg=PA13 "Chapter 2: The Basic Photosynthetic Process"]. In Singhal GS, Renger G, Sopory SK, Irrgang KD, Govindjee (eds.). Concepts in Photobiology: Photosynthesis and Photomorphogenesis. Boston: Kluwer Academic Publishers. p. 13. ISBN 978-0-7923-5519-9.
  15. ^ Anaerobic Photosynthesis, Chemical & Engineering News, 86, 33, August 18, 2008, p. 36
  16. ^ Kulp TR, Hoeft SE, Asao M, Madigan MT, Hollibaugh JT, Fisher JC, Stolz JF, Culbertson CW, Miller LG, Oremland RS (Aug 2008). "Arsenic(III) fuels anoxygenic photosynthesis in hot spring biofilms from Mono Lake, California". Science. 321 (5891): 967–970. Bibcode:2008Sci...321..967K. doi:10.1126/science.1160799. PMID 18703741.
  17. ^ "Scientists discover unique microbe in California's largest lake". Retrieved 2009-07-20.
  18. ^ Plants: Diversity and Evolution, page 14, Martin Ingrouille, Bill Eddie
  19. ^ Oakley, Todd (19 December 2008). "Evolutionary Novelties: Opsins: An amazing evolutionary convergence".
  20. ^ Tavano CL, Donohue TJ (Dec 2006). "Development of the bacterial photosynthetic apparatus". Current Opinion in Microbiology. 9 (6): 625–631. doi:10.1016/j.mib.2006.10.005. PMC 2765710. PMID 17055774.
  21. ^ a b Mullineaux CW (1999). "The thylakoid membranes of cyanobacteria: structure, dynamics and function". Australian Journal of Plant Physiology. 26 (7): 671–677. doi:10.1071/PP99027.
  22. ^ Sener MK, Olsen JD, Hunter CN, Schulten K (Oct 2007). "Atomic-level structural and functional model of a bacterial photosynthetic membrane vesicle". Proceedings of the National Academy of Sciences of the United States of America. 104 (40): 15723–15728. Bibcode:2007PNAS..10415723S. doi:10.1073/pnas.0706861104. PMC 2000399. PMID 17895378.
  23. ^ Campbell NA, Williamson B, Heyden RJ (2006). Biology Exploring Life. Upper Saddle River, NJ: Prentice Hall. ISBN 978-0-13-250882-7.
  24. ^ Ziehe, D; Dünschede, B; Schünemann, D (Dec 2018). "Molecular mechanism of SRP-dependent light-harvesting protein transport to the thylakoid membrane in plants". Photosynthesis Research. 138 (3): 303–313. doi:10.1007/s11120-018-0544-6. PMC 6244792. PMID 29956039.
  25. ^ a b Raven PH, Evert RF, Eichhorn SE (2005). Biology of Plants (7th ed.). New York: W. H. Freeman and Company. pp. 124–127. ISBN 978-0-7167-1007-3.
  26. ^ Dolai U (2017). "Chemical Scheme of Water-Splitting Process during Photosynthesis by the Way of Experimental Analysis". IOSR Journal of Pharmacy and Biological Sciences. 12 (6): 65–67. doi:10.9790/3008-1206026567 (inactive 2019-02-17).
  27. ^ "Yachandra Group Home page".
  28. ^ Pushkar Y, Yano J, Sauer K, Boussac A, Yachandra VK (Feb 2008). "Structural changes in the Mn4Ca cluster and the mechanism of photosynthetic water splitting". Proceedings of the National Academy of Sciences of the United States of America. 105 (6): 1879–1884. Bibcode:2008PNAS..105.1879P. doi:10.1073/pnas.0707092105. PMC 2542863. PMID 18250316.
  29. ^ a b Williams BP, Johnston IG, Covshoff S, Hibberd JM (September 2013). "Phenotypic landscape inference reveals multiple evolutionary paths to C4 photosynthesis". eLife. 2: e00961. doi:10.7554/eLife.00961. PMC 3786385. PMID 24082995.
  30. ^ Taiz L, Geiger E (2006). Plant Physiology (4th ed.). Sinauer Associates. ISBN 978-0-87893-856-8.
  31. ^ Monson RK, Sage RF (1999). "The Taxonomic Distribution of C
    4
    Photosynthesis"
    . C₄ plant biology. Boston: Academic Press. pp. 551–580. ISBN 978-0-12-614440-6.
  32. ^ Dodd AN, Borland AM, Haslam RP, Griffiths H, Maxwell K (Apr 2002). "Crassulacean acid metabolism: plastic, fantastic". Journal of Experimental Botany. 53 (369): 569–580. doi:10.1093/jexbot/53.369.569. PMID 11886877.
  33. ^ Badger MR, Price GD (Feb 2003). "CO2 concentrating mechanisms in cyanobacteria: molecular components, their diversity and evolution". Journal of Experimental Botany. 54 (383): 609–622. doi:10.1093/jxb/erg076. PMID 12554704.
  34. ^ Badger MR, Andrews JT, Whitney SM, Ludwig M, Yellowlees DC, Leggat W, Price GD (1998). "The diversity and coevolution of Rubisco, plastids, pyrenoids, and chloroplast-based CO2-concentrating mechanisms in algae". Canadian Journal of Botany. 76 (6): 1052–1071. doi:10.1139/b98-074.
  35. ^ Miyamoto K. "Chapter 1 – Biological energy production". Renewable biological systems for alternative sustainable energy production (FAO Agricultural Services Bulletin – 128). Food and Agriculture Organization of the United Nations. Retrieved 2009-01-04.
  36. ^ Maxwell K, Johnson GN (Apr 2000). "Chlorophyll fluorescence--a practical guide". Journal of Experimental Botany. 51 (345): 659–668. doi:10.1093/jexbot/51.345.659. PMID 10938857.
  37. ^ Maxwell K, Johnson GN (2000). "Chlorophyll fluorescence – a practical guide". Journal of Experimental Botany. 51 (345): 659–668. doi:10.1093/jxb/51.345.659.
  38. ^ Govindjee R. "What is Photosynthesis". Biology at Illinois.
  39. ^ a b Rosenqvist E, van Kooten O (2006). "Chapter 2: Chlorophyll Fluorescence: A General Description and Nomenclature". In DeEll JA, Toivonen PM (eds.). Practical Applications of Chlorophyll Fluorescence in Plant Biology. Dordrecht, the Netherlands: Kluwer Academic Publishers. pp. 39–78.
  40. ^ Baker NR, Oxborough K (2004). "Chapter 3: Chlorophyll fluorescence as a probe of photosynthetic productivity". In Papaqeorgiou G, Govindjee (eds.). Chlorophylla Fluorescence a Signature of Photosynthesis. Dordrecht, The Netherlands: Springer. pp. 66–79.
  41. ^ Flexas J, Escalnona JM, Medrano H (January 1999). "Water stress induces different levels of photosynthesis and electron transport rate regulation in grapevines". Plant, Cell and Environment. 22 (1): 39–48. doi:10.1046/j.1365-3040.1999.00371.x.
  42. ^ Fryer MJ, Andrews JR, Oxborough K, Blowers DA, Baker NR (1998). "Relationship between CO2 Assimilation, Photosynthetic Electron Transport, and Active O2 Metabolism in Leaves of Maize in the Field during Periods of Low Temperature". Plant Physiology. 116 (2): 571–580. doi:10.1104/pp.116.2.571. PMC 35114. PMID 9490760.
  43. ^ Earl H, Said Ennahli S (2004). "Estimating photosynthetic electron transport via chlorophyll fluorometry without Photosystem II light saturation". Photosynthesis Research. 82 (2): 177–186. doi:10.1007/s11120-004-1454-3. PMID 16151873.
  44. ^ Genty B, Briantais J, Baker NR (1989). "MThe relationship between the quantum yield of photosynthetic electron transport and quenching of chlorophyll fluorescence". Biochimica et Biophysica Acta. 990 (1): 87–92. doi:10.1016/s0304-4165(89)80016-9.
  45. ^ a b Baker NR (2008). "Chlorophyll Fluorescence: A Probe of Photosynthesis In Vivo". Annu. Rev. Plant Biol. 59: 89–113. doi:10.1146/annurev.arplant.59.032607.092759. PMID 18444897.
  46. ^ a b c Bernacchi CJ, Portis AR, Nakano H, von Caemmerer S, Long SP (2002). "Temperature response of mesophyll conductance. Implications for the determination of Rubisco enzyme kinetics and for limitations to photosynthesis in vivo". Plant Physiology. 130 (4): 1992–1998. doi:10.1104/pp.008250. PMC 166710. PMID 12481082.
  47. ^ a b c d Ribas-Carbo M, Flexas J, Robinson SA, Tcherkez GG (2010). "In vivo measurement of plant respiration". University of Wollongong Research Online.
  48. ^ a b c d Long SP, Bernacchi CJ (2003). "Gas exchange measurements, what can they tell us about the underlying limitations to photosynthesis? Procedures and sources of error". Journal of Experimental Botany. 54 (392): 2393–2401. doi:10.1093/jxb/erg262. PMID 14512377.
  49. ^ Bernacchi CJ, Portis A (2002). "R., Nakano H., von Caemmerer S., and Long S.P. (2002) Temperature Response of Mesophyll Conductance. Implications for the Determination of Rubisco Enzyme Kinetics and for Limitations to Photosynthesis in Vivo". Plant Physiology. 130 (4): 1992–1998. doi:10.1104/pp.008250. PMC 166710. PMID 12481082.
  50. ^ YIN X, Struik PC (2009). "Theoretical reconsiderations when estimating the mesophyll conductanceto CO2 diffusion in leaves of C3 plants by analysis of combined gas exchange and chlorophyll fluorescence measurements pce_2016 1513..1". Plant, Cell and Environment. 32 (11): 1513–1524 [1524]. doi:10.1111/j.1365-3040.2009.02016.x. PMID 19558403.
  51. ^ Schreiber U, Klughammer C, Kolbowski J (2012). "Assessment of wavelength-dependent parameters of photosynthetic electron transport with a new type of multi-color PAM chlorophyll fluorometer". Photosynthesis Research. 113 (1–3): 127–144. doi:10.1007/s11120-012-9758-1. PMC 3430841. PMID 22729479.
  52. ^ Palmer J (21 June 2013). "Plants 'seen doing quantum physics'". BBC News.
  53. ^ Lloyd S (10 March 2014). "Quantum Biology: Better Living Through Quantum Mechanics – The Nature of Reality". Nova: PBS Online, WGBH Boston.
  54. ^ Hildner R, Brinks D, Nieder JB, Cogdell RJ, van Hulst NF (Jun 2013). "Quantum coherent energy transfer over varying pathways in single light-harvesting complexes". Science. 340 (6139): 1448–1451. Bibcode:2013Sci...340.1448H. doi:10.1126/science.1235820. PMID 23788794.
  55. ^ Gale J (2009). Astrobiology of Earth : The emergence, evolution and future of life on a planet in turmoil. OUP Oxford. pp. 112–113. ISBN 978-0-19-154835-2.
  56. ^ Davis K (2 October 2004). "Photosynthesis got a really early start". New Scientist.
  57. ^ Hooper R (19 August 2006). "Revealing the dawn of photosynthesis". New Scientist.
  58. ^ Caredona, Tanai (6 March 2018). "Early Archean origin of heterodimeric Photosystem I". Elsevier. 4 (3): e00548. doi:10.1016/j.heliyon.2018.e00548. PMC 5857716. PMID 29560463. Retrieved 23 March 2018.
  59. ^ Howard, Victoria (7 March 2018). "Photosynthesis Originated A Billion Years Earlier Than We Thought, Study Shows". Astrobiology Magazine. Retrieved 23 March 2018.
  60. ^ Venn AA, Loram JE, Douglas AE (2008). "Photosynthetic symbioses in animals". Journal of Experimental Botany. 59 (5): 1069–1080. doi:10.1093/jxb/erm328. PMID 18267943.
  61. ^ Rumpho ME, Summer EJ, Manhart JR (May 2000). "Solar-powered sea slugs. Mollusc/algal chloroplast symbiosis". Plant Physiology. 123 (1): 29–38. doi:10.1104/pp.123.1.29. PMC 1539252. PMID 10806222.
  62. ^ Muscatine L, Greene RW (1973). Chloroplasts and algae as symbionts in molluscs. International Review of Cytology. 36. pp. 137–169. doi:10.1016/S0074-7696(08)60217-X. ISBN 978-0-12-364336-0. PMID 4587388.
  63. ^ Rumpho ME, Worful JM, Lee J, Kannan K, Tyler MS, Bhattacharya D, Moustafa A, Manhart JR (Nov 2008). "Horizontal gene transfer of the algal nuclear gene psbO to the photosynthetic sea slug Elysia chlorotica". Proceedings of the National Academy of Sciences of the United States of America. 105 (46): 17867–17871. Bibcode:2008PNAS..10517867R. doi:10.1073/pnas.0804968105. PMC 2584685. PMID 19004808.
  64. ^ Douglas SE (Dec 1998). "Plastid evolution: origins, diversity, trends". Current Opinion in Genetics & Development. 8 (6): 655–661. doi:10.1016/S0959-437X(98)80033-6. PMID 9914199.
  65. ^ Reyes-Prieto A, Weber AP, Bhattacharya D (2007). "The origin and establishment of the plastid in algae and plants". Annual Review of Genetics. 41: 147–168. doi:10.1146/annurev.genet.41.110306.130134. PMID 17600460.
  66. ^ Raven JA, Allen JF (2003). "Genomics and chloroplast evolution: what did cyanobacteria do for plants?". Genome Biology. 4 (3): 209. doi:10.1186/gb-2003-4-3-209. PMC 153454. PMID 12620099.
  67. ^ Allen JF (December 2017). "The CoRR hypothesis for genes in organelles". J. Theor. Biol. 434: 50–57. doi:10.1016/j.jtbi.2017.04.008. PMID 28408315.
  68. ^ Tomitani A, Knoll AH, Cavanaugh CM, Ohno T (Apr 2006). "The evolutionary diversification of cyanobacteria: molecular-phylogenetic and paleontological perspectives". Proceedings of the National Academy of Sciences of the United States of America. 103 (14): 5442–5447. Bibcode:2006PNAS..103.5442T. doi:10.1073/pnas.0600999103. PMC 1459374. PMID 16569695.
  69. ^ "Cyanobacteria: Fossil Record". Ucmp.berkeley.edu. Retrieved 2010-08-26.
  70. ^ Smith A (2010). Plant biology. New York: Garland Science. p. 5. ISBN 978-0-8153-4025-6.
  71. ^ Herrero A, Flores E (2008). The Cyanobacteria: Molecular Biology, Genomics and Evolution (1st ed.). Caister Academic Press. ISBN 978-1-904455-15-8.
  72. ^ Walker DA (2002). "'And whose bright presence' – an appreciation of Robert Hill and his reaction" (PDF). Photosynthesis Research. 73 (1–3): 51–54. doi:10.1023/A:1020479620680. PMID 16245102.
  73. ^ Otto Warburg – Biography. Nobelprize.org (1970-08-01). Retrieved on 2011-11-03.
  74. ^ Kandler, Otto (1950). "Über die Beziehungen zwischen Phosphathaushalt und Photosynthese. I. Phosphatspiegelschwankungen bei Chlorella pyrenoidosa als Folge des Licht-Dunkel-Wechsels" [On the relationship between the phosphate metabolism and photosynthesis I. Variations in phosphate levels in Chlorella pyrenoidosa as a consequence of light-dark changes] (PDF). Zeitschrift für Naturforschung. 5b (8): 423–437. doi:10.1515/znb-1950-0806.
  75. ^ Arnon, Daniel I.; Allen, M.B.; Whatley, F.R. (1954). "Photosynthesis by isolated chloroplasts. II. Photophosphorylation, the conversion of light into phosphate bond energy". J Am Chem Soc. 76 (24): 6324–6329. doi:10.1021/ja01653a025.
  76. ^ Arnon, Daniel I. (1956). "Phosphorus metabolism and photosynthesis". Review of Plant Physiology. 7: 325–354. doi:10.1146/annurev.pp.07.060156.001545.
  77. ^ Gest H (2002). "History of the word photosynthesis and evolution of its definition". Photosynthesis Research. 73 (1–3): 7–10. doi:10.1023/A:1020419417954. PMID 16245098.
  78. ^ Calvin M (July 1989). "Forty years of photosynthesis and related activities". Photosynthesis Research. 21 (1): 3–16. doi:10.1007/BF00047170 (inactive 2019-02-17). PMID 24424488.
  79. ^ Verduin J (1953). "A table of photosynthesis rates under optimal, near natural conditions". Am. J. Bot. 40 (9): 675–679. doi:10.1002/j.1537-2197.1953.tb06540.x. JSTOR 2439681.
  80. ^ Verduin J, Whitwer EE, Cowell BC (1959). "Maximal photosynthetic rates in nature". Science. 130 (3370): 268–269. Bibcode:1959Sci...130..268V. doi:10.1126/science.130.3370.268. PMID 13668557.
  81. ^ Hesketh JD, Musgrave R (1962). "Photosynthesis under field conditions. IV. Light studies with individual corn leaves". Crop Sci. 2 (4): 311–315. doi:10.2135/cropsci1962.0011183x000200040011x.
  82. ^ Hesketh JD, Moss DN (1963). "Variation in the response of photosynthesis to light". Crop Sci. 3 (2): 107–110. doi:10.2135/cropsci1963.0011183X000300020002x.
  83. ^ a b El-Sharkawy, MA, Hesketh JD (1965). "Photosynthesis among species in relation to characteristics of leaf anatomy and CO2 diffusion resistances". Crop Sci. 5 (6): 517–521. doi:10.2135/cropsci1965.0011183x000500060010x.
  84. ^ a b c El-Sharkawy MA, Hesketh JD (1986). "Citation Classic-Photosynthesis among species in relation to characteristics of leaf anatomy and CO2 diffusion resistances" (PDF). Curr. Cont./Agr.Biol.Environ. 27: 14.
  85. ^ Haberlandt G (1904). Physiologische Pflanzanatomie. Leipzig: Engelmann.
  86. ^ El-Sharkawy MA (1965). Factors Limiting Photosynthetic Rates of Different Plant Species (Ph.D. thesis). The University of Arizona, Tucson, USA.
  87. ^ Karpilov YS (1960). "The distribution of radioactvity in carbon-14 among the products of photosynthesis in maize". Proc. Kazan Agric. Inst. 14: 15–24.
  88. ^ Kortschak HP, Hart CE, Burr GO (1965). "Carbon dioxide fixation in sugarcane leaves". Plant Physiol. 40 (2): 209–213. doi:10.1104/pp.40.2.209. PMC 550268. PMID 16656075.
  89. ^ Hatch MD, Slack CR (1966). "Photosynthesis by sugar-cane leaves. A new carboxylation reaction and the pathway of sugar formation". Biochem. J. 101 (1): 103–111. doi:10.1042/bj1010103. PMC 1270070. PMID 5971771.
  90. ^ Chapin FS, Matson PA, Mooney HA (2002). Principles of Terrestrial Ecosystem Ecology. New York: Springer. pp. 97–104. ISBN 978-0-387-95443-1.
  91. ^ Jones HG (2014). Plants and Microclimate: a Quantitative Approach to Environmental Plant Physiology (Third ed.). Cambridge: Cambridge University Press. ISBN 978-0-521-27959-8.

Further reading

Books

Papers

External links

Anoxygenic photosynthesis

Bacterial anoxygenic photosynthesis is distinguished from the more familiar terrestrial plant oxygenic photosynthesis by the nature of the terminal reductant (e.g. hydrogen sulfide rather than water) and in the byproduct generated (e.g. elemental sulfur instead of molecular oxygen). As its name implies, anoxygenic photosynthesis does not produce oxygen as a byproduct of the reaction. . Several groups of bacteria can conduct anoxygenic photosynthesis: green sulfur bacteria (GSB), red and green filamentous phototrophs (FAPs e.g. Chloroflexi), purple bacteria, Acidobacteria, and heliobacteria.The pigments used to carry out anaerobic photosynthesis are similar to chlorophyll but differ in molecular detail and peak wavelength of light absorbed. Bacteriochlorophylls a through g absorb electromagnetic photons maximally in the near-infrared within their natural membrane milieu. This differs from chlorophyll a, the predominant plant and cyanobacteria pigment, which has peak absorption wavelength approximately 100 nanometers shorter (in the red portion of the visible spectrum).

Some archaea (e.g. Halobacterium) capture light energy for metabolic function and are thus phototrophic but none are known to "fix" carbon (i.e. be photosynthetic). Instead of a chlorophyll-type receptor and electron transport chain, proteins such as halorhodopsin capture light energy with the aid of diterpenes to move ions against the gradient and produce ATP via chemiosmosis in the manner of mitochondria.

There are two main types of anaerobic photosynthetic electron transport chains in bacteria. The type I reaction centers found in GSB, Chloracidobacterium, and Heliobacteria and the type II reaction centers found in FAPs and Purple Bacteria

Type I Reaction Centers

The electron transport chain of green sulfur bacteria — such as is present in model organism Chlorobaculum tepidum — uses the reaction centre bacteriochlorophyll pair, P840. When light is absorbed by the reaction center, P840 enters an excited state with a large negative reduction potential, and so readily donates the electron to bacteriochlorophyll 663 which passes it on down the electron chain. The electron is transferred through a series of electron carriers and complexes until it is used to reduce NAD+. P840 regeneration is accomplished with the oxidation of sulfide ion from hydrogen sulfide (or hydrogen or ferrous iron) by cytochrome c555.

Type II Reaction Centers

Although the type II reaction centers are structurally and sequentially analogous to Photosystem II (PSII) in plant chloroplasts and cyanobacteria, known organisms that exhibit anoxygenic photosynthesis do not have a region analogous to the oxygen-evolving complex of PSII.

The electron transport chain of purple non-sulfur bacteria begins when the reaction centre bacteriochlorophyll pair, P870, becomes excited from the absorption of light. Excited P870 will then donate an electron to bacteriopheophytin, which then passes it on to a series of electron carriers down the electron chain. In the process, it will generate an electro-chemical gradient which can then be used to synthesize ATP by chemiosmosis. P870 has to be regenerated (reduced) to be available again for a photon reaching the reaction-center to start the process anew. Molecular hydrogen in the bacterial environment is the usual electron donor.

Artificial photosynthesis

Artificial photosynthesis is a chemical process that biomimics the natural process of photosynthesis to convert sunlight, water, and carbon dioxide into carbohydrates and oxygen. The term artificial photosynthesis is commonly used to refer to any scheme for capturing and storing the energy from sunlight in the chemical bonds of a fuel (a solar fuel). Photocatalytic water splitting converts water into hydrogen and oxygen and is a major research topic of artificial photosynthesis. Light-driven carbon dioxide reduction is another process studied that replicates natural carbon fixation.

Research of this topic includes the design and assembly of devices for the direct production of solar fuels, photoelectrochemistry and its application in fuel cells, and the engineering of enzymes and photoautotrophic microorganisms for microbial biofuel and biohydrogen production from sunlight.

Autotroph

An autotroph or primary producer, is an organism that produces complex organic compounds (such as carbohydrates, fats, and proteins) from simple substances present in its surroundings, generally using energy from light (photosynthesis) or inorganic chemical reactions (chemosynthesis). They are the producers in a food chain, such as plants on land or algae in water (in contrast to heterotrophs as consumers of autotrophs). They do not need a living source of energy or organic carbon. Autotrophs can reduce carbon dioxide to make organic compounds for biosynthesis and also create a store of chemical energy. Most autotrophs use water as the reducing agent, but some can use other hydrogen compounds such as hydrogen sulfide. Some autotrophs, such as green plants and algae, are phototrophs, meaning that they convert electromagnetic energy from sunlight into chemical energy in the form of reduced carbon.

Autotrophs can be photoautotrophs or chemoautotrophs. Phototrophs use light as an energy source, while chemotrophs use electron donors as a source of energy, whether from organic or inorganic sources; however in the case of autotrophs, these electron donors come from inorganic chemical sources. Such chemotrophs are lithotrophs. Lithotrophs use inorganic compounds, such as hydrogen sulfide, elemental sulfur, ammonium and ferrous iron, as reducing agents for biosynthesis and chemical energy storage. Photoautotrophs and lithoautotrophs use a portion of the ATP produced during photosynthesis or the oxidation of inorganic compounds to reduce NADP+ to NADPH to form organic compounds.

C. B. van Niel

Cornelius Bernardus van Niel (November 4, 1897, Haarlem – March 10, 1985, Carmel, California) was a Dutch-American microbiologist. He introduced the study of general microbiology to the United States and made key discoveries explaining the chemistry of photosynthesis.

C3 carbon fixation

C3 carbon fixation is the most common of three metabolic pathways for carbon fixation in photosynthesis, along with C4 and CAM. This process converts carbon dioxide and ribulose bisphosphate (RuBP, a 5-carbon sugar) into two molecules of 3-phosphoglycerate through the following reaction:

CO2 + H2O + RuBP → (2) 3-phosphoglycerateThis reaction occurs in all plants as the first step of the Calvin–Benson cycle. (In C4 and CAM plants, carbon dioxide is drawn out of malate and into this reaction rather than directly from the air.)

Plants that survive solely on C3 fixation (C3 plants) tend to thrive in areas where sunlight intensity is moderate, temperatures are moderate, carbon dioxide concentrations are around 200 ppm or higher, and groundwater is plentiful. The C3 plants, originating during Mesozoic and Paleozoic eras, predate the C4 plants and still represent approximately 95% of Earth's plant biomass, including important food crops such as rice, wheat, soybeans and barley.

C3 plants cannot grow in very hot areas because RuBisCO incorporates more oxygen into RuBP as temperatures increase. This leads to photorespiration (also known as the oxidative photosynthetic carbon cycle, or C2 photosynthesis), which leads to a net loss of carbon and nitrogen from the plant and can therefore limit growth.

C3 plants lose up to 97% of the water taken up through their roots to transpiration. In dry areas, C3 plants shut their stomata to reduce water loss, but this stops CO2 from entering the leaves and therefore reduces the concentration of CO2 in the leaves. This lowers the CO2:O2 ratio and therefore also increases photorespiration. C4 and CAM plants have adaptations that allow them to survive in hot and dry areas, and they can therefore out-compete C3 plants in these areas.

The isotopic signature of C3 plants shows higher degree of 13C depletion than the C4 plants, due to variation in fractionation of carbon isotopes in oxygenic photosynthesis across plant types.

Scientists have designed new metabolism pathways which reduces the losses to photorespiration, by more efficiently metabolizing the toxic glycolate produced. This resulted in over 40% increase in biomass production in their model organism (the tobacco plant) in their test conditions. The scientists are optimistic that this optimization can also be implemented in other C3 crops like wheat.

C4 carbon fixation

C4 carbon fixation or the Hatch–Slack pathway is a photosynthetic process in some plants. It is the first step in extracting carbon from carbon dioxide to be able to use it in sugar and other biomolecules. It is one of three known processes for carbon fixation. "C4" refers to the four-carbon molecule that is the first product of this type of carbon fixation.

C4 fixation is an elaboration of the more common C3 carbon fixation and is believed to have evolved more recently. C4 overcomes the tendency of the enzyme RuBisCO to wastefully fix oxygen rather than carbon dioxide in the process of photorespiration. This is achieved by ensuring that RuBisCO works in an environment where there is a lot of carbon dioxide and very little oxygen. CO2 is shuttled via malate or aspartate from mesophyll cells to bundle-sheath cells. In these bundle-sheath cells CO2 is released by decarboxylation of the malate. C4 plants use PEP carboxylase to capture more CO2 in the mesophyll cells. PEP (phosphoenolpyruvate, three carbons) binds to CO2 to make oxaloacetic acid (OAA). OAA then makes malate (four carbons). Malate enters bundle sheath cells and releases the CO2. These additional steps, however, require more energy in the form of ATP. Using this extra energy, C4 plants are able to more efficiently fix carbon in drought, high temperatures, and limitations of nitrogen or CO2. Since the more common C3 pathway does not require this extra energy, it is more efficient in the other conditions.

The naming Hatch–Slack pathway is in honor of Marshall Davidson Hatch and C. R. Slack, who elucidated it in Australia in 1966.

Calvin cycle

The calvin cycle, light-independent reactions, dark reactions, or photosynthetic carbon reduction (PCR) cycle of photosynthesis are the chemical reactions that convert carbon dioxide and other compounds into glucose. These reactions occur in the stroma, the fluid-filled area of a chloroplast outside the thylakoid membranes. These reactions take the products (ATP and NADPH) of light-dependent reactions and perform further chemical processes on them. There are three phases to the light-independent reactions, collectively called the Calvin cycle: carbon fixation, reduction reactions, and ribulose 1,5-bisphosphate (RuBP) regeneration.

This process occurs only when light is available. Plants do not carry out the Calvin cycle during nighttime. They instead release sucrose into the phloem from their starch reserves to provide energy for the plant. This process happens when light is available independent of the kind of photosynthesis (C3 carbon fixation, C4 carbon fixation, and Crassulacean Acid Metabolism (CAM)); CAM plants store malic acid in their vacuoles every night and release it by day to make this process work. They are also known as dark reactions.

Carbon fixation

Carbon fixation or сarbon assimilation is the conversion process of inorganic carbon (carbon dioxide) to organic compounds by living organisms. The most prominent example is photosynthesis, although chemosynthesis is another form of carbon fixation that can take place in the absence of sunlight. Organisms that grow by fixing carbon are called autotrophs. Autotrophs include photoautotrophs, which synthesize organic compounds using the energy of sunlight, and lithoautotrophs, which synthesize organic compounds using the energy of inorganic oxidation. Heterotrophs are organisms that grow using the carbon fixed by autotrophs. The organic compounds are used by heterotrophs to produce energy and to build body structures. "Fixed carbon", "reduced carbon", and "organic carbon" are equivalent terms for various organic compounds.

Cyanobacteria

Cyanobacteria , also known as Cyanophyta, are a phylum of bacteria that obtain their energy through photosynthesis and are the only photosynthetic prokaryotes able to produce oxygen. The name cyanobacteria comes from the color of the bacteria (Greek: κυανός, translit. kyanós, lit. 'blue'). Cyanobacteria, which are prokaryotes, are also called "blue-green algae", though the term algae in modern usage is restricted to eukaryotes. The cyanobacteria appears to have originated in freshwater or a terrestrial environment.Unlike heterotrophic prokaryotes, cyanobacteria have internal membranes. These are flattened sacs called thylakoids where photosynthesis is performed.Phototrophic eukaryotes perform photosynthesis by plastids that may have their ancestry in cyanobacteria, acquired long ago via a process called endosymbiosis. These endosymbiotic cyanobacteria in eukaryotes may have evolved or differentiated into specialized organelles such as chloroplasts, etioplasts and leucoplasts.

By producing and releasing oxygen (as a byproduct of photosynthesis), cyanobacteria are thought to have converted the early oxygen-poor, reducing atmosphere into an oxidizing one, causing the Great Oxygenation Event and the "rusting of the Earth", which dramatically changed the composition of the Earth's life forms and led to the near-extinction of anaerobic organisms.

Daniel I. Arnon

Daniel Israel Arnon (November 14, 1910 – December 20, 1994) was a Polish-born American plant physiologist whose research led to greater insights into the operation of photosynthesis in plants. In 1973, he was awarded the National Medal of Science for "his fundamental research into the mechanism of green plant utilization of light to produce chemical energy and oxygen and for contributions to our understanding of plant nutrition."

Arnon was born on November 14, 1910, in Warsaw. Summers spent on the family's farm helped foster Arnon's interest in agriculture. His father had lost the family's food wholesale business after World War I and Arnon's readings of the works of Jack London led him to save up his money to head to California. He enrolled in the University of California, Berkeley from Poland, and would spend his entire professional career at the school, until his retirement in 1978. He ultimately earned his Ph.D. in plant physiology at UC Berkeley under Dennis R. Hoagland and some of his earliest research focused on growing plants in nutrient-enriched water rather than in the soil. During World War II, Arnon served in the United States Army in the Pacific Theater of Operations, where he used his prior experience with plant nutrition on Ponape Island, where there was no arable land available and he was able to grow food to feed the troops stationed there using gravel and nutrient-enriched water.After returning from military service, Arnon performed research on chloroplasts and their role in the photosynthesis process. His work was able to demonstrate how energy from sunlight is used to form adenosine triphosphate, the energy transport messenger within living cells, by adding a third phosphorus group to adenosine diphosphate. In 1954, Arnon reproduced the process in a laboratory, making him the first to successfully demonstrate the chemical function of photosynthesis, producing sugar and starch from inputs of carbon dioxide and water. He was elected a Fellow of the American Academy of Arts and Sciences in 1962.A resident of Kensington, California, Arnon died at age 84 on December 20, 1994, in Berkeley, California, of complications resulting from cardiac arrest. He had three daughters and two sons. His wife, the former Lucile Soule, died in 1986. in 1954

Ecosystem

An ecosystem is a community of living organisms in conjunction with the nonliving components of their environment, interacting as a system. These biotic and abiotic components are linked together through nutrient cycles and energy flows. Energy enters the system through photosynthesis and is incorporated into plant tissue. By feeding on plants and on one-another, animals play an important role in the movement of matter and energy through the system. They also influence the quantity of plant and microbial biomass present. By breaking down dead organic matter, decomposers release carbon back to the atmosphere and facilitate nutrient cycling by converting nutrients stored in dead biomass back to a form that can be readily used by plants and other microbes.Ecosystems are controlled by external and internal factors. External factors such as climate, the parent material which forms the soil and topography, control the overall structure of an ecosystem, but are not themselves influenced by the ecosystem.Ecosystems are dynamic entities—they are subject to periodic disturbances and are in the process of recovering from some past disturbance. Ecosystems in similar environments that are located in different parts of the world can end up doing things very differently simply because they have different pools of species present. Internal factors not only control ecosystem processes but are also controlled by them and are often subject to feedback loops.Resource inputs are generally controlled by external processes like climate and parent material. Resource availability within the ecosystem is controlled by internal factors like decomposition, root competition or shading. Although humans operate within ecosystems, their cumulative effects are large enough to influence external factors like climate.Biodiversity affects ecosystem functioning, as do the processes of disturbance and succession. Ecosystems provide a variety of goods and services upon which people depend.

Evolution of photosynthesis

The evolution of photosynthesis refers to the origin and subsequent evolution of photosynthesis, the process by which light energy synthesizes sugars from carbon dioxide, releasing oxygen as a waste product. The process of photosynthesis was discovered by Jan Ingenhousz, a Dutch-born British physician and scientist, first publishing about it in 1779.The first photosynthetic organisms probably evolved early in the evolutionary history of life and most likely used reducing agents such as hydrogen or electrons, rather than water. There are three major metabolic pathways by which photosynthesis is carried out: C3 photosynthesis, C4 photosynthesis, and CAM photosynthesis. C3 photosynthesis is the oldest and most common form. C3 is a plant that uses the calvin cycle for the initial steps that incorporate CO2 into organic material. C4 is a plant that prefaces the calvin cycle with reactions that incorporate CO2 into four-carbon compounds. CAM is a plant that uses crassulacean acid metabolism, an adaptation for photosynthesis in arid conditions. C4 and CAM Plants have special adaptations that save water.

Excretion

Excretion is a process by which metabolic waste

is eliminated from an organism. In vertebrates this is primarily carried out by the lungs, kidneys and skin. This is in contrast with secretion, where the substance may have specific tasks after leaving the cell. Excretion is an essential process in all forms of life. For example, in mammals urine is expelled through the urethra, which is part of the excretory system. In unicellular organisms, waste products are discharged directly through the surface of the cell.

During life activities such as cellular respiration, several chemical reactions take place in the body. These are known as metabolism. These chemical reactions produce waste products such as carbon dioxide, water, salts, urea and uric acid. Accumulation of these wastes beyond a level inside the body is harmful to the body. The excretory organs remove these wastes. This process of removal of metabolic waste from the body is known as excretion.

Green plants produce carbon dioxide and water as respiratory products. In green plants, the carbon dioxide released during respiration gets utilized during photosynthesis. Oxygen is a by product generated during photosynthesis, and exits through stomata, root cell walls, and other routes. Plants can get rid of excess water by transpiration and guttation. It has been shown that the leaf acts as an 'excretophore' and, in addition to being a primary organ of photosynthesis, is also used as a method of excreting toxic wastes via diffusion. Other waste materials that are exuded by some plants — resin, saps, latex, etc. are forced from the interior of the plant by hydrostatic pressures inside the plant and by absorptive forces of plant cells. These latter processes do not need added energy, they act passively. However, during the pre-abscission phase, the metabolic levels of a leaf are high. Plants also excrete some waste substances into the soil around them.

In animals, the main excretory products are carbon dioxide, ammonia (in ammoniotelics), urea (in ureotelics), uric acid (in uricotelics), guanine (in Arachnida) and creatine. The liver and kidneys clear many substances from the blood (for example, in renal excretion), and the cleared substances are then excreted from the body in the urine and feces.

Aquatic animals usually excrete ammonia directly into the external environment, as this compound has high solubility and there is ample water available for dilution. In terrestrial animals ammonia-like compounds are converted into other nitrogenous materials as there is less water in the environment and ammonia itself is toxic.

Birds excrete their nitrogenous wastes as uric acid in the form of a paste. Although this process is metabolically more expensive, it allows more efficient water retention and it can be stored more easily in the egg. Many avian species, especially seabirds, can also excrete salt via specialized nasal salt glands, the saline solution leaving through nostrils in the beak.

In insects, a system involving Malpighian tubules is utilized to excrete metabolic waste. Metabolic waste diffuses or is actively transported into the tubule, which transports the wastes to the intestines. The metabolic waste is then released from the body along with fecal matter.

The excreted material may be called ejecta. In pathology the word ejecta is more commonly used.

Johann Deisenhofer

Johann Deisenhofer (born September 30, 1943) is a German biochemist who, along with Hartmut Michel and Robert Huber, received the Nobel Prize for Chemistry in 1988 for their determination of the first crystal structure of an integral membrane protein, a membrane-bound complex of proteins and co-factors that is essential to photosynthesis.

Photosynthetic efficiency

The photosynthetic efficiency is the fraction of light energy converted into chemical energy during photosynthesis in plants and algae. Photosynthesis can be described by the simplified chemical reaction

6H2O + 6CO2 + energy → C6H12O6 + 6O2where C6H12O6 is glucose (which is subsequently transformed into other sugars, cellulose, lignin, and so forth). The value of the photosynthetic efficiency is dependent on how light energy is defined – it depends on whether we count only the light that is absorbed, and on what kind of light is used (see Photosynthetically active radiation). It takes eight (or perhaps 10 or more) photons to utilize one molecule of CO2. The Gibbs free energy for converting a mole of CO2 to glucose is 114 kcal, whereas eight moles of photons of wavelength 600 nm contains 381 kcal, giving a nominal efficiency of 30%. However, photosynthesis can occur with light up to wavelength 720 nm so long as there is also light at wavelengths below 680 nm to keep Photosystem II operating (see Chlorophyll). Using longer wavelengths means less light energy is needed for the same number of photons and therefore for the same amount of photosynthesis. For actual sunlight, where only 45% of the light is in the photosynthetically active wavelength range, the theoretical maximum efficiency of solar energy conversion is approximately 11%. In actuality, however, plants do not absorb all incoming sunlight (due to reflection, respiration requirements of photosynthesis and the need for optimal solar radiation levels) and do not convert all harvested energy into biomass, which results in a maxium overall photosynthetic efficiency of 3 to 6% of total solar radiation. If photosynthesis is inefficient, excess light energy must be dissipated to avoid damaging the photosynthetic apparatus. Energy can be dissipated as heat (non-photochemical quenching), or emitted as chlorophyll fluorescence.

Photosystem

Photosystems are functional and structural units of protein complexes involved in photosynthesis that together carry out the primary photochemistry of photosynthesis: the absorption of light and the transfer of energy and electrons. Photosystems are found in the thylakoid membranes of plants, algae and cyanobacteria. They are located in the chloroplasts of plants and algae, and in the cytoplasmic membrane of photosynthetic bacteria. There are two kinds of photosystems: II and I.

Plastid

The plastid (Greek: πλαστός; plastós: formed, molded – plural plastids) is a membrane-bound organelle found in the cells of plants, algae, and some other eukaryotic organisms. Plastids were discovered and named by Ernst Haeckel, but A. F. W. Schimper was the first to provide a clear definition. Plastids are the site of manufacture and storage of important chemical compounds used by the cells of autotrophic eukaryotes. They often contain pigments used in photosynthesis, and the types of pigments in a plastid determine the cell's color. They have a common evolutionary origin and possess a double-stranded DNA molecule that is circular, like that of prokaryotic cells.

Primary producers

Primary producers convert an abiotic source of energy (e.g. light) into energy stored in organic compounds, which can be used by other organisms (e.g. heterotrophs). The primary producers can convert the energy in the light (phototroph and photoautotroph) or the energy in inorganic chemical compounds (chemolithotrophs) to build organic molecules, which is usually accumulated in the form of biomass and will be used as carbon and energy source by other organisms (e.g. heterotrophs and mixotrophs). The photoautotrophs are the main primary producers, converting the energy of the light into chemical energy through photosynthesis, ultimately building organic molecules from carbon dioxide, an inorganic carbon source. Examples of chemolithotrophs are some archaea and bacteria (unicellular organisms) that produce biomass from the oxidation of inorganic chemical compounds, these organisms are called chemoautotrophs, and are frequently found in hydrothermal vents in the deep ocean. Primary producers ares at the lowest trophic level, and are the reasons why Earth is sustainable for life to this day.

Terence McKenna

Terence Kemp McKenna (November 16, 1946 – April 3, 2000) was an American ethnobotanist, mystic, psychonaut, lecturer, author, and an advocate for the responsible use of naturally occurring psychedelic plants. He spoke and wrote about a variety of subjects, including psychedelic drugs, plant-based entheogens, shamanism, metaphysics, alchemy, language, philosophy, culture, technology, environmentalism, and the theoretical origins of human consciousness. He was called the "Timothy Leary of the '90s", "one of the leading authorities on the ontological foundations of shamanism", and the "intellectual voice of rave culture".McKenna formulated a concept about the nature of time based on fractal patterns he claimed to have discovered in the I Ching, which he called novelty theory, proposing this predicted the end of time, and a transition of consciousness in the year 2012. His promotion of novelty theory and its connection to the Maya calendar is credited as one of the factors leading to the widespread beliefs about 2012 eschatology. Novelty theory is considered pseudoscience.

Subdisciplines
Plant groups
Plant morphology
(glossary)
Plant growth and habit
Reproduction
Plant taxonomy
Practice
  • Lists
  • Related topics
General
Producers
Consumers
Decomposers
Microorganisms
Food webs
Example webs
Processes
Defense,
counter
Ecology: Modelling ecosystems: Other components
Population
ecology
Species
Species
interaction
Spatial
ecology
Niche
Other
networks
Other

This page is based on a Wikipedia article written by authors (here).
Text is available under the CC BY-SA 3.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.