In nature, free oxygen is produced by the light-driven splitting of water during oxygenic photosynthesis. According to some estimates, green algae and cyanobacteria in marine environments provide about 70% of the free oxygen produced on Earth, and the rest is produced by terrestrial plants. Other estimates of the oceanic contribution to atmospheric oxygen are higher, while some estimates are lower, suggesting oceans produce ~45% of Earth's atmospheric oxygen each year. A simplified overall formula for photosynthesis is: or simply
Photosynthesis and respiration
Oxygen
Oxygen | Photosynthesis and respiration
Light-driven splitting of water during oxygenic photosynthesis Sorry. I cannot find the answer based on the context. Using the energy of sunlight to produce oxygen from water and carbon dioxide. Photosynthesis uses the energy of sunlight to produce oxygen from water and carbon dioxide.
oxygen (o2) is formed in photosynthesis as a direct result of
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In nature, free oxygen is produced by the light-driven splitting of water during oxygenic photosynthesis. According to some estimates, green algae and cyanobacteria in marine environments provide about 70% of the free oxygen produced on Earth, and the rest is produced by terrestrial plants. Other estimates of the oceanic contribution to atmospheric oxygen are higher, while some estimates are lower, suggesting oceans produce ~45% of Earth's atmospheric oxygen each year. A simplified overall formula for photosynthesis is: or simply
Oxygen | Photosynthesis and respiration
In nature, free oxygen is produced by the light-driven splitting of water during oxygenic photosynthesis. Green algae and cyanobacteria in marine environments provide about 70% of the free oxygen produced on earth. The remainder is produced by terrestrial plants, although for example, almost all oxygen produced in tropical forests is consumed by organisms living there. A simplified overall formula for photosynthesis is: Photolytic oxygen evolution during photosynthesis occurs via the light-dependent oxidation of water to molecular oxygen and can be written as the following simplified chemical reaction: 2HO → 4e + 4H + O
Dioxygen in biological reactions | Photosynthesis
Plants use carbon dioxide gas in the process of photosynthesis, and exhale oxygen gas as waste. The chemical equation of photosynthesis is 6 CO (carbon dioxide) and 6 HO (water), which in the presence of sunlight makes CHO (glucose) and 6 O (oxygen). Photosynthesis uses electrons on the carbon atoms as the repository for the energy obtained from sunlight. Respiration is the opposite of photosynthesis. It reclaims the energy to power chemical reactions in cells. In so doing the carbon atoms and their electrons are combined with oxygen forming CO which is easily removed from both the cells and the organism.
Respiratory system | Plants
Oxygen is a waste product of light-dependent reactions, but the majority of organisms on Earth use oxygen for cellular respiration, including photosynthetic organisms.
Photosynthesis | Water photolysis
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:
Photosynthesis | Overview
Photosynthesis and respiration are performed by algae and by macrophytes. Respiration is also performed by bacteria and animals. Assuming steady state (net daily average) the change in deficit will be where Note that BOD only includes respiration of microorganisms e.g. algae and bacteria and not by macrophytes and animals. Due to the variation of light over time, the variation of the photosynthetic oxygen can be described by a periodical function over time, where time is after sunrise and before sunset where
Streeter–Phelps equation | Photosynthesis and respiration
Molecular oxygen, O, is essential for cellular respiration in all aerobic organisms, providing most of the chemical energy released. Oxygen is used as an electron acceptor in mitochondria to generate chemical energy in the form of adenosine triphosphate (ATP) during oxidative phosphorylation. The reaction for the aerobic respiration is essentially the reverse of photosynthesis, except that now there is a large release of the chemical energy of O, which is stored in ATP molecules (up to 38 ATP molecules are formed from one molecule of glucose). The simplified version of this reaction is:
Dioxygen in biological reactions | Aerobic respiration
Photosynthetic oxygen evolution is the fundamental process by which oxygen is generated in earth's biosphere. The reaction is part of the light-dependent reactions of photosynthesis in cyanobacteria and the chloroplasts of green algae and plants. It utilizes the energy of light to split a water molecule into its protons and electrons for photosynthesis. Free oxygen, generated as a by-product of this reaction, is released into the atmosphere. Water oxidation is catalyzed by a manganese-containing cofactor contained in photosystem II known as the oxygen-evolving complex (OEC) or water-splitting complex. Manganese is an important cofactor, and calcium and chloride are also required for the reaction to occur.
Oxygen evolution | Oxygen evolution in nature
All aerobic organisms in the aquatic environment take up dissolved oxygen during aerobic respiration, while carbon dioxide is released as a byproduct of this reaction. Because photosynthesis is light-limited, both photosynthesis and respiration occur during the daylight hours, while only respiration occurs during dark hours or in dark portions of an ecosystem. The balance between dissolved oxygen production and consumption is calculated as the aquatic metabolism rate. Vertical changes in the concentrations of dissolved oxygen are affected by both wind mixing of surface waters and the balance between photosynthesis and respiration of organic matter. These vertical changes, known as profiles, are based on similar principles as thermal stratification and light penetration.
Limnology | Oxygen and carbon dioxide
More recent studies, reported in March 2018, also suggest that photosynthesis may have begun about 3.4 billion years ago. 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 () in the photosynthetic reaction center.
Photosynthesis | Evolution
The overall general equation for the light-independent reactions is the following:3 CO + 9 ATP + 6 NADPH + 6 H → CHO-phosphate + 9 ADP + 8 P + 6 NADP + 3 HOThe 3-carbon products (CHO-phosphate) of the Calvin cycle are later converted to glucose or other carbohydrates such as starch, sucrose, and cellulose.
Fractionation of carbon isotopes in oxygenic photosynthesis | Photosynthesis reactions
In the study of plants' photorespiration, the labeling of atmosphere by oxygen-18 allows us to measure the oxygen uptake by the photorespiration pathway. Labeling by gives the unidirectional flux of uptake, while there is a net photosynthetic evolution. It was demonstrated that, under preindustrial atmosphere, most plants reabsorb, by photorespiration, half of the oxygen produced by photosynthesis. Then, the yield of photosynthesis was halved by the presence of oxygen in atmosphere.
Oxygen-18 | Plant physiology
Animals especially but also fungi and bacteria ultimately depend upon photosynthesizing plants and phytoplankton for food and oxygen.
Period 2 element | Oxygen
This test proved it was oxygen, or, as Joseph Priestley had called it, 'de-phlogisticated air'. In 1932, Robert Emerson and his student, William Arnold, used a repetitive flash technique to precisely measure small quantities of oxygen evolved by chlorophyll in the algae "Chlorella". Their experiment proved the existence of a photosynthetic unit. Gaffron and Wohl later interpreted the experiment and realized that the light absorbed by the photosynthetic unit was transferred. This reaction occurs at the reaction center of photosystem II and takes place in cyanobacteria, algae and green plants.
Photosynthetic reaction centre | Oxygenic photosynthesis
The electrons lost from Photosystem II get replaced by the oxidation of water, which is "split" into protons and oxygen by the oxygen-evolving complex (OEC, also known as WOC, or the water-oxidizing complex). To generate one molecule of diatomic oxygen, 10 photons must be absorbed by photosystems I and II, four electrons must move through the two photosystems, and 2 NADPH are generated (later used for carbon dioxide fixation in the Calvin Cycle).
Chemiosmosis | In plants
This discovery was made by Robin Hill, and subsequently the light-driven release of oxygen in the absence of CO was called the "Hill reaction". Our current knowledge of the mechanism of oxygen evolution during photosynthesis was further established in experiments tracing isotopes of oxygen from water to oxygen gas.
Oxygen evolution | History of discovery
The evolution of oxygen during the light-dependent steps in photosynthesis (Hill reaction) was proposed and proven by British biochemist Robin Hill. He demonstrated that isolated chloroplasts would make oxygen (O) but not fix carbon dioxide (CO). This is evidence that the light and dark reactions occur at different sites within the cell. Hill's finding was that the origin of oxygen in photosynthesis is water (HO) not carbon dioxide (CO) as previously believed. Hill's observation of chloroplasts in dark conditions and in the absence of CO, showed that the artificial electron acceptor was oxidized but not reduced, terminating the process, but without production of oxygen and sugar.
Hill reaction | History
The chemical pathway of oxygenic photosynthesis fixes carbon in two stages: the light-dependent reactions and the light-independent reactions. The light-dependent reactions capture light energy to transfer electrons from water and convert NADP, ADP, and inorganic phosphate into the energy-storage molecules NADPH and ATP. The overall equation for the light-dependent reactions is generally:2 HO + 2 NADP + 3 ADP + 3 P + light → 2 NADPH + 2 H + 3 ATP + OThe light-independent reactions undergo the Calvin-Benson cycle, in which the energy from NADPH and ATP is used to convert carbon dioxide and water into organic compounds via the enzyme RuBisCO.
Fractionation of carbon isotopes in oxygenic photosynthesis | Photosynthesis reactions
In general outline and in effect, photosynthesis is the opposite of cellular respiration, in which glucose and other compounds are oxidized to produce and water, and to release exothermic chemical energy to drive the organism's metabolism. However, the two processes take place through a different sequence of chemical reactions and in different cellular compartments.
Carbon dioxide in Earth's atmosphere | Atmospheric carbon dioxide and photosynthesis
This led to many organisms adapting from fermentation to respiration, leading to organisms evolving photosynthesis and what is termed the Cambrian explosion of species. It has also been suggested that this increased oxygen level reduced the influence of Ultraviolet radiation. It is now well documented that oxygen level reached at least 10% of the present value 2.4 billion years ago (for details see Great Oxygenation Event).
Pasteur point | Introduction
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.
Photosynthesis | Overview
To supply the electrons in the circuit, hydrogen is stripped from water, leaving oxygen as a waste product. Some organisms, including purple bacteria and green sulfur bacteria, use an anoxygenic form of photosynthesis that uses alternatives to hydrogen stripped from water as electron donors; examples are hydrogen sulfide, sulfur and iron. Such extremophile organisms are restricted to otherwise inhospitable environments such as hot springs and hydrothermal vents. The simpler anoxygenic form arose about 3.8 Ga, not long after the appearance of life. The timing of oxygenic photosynthesis is more controversial; it had certainly appeared by about 2.4 Ga, but some researchers put it back as far as 3.2 Ga.
History of Earth | Oxygen revolution
The inequality is balanced by photosynthesis. Photosynthesis emits oxygen with the same isotopic composition (i.e. the ratio between O and O) as the water (HO) used in the reaction, which is independent of the atmospheric ratio. Thus when atmospheric O levels are high enough, photosynthesis will act as a reducing factor. However, as a complicating factor, the degree of fractionation (i.e. change in isotope ratio) occurring due to photosynthesis is not "entirely" dependent on the water drawn up by the plant, as fractionation can occur as a result of preferential evaporation of and other small but significant processes.
Dole effect | Introduction
Different organisms fix carbon through different mechanisms, which are reflected in the varying isotope compositions across photosynthetic pathways (see table below, and explanation of notation in "Carbon Isotope Measurement" section). The following sections will outline the different oxygenic photosynthetic pathways and what contributes to their associated delta values.
Fractionation of carbon isotopes in oxygenic photosynthesis | Introduction
By studying purple sulfur bacteria and green sulfur bacteria he was the first scientist to demonstrate, in 1931, that photosynthesis is a light-dependent redox reaction in which hydrogen from an oxidizable compound reduces carbon dioxide to cellular materials. Expressed as: where A is the electron acceptor. His discovery predicted that HO is the hydrogen donor in green plant photosynthesis and is oxidized to O. The chemical summation of photosynthesis was a milestone in the understanding of the chemistry of photosynthesis. This was later experimentally verified by Robert Hill. In a nutshell, van Niel proved that plants give off oxygen as a result of splitting water molecules during photosynthesis, not carbon dioxide molecules as thought before.
C. B. van Niel | Photosynthesis (1931)
In hot and dry conditions, plants close their stomata to prevent water loss. Under these conditions, 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 concentration in the leaves under these conditions. Plants that use the C 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.
Photosynthesis | On land
The oxygen used for oxidizing reducing compounds by organisms at hydrothermal vents at the bottom of the ocean is the result of photosynthesis at the Oceans' surface.
Abiogenesis | Chemiosmosis
In August 2020 researchers at Cambridge University announced a stand alone advanced ‘photosheet’ technology that converts sunlight, carbon dioxide and water into oxygen and formic acid with no other inputs.
Formic acid | Artificial photosynthesis
Oxygen is a component of many organic and inorganic molecules within the plant, and is acquired in many forms. These include: O and CO (mainly from the air via leaves) and HO, NO, HPO and SO (mainly from the soil water via roots). Plants produce oxygen gas (O) along with glucose during photosynthesis but then require O to undergo aerobic cellular respiration and break down this glucose to produce ATP.
Plant nutrition | Oxygen
Factors which influence this include the atmospheric abundance of the two gases, the supply of the gases to the site of fixation (i.e. in land plants: whether the stomata are open or closed), the length of the liquid phase (how far these gases have to diffuse through water in order to reach the reaction site). For example, when the stomata are closed to prevent water loss during drought: this limits the supply, while production within the leaf will continue. In algae (and plants which photosynthesise underwater); gases have to diffuse significant distances through water, which results in a decrease in the availability of relative to .
Photorespiration | Altered substrate availability: lowered CO or increased O
Oxygen evolution is the process of generating molecular oxygen (O) by a chemical reaction, usually from water. Oxygen evolution from water is effected by oxygenic photosynthesis, electrolysis of water, and thermal decomposition of various oxides. The biological process supports aerobic life. When relatively pure oxygen is required industrially, it is isolated by distillation of liquified air.
Oxygen evolution | Introduction
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).
Anoxygenic photosynthesis | Introduction
Dioxygen provides the energy released in combustion and aerobic cellular respiration, and many major classes of organic molecules in living organisms contain oxygen atoms, such as proteins, nucleic acids, carbohydrates, and fats, as do the major constituent inorganic compounds of animal shells, teeth, and bone. Most of the mass of living organisms is oxygen as a component of water, the major constituent of lifeforms. Oxygen is continuously replenished in Earth's atmosphere by photosynthesis, which uses the energy of sunlight to produce oxygen from water and carbon dioxide. Oxygen is too chemically reactive to remain a free element in air without being continuously replenished by the photosynthetic action of living organisms.
Oxygen | Introduction
At the compensation point, the rate of photosynthesis is equal to the rate of respiration. Products of photosynthesis are used up in respiration so that the organism is neither consuming nor building biomass. The net gaseous exchange is also zero at this point.
Compensation point | Rate
The main source of atmospheric free oxygen is photosynthesis, which produces sugars and free oxygen from carbon dioxide and water: Photosynthesizing organisms include the plant life of the land areas as well as the phytoplankton of the oceans. The tiny marine cyanobacterium "Prochlorococcus" was discovered in 1986 and accounts for up to half of the photosynthesis of the open ocean.
Oxygen cycle | Biological production
The overall reaction catalysed by photosystem II is: Q represents plastoquinone, the oxidized form of Q. QH represents plastoquinol, the reduced form of Q. This process of reducing quinone is comparable to that which takes place in the bacterial reaction center. Photosystem II obtains electrons by oxidizing water in a process called photolysis. Molecular oxygen is a byproduct of this process, and it is this reaction that supplies the atmosphere with oxygen. The fact that the oxygen from green plants originated from water was first deduced by the Canadian-born American biochemist Martin David Kamen. He used a stable isotope of oxygen, O to trace the path of the oxygen from water to gaseous molecular oxygen.
Photosynthetic reaction centre | Photosystem II
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.
Anoxygenic photosynthesis | Type II reaction centers
Ingenhousz suggested in 1796 that CO (carbon dioxide) is split during photosynthesis to release oxygen, while the carbon combined with water to form carbohydrates. While this hypothesis was attractive and reasonable and thus widely accepted for a long time, it was later proven incorrect. Graduate student C.B. Van Niel at Stanford University found that purple sulfur bacteria reduce carbon to carbohydrates, but accumulate sulfur instead of releasing oxygen. He boldly proposed that, in analogy to the sulfur bacteria's forming elemental sulfur from HS (hydrogen sulfide), plants would form oxygen from HO (water). In 1937, this hypothesis was corroborated by the discovery that plants are capable of producing oxygen in the absence of CO.
Oxygen evolution | History of discovery
As an example of a biotic fractionation event, respiration has led to an enrichment of O in the atmosphere relative to O. The isotopic ratio of O/O is +23.5‰ relative to V-SMOW, and this ratio should also be observed in oxygen's consumption ratios. In one of the first major studies of plant oxygen consumption and fractionation, Guy, Fogel, and Berry defined the oxygen fractionation effects of various plant functions. They found that plants do not fraction oxygen isotopes in the photolysis of water in spinach thylakoids, but that they did discriminate against O during oxygen uptake by 21.3‰ during for the oxygenation of Rubisco in spinach and by 22.7‰ during the photorespiration of phosphoglycate by glycate oxidase.
Marilyn Fogel | Modern <a href="ecosystem">ecosystem</a>s
In hundreds of experiments with carbon-11 produced from deuterons and boron-10 by Kamen in the Radiation Laboratory's 37-inch cyclotron, Ruben and Kamen, with collaborators from botany, microbiology, physiology and organic chemistry, pursued the path of carbon dioxide in plants, algae, and bacteria. Their results, confused by absorption of the products on proteinaceous residues, initially failed to reveal the path of carbon in photosynthesis but succeeded in exciting the interest of scientists worldwide in the search and revelation of metabolic processes, beginning a revolution in biochemistry and medicine. Ruben's experiments using 'heavy water', HO, to yield O gas had shown that the oxygen gas produced in photosynthesis comes from water.
Sam Ruben | Research
Many important biological processes involve redox reactions. Cellular respiration, for instance, is the oxidation of glucose (CHO) to CO and the reduction of oxygen to water. The summary equation for cell respiration is: The process of cell respiration also depends heavily on the reduction of NAD to NADH and the reverse reaction (the oxidation of NADH to NAD). Photosynthesis and cellular respiration are complementary, but photosynthesis is not the reverse of the redox reaction in cell respiration:
Redox | Redox reactions in biology
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), which are the only prokaryotes performing oxygenic photosynthesis. The geological record indicates that this transforming event took place early in Earth's history, at least 2450–2320 million years ago (Ma), and, it is speculated, much earlier. Because the Earth's atmosphere contained almost no oxygen during the estimated development of photosynthesis, it is believed that the first photosynthetic cyanobacteria did not generate oxygen. Available evidence from geobiological studies of Archean (>2500 Ma) sedimentary rocks indicates that life existed 3500 Ma, but the question of when oxygenic photosynthesis evolved is still unanswered.
Photosynthesis | Cyanobacteria and the evolution of photosynthesis
Before photosynthesis evolved, Earth's atmosphere had no free oxygen (O). Photosynthetic prokaryotic organisms that produced O as a waste product lived long before the first build-up of free oxygen in the atmosphere, perhaps as early as 3.5 billion years ago. The oxygen they produced would have been rapidly removed from the oceans by weathering of reducing minerals, most notably iron. This rusting led to the deposition of iron oxide on the ocean floor, forming banded iron formations. Thus, the oceans rusted and turned red. Oxygen only began to persist in the atmosphere in small quantities about 50 million years before the start of the Great Oxygenation Event.
Geological history of oxygen | Introduction
Currently, most of the oxygen on planet earth derives from oxygenic photosynthesis. Some phototrophic prokaryotes such as cyanobacteria developed the ability to carry out oxygenic photosynthesis around 2.7 billion years ago. Moreover, 2 billion years ago planet earth was struck with "The Great Oxygenation event" also known as the oxygen crisis. Rising levels of atmospheric , mainly due to oxygenic photosynthesis carried out by cyanobacteria nearly caused the mass death of anaerobic organisms. Having to face the evolutionary pressure of dropping ambient levels, cyanobacteria coped by evolving carbon concentrating mechanisms (CCM). Thus, these carbon concentrating mechanisms, also known as "DIC-pumps", are found in most photosynthetic microorganisms, such as unicellular green algea and cyanobacteria.
SbtB protein | Carbon concentrating mechanisms
Ecosystem respiration happens when a cell takes glucose and oxygen and uses it to produce carbon dioxide, energy, and water. This transaction is important not only for the benefit of the cells, but for the carbon dioxide output provided, which is key in the process of photosynthesis. Without respiration, actions necessary to life, such as metabolic processes and photosynthesis, would cease.
Ecosystem respiration | Overview
The large fractionation of C in photosynthesis is due to the carboxylation reaction, which is carried out by the enzyme ribulose-1,5-bisphosphate carboxylase oxygenase, or RuBisCO. RuBisCO catalyzes the reaction between a five-carbon molecule, ribulose-1,5-bisphosphate (abbreviated as RuBP) and CO to form two molecules of 3-phosphoglyceric acid (abbreviated as PGA). PGA reacts with NADPH to produce 3-phosphoglyceraldehyde. Isotope fractionation due to Rubsico (form I) carboxylation alone is predicted to be a 28‰ depletion, on average. However, fractionation values vary between organisms, ranging from an 11‰ depletion observed in coccolithophorid algae to a 29‰ depletion observed in spinach. RuBisCO causes a kinetic isotope effect because CO and CO compete for the same active site and C has an intrinsically lower reaction rate.
Fractionation of carbon isotopes in oxygenic photosynthesis | Fractionation via RuBisCO
Respiration occurs by both plants and animals throughout the water column, resulting in the destruction, or usage, of organic matter, but photosynthesis can only take place via photosynthetic algae in the presence of light, nutrients and CO. In well-mixed water columns plankton are evenly distributed, but a net production only occurs above the compensation depth. Below the compensation depth there is a net loss of organic matter. The total population of photosynthetic organisms cannot increase if the loss exceeds the net production. The compensation depth between photosynthesis and respiration of phytoplankton in the ocean must be dependent on some factors: the illumination at the surface, the transparency of the water, the biological character of the plankton present, and the temperature.
Compensation point | The marine environment
The stoichiometry this reaction follows: The protons are released into the thylakoid lumen, thus contributing to the generation of a proton gradient across the thylakoid membrane. This proton gradient is the driving force for ATP synthesis via photophosphorylation and coupling the absorption of light energy and oxidation of water to the creation of chemical energy during photosynthesis.
Oxygen evolution | Oxygen evolution in nature
Earth's earliest atmosphere contained no free oxygen (O); the oxygen that animals breathe today, both in the air and dissolved in water, is the product of billions of years of photosynthesis. Cyanobacteria were the first organisms to evolve the ability to photosynthesize, introducing a steady supply of oxygen into the environment. Initially, oxygen levels did not increase substantially in the atmosphere. The oxygen quickly reacted with iron and other minerals in the surrounding rock and ocean water. Once a saturation point was reached for the reactions in rock and water, oxygen was able to exist as a gas in its diatomic form.
Cambrian explosion | Increase in oxygen levels
In C4 plants, carbon is converted to bicarbonate, fixed into oxaloacetate via the enzyme phosphoenolpyruvate (PEP) carboxylase, and is then converted to malate. The malate is transported from the mesophyll to bundle sheath cells, which are impermeable to CO. The internal CO is concentrated in these cells as malate is reoxidized then decarboxylated back into CO and pyruvate. This enables RuBisCO to perform catalysis while internal CO is sufficiently high to avoid the competing photorespiration reaction. The delta value in the C4 pathway is -12 to -16‰ depleted in C due to the combined effects of PEP carboxylase and RuBisCO.
Fractionation of carbon isotopes in oxygenic photosynthesis | In C4 plants
Experimentation with respiratory oxidase inhibitors (for instance, cyanide) on unicellular algae has revealed interactive pathways to be present between chloroplasts and mitochondria. Metabolic pathways responsible for photosynthesis are present in chloroplasts, whereas respiratory metabolic pathways are present in mitochondria. In these pathways, metabolic carriers (like phosphate) exchange NAD(P)H molecules between photosynthetic and respiratory ETCs. Evidence using mass spectrometry on algae and photosynthetic mutants of "Chlamydomonas", discovered that oxygen molecules were also being exchanged between photosynthetic and chlororespiratory ETCs. The mutant "Chlamydomonas" plant species, lacks photosystems one and two (PS I and PS II), so when the plant underwent flash-induced PS I activity, it resulted in no effect on mitochondrial pathways of respiration.
Chlororespiration | The relation between chlororespiration, photosynthesis and respiration
Addition of molecular oxygen to ribulose-1,5-bisphosphate produces 3-phosphoglycerate (PGA) and 2-phosphoglycolate (2PG, or PG). PGA is the normal product of carboxylation, and productively enters the Calvin cycle. Phosphoglycolate, however, inhibits certain enzymes involved in photosynthetic carbon fixation (hence is often said to be an 'inhibitor of photosynthesis'). It is also relatively difficult to recycle: in higher plants it is salvaged by a series of reactions in the peroxisome, mitochondria, and again in the peroxisome where it is converted into glycerate. Glycerate reenters the chloroplast and by the same transporter that exports glycolate. A cost of 1 ATP is associated with conversion to 3-phosphoglycerate (PGA) (Phosphorylation), within the chloroplast, which is then free to re-enter the Calvin cycle.
Photorespiration | Photorespiratory reactions
Plants use both processes, photosynthesis to capture the energy and oxidative metabolism to use it. Plant respiration is limited by the process of diffusion. Plants take in carbon dioxide through holes, known as stomata, that can open and close on the undersides of their leaves and sometimes other parts of their anatomy. Most plants require some oxygen for catabolic processes (break-down reactions that release energy). But the quantity of O used per hour is small as they are not involved in activities that require high rates of aerobic metabolism. Their requirement for air, however, is very high as they need CO for photosynthesis, which constitutes only 0.04% of the environmental air.
Respiratory system | Plants
A chronology of oxygen accumulation suggests that free oxygen was first produced by prokaryotic and then later by eukaryotic organisms in the ocean. These organisms carried out photosynthesis, producing oxygen as a waste product. In one interpretation, the first oxygen-producing cyanobacteria could have arisen before the GOE, from 2.7–2.4 Ga and perhaps even earlier. However, oxygenic photosynthesis also produces organic carbon that must be segregated from oxygen to allow oxygen accumulation in the surface environment, otherwise the oxygen back-reacts with the organic carbon and does not accumulate. The burial of organic carbon, sulfide, and minerals containing ferrous iron (Fe) is a primary factor in oxygen accumulation.
Great Oxidation Event | Oxygen accumulation
While there are many abiotic sources and sinks for O, the presence of the profuse concentration of free oxygen in modern Earth's atmosphere and ocean is attributed to O production from the biological process of oxygenic photosynthesis in conjunction with a biological sink known as the biological pump and a geologic process of carbon burial involving plate tectonics. Biology is the main driver of O flux on modern Earth, and the evolution of oxygenic photosynthesis by bacteria, which is discussed as part of The Great Oxygenation Event, is thought to be directly responsible for the conditions permitting the development and existence of all complex eukaryotic metabolism.
Oxygen cycle | Sources and sinks
In plants, algae and cyanobacteria, photosynthesis releases oxygen. This is called "oxygenic photosynthesis". Although there are some differences between oxygenic photosynthesis in plants, algae, and cyanobacteria, the overall process is quite similar in these organisms. However, there are some types of bacteria that carry out anoxygenic photosynthesis, which consumes but does not release oxygen. Carbon dioxide is converted into sugars in a process called carbon fixation. Carbon fixation is an endothermic redox reaction, so photosynthesis needs to supply both the source of energy to drive this process and the electrons needed to convert into a carbohydrate. This addition of the electrons is a reduction reaction.
Carbon dioxide in Earth's atmosphere | Atmospheric carbon dioxide and photosynthesis
The overall process of photosynthesis takes place in four stages:
Photosynthesis | Order and kinetics
Photosynthesis releases oxygen into the atmosphere, while respiration, decay, and combustion remove it from the atmosphere. In the present equilibrium, production and consumption occur at the same rate. Free oxygen also occurs in solution in the world's water bodies. The increased solubility of at lower temperatures (see Physical properties) has important implications for ocean life, as polar oceans support a much higher density of life due to their higher oxygen content. Water polluted with plant nutrients such as nitrates or phosphates may stimulate growth of algae by a process called eutrophication and the decay of these organisms and other biomaterials may reduce the content in eutrophic water bodies.
Oxygen | Occurrence
Photorespiration can occur when the oxygen concentration is too high. RuBisCO cannot distinguish between oxygen and carbon dioxide very well, so it can accidentally add O instead of CO to RuBP. This process reduces the efficiency of photosynthesis—it consumes ATP and oxygen, releases CO, and produces no sugar. It can waste up to half the carbon fixed by the Calvin cycle. Several mechanisms have evolved in different lineages that raise the carbon dioxide concentration relative to oxygen within the chloroplast, increasing the efficiency of photosynthesis. These mechanisms are called carbon dioxide concentrating mechanisms, or CCMs. These include Crassulacean acid metabolism, carbon fixation, and pyrenoids.
Chloroplast | Photorespiration
Photorespiration (also known as the oxidative photosynthetic carbon cycle, or C photosynthesis) refers to a process in plant metabolism where the enzyme RuBisCO oxygenates RuBP, wasting some of the energy produced by photosynthesis. The desired reaction is the addition of carbon dioxide to RuBP (carboxylation), a key step in the Calvin–Benson cycle, but approximately 25% of reactions by RuBisCO instead add oxygen to RuBP (oxygenation), creating a product that cannot be used within the Calvin–Benson cycle. This process reduces the efficiency of photosynthesis, potentially reducing photosynthetic output by 25% in plants. Photorespiration involves a complex network of enzyme reactions that exchange metabolites between chloroplasts, leaf peroxisomes and mitochondria.
Photorespiration | Introduction
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 "phōs" (), "light", and "sunthesis" (), "putting together". 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 most of the energy necessary for life on Earth.
Photosynthesis | Introduction
These cells are part of the characteristic "kranz leaf anatomy", which spatially separates photosynthetic cell-types in a concentric arrangement to accumulate CO near RuBisCO. These chemical and anatomical mechanisms improve the ability of RuBisCO to fix carbon, rather than perform its wasteful oxygenase activity. The RuBisCO oxygenase activity, called photorespiration, causes the RuBP substrate to be lost to oxigenation, and consumes energy in doing so. The adaptations of C4 plants provide an advantage over the C3 pathway, which loses efficiency due to photorespiration. The ratio of photorespiration to photosynthesis in a plant varies with environmental conditions, since decreased CO and elevated O concentrations would increase the efficiency of photorespiration.
Fractionation of carbon isotopes in oxygenic photosynthesis | In C4 plants
Thus, to make 1 g of glucose requires the removal of all the CO from "at least" 18.7 liters of air at sea level. But inefficiencies in the photosynthetic process cause considerably greater volumes of air to be used.
Respiratory system | Plants
Estimates of lake metabolism typically rely on the measurement of dissolved oxygen or carbon dioxide, or measurements of a carbon or oxygen tracer to estimate production and consumption of organic carbon. Oxygen is produced and carbon dioxide consumed through photosynthesis and oxygen is consumed and carbon dioxide produced through respiration. Photosynthesis: formula_1 Respiration: formula_2
Lake metabolism | Concept
How did photosynthesis arise in bacteria? The answer to this question is complicated by the fact that there are several types of light-harvesting energy capture systems. "Chloroflexus aurantiacus" has been of interest in the search for origins of the so-called type II photosynthetic reaction center. One idea is that bacteria with respiratory electron transport evolved photosynthesis by coupling a light-harvesting energy capture system to the pre-existing respiratory electron transport chain. Thus, rare organisms like "Chloroflexus aurantiacus" that can survive using either respiration or photosynthesis are of interest in on-going attempts to trace the evolution of photosynthesis.
Chloroflexus aurantiacus | The "respiration early" hypothesis
Oxygen is emitted and carbon dioxide is absorbed by plants and microorganisms performing photosynthesis, but CO levels are most strongly affected by wild fires and human activity.
Homosphere | Variations in concentration
Photosynthesis converts carbon dioxide to carbohydrates via several metabolic pathways that provide energy to an organism and preferentially react with certain stable isotopes of carbon. The selective enrichment of one stable isotope over another creates distinct isotopic fractionations that can be measured and correlated with among oxygenic phototrophs. The degree of carbon isotope fractionation is influenced by several factors, including the metabolism, anatomy, growth rate, and environmental conditions of the organism. Understanding these variations in carbon fractionation across species is useful for biogeochemical studies, including the reconstruction of paleoecology, plant evolution, and the characterization of food chains. Oxygenic photosynthesis is a metabolic pathway facilitated by autotrophs, including plants, algae, and cyanobacteria.
Fractionation of carbon isotopes in oxygenic photosynthesis | Introduction
This is especially a concern in herbivorous animals (see Photosensitivity in animals).
Singlet oxygen | Biochemistry
Primary production is the process by which autotrophs use light to convert carbon from aqueous carbon dioxide to sugar for cellular growth. Light catalyzes the photosynthetic process and nutrients are incorporated into organic material. For photosynthesis to occur, macronutrients such as nitrate and phosphate must be available in sufficient ratios and bioavailable forms for biological utilization. The molecular ratio of 106(Carbon):16(Nitrogen):1(Phosphorus) was discovered by Redfield, Ketcham, and Richards (RKR) and is known as the Redfield Ratio. Photosynthesis (forward) and respiration (reverse) is represented by the equation: Photosynthesis can be limited by deficiencies of certain macronutrients. However, in the North Pacific, the Equatorial Pacific, and the Southern Ocean macronutrients are found in sufficient ratios, quantities and bioavailable forms to support greater levels of primary production than found.
High-nutrient, low-chlorophyll regions | Primary production
There may have been a gap of up to 900 million years between the start of photosynthetic oxygen production and the geologically rapid increase in atmospheric oxygen about 2.5–2.4 billion years ago. Several hypotheses propose to explain this time lag.
Great Oxidation Event | Hypotheses
This observation allowed Hill to conclude that oxygen is released during the light-dependent steps (Hill reaction) of photosynthesis. Hill also discovered Hill reagents, artificial electron acceptors that participate in the light reaction, such as Dichlorophenolindophenol (DCPIP), a dye that changes color when reduced. These dyes permitted the finding of electron transport chains during photosynthesis.
Hill reaction | History
Öquist's field of research has been the adaptation of photosynthesis in cyanobacteria, algae and higher plants when they are submitted to variations in light intensity, temperature, supply of water or supply of nutrition.
Gunnar Öquist | Introduction
The photosynthesis conducted by land plants and algae is the ultimate source of energy and organic material in nearly all ecosystems. Photosynthesis, at first by cyanobacteria and later by photosynthetic eukaryotes, radically changed the composition of the early Earth's anoxic atmosphere, which as a result is now 21% oxygen. Animals and most other organisms are aerobic, relying on oxygen; those that do not are confined to relatively rare anaerobic environments. Plants are the primary producers in most terrestrial ecosystems and form the basis of the food web in those ecosystems. Many animals rely on plants for shelter as well as oxygen and food.
Plant | Ecology
About half of the GPP is consumed in plant respiration. The remainder, that portion of GPP that is not used up by respiration, is known as the net primary production (NPP). 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.
Ecosystem | Primary production
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. The pathway of CO 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 CO·m·s, with the conclusion that all terrestrial plants having the same photosynthetic capacities that were light saturated at less than 50% of sunlight.
Photosynthesis | C3 : C4 photosynthesis research
This mass oxygenation of the atmosphere resulted in rapid buildup of free oxygen. At current rates of primary production, today's concentration of oxygen could be produced by photosynthetic organisms in 2,000 years. In the absence of plants, the rate of oxygen production by photosynthesis was slower in the Precambrian, and the concentrations of O attained were less than 10% of today's and probably fluctuated greatly; oxygen may even have disappeared from the atmosphere again around 1.9 billion years ago. These fluctuations in oxygen concentration had little direct effect on life, with mass extinctions not observed until the appearance of complex life around the start of the Cambrian period, .
Geological history of oxygen | Introduction
The (light) compensation point is the light intensity on the light curve where the rate of photosynthesis exactly matches the rate of cellular respiration. At this point, the uptake of CO through photosynthetic pathways is equal to the respiratory release of carbon dioxide, and the uptake of O by respiration is equal to the photosynthetic release of oxygen.. In assimilation terms, at the compensation point, the net carbon dioxide assimilation is zero. Leaves release CO by photorespiration and cellular respiration, but CO is also converted into carbohydrate by photosynthesis. Assimilation is therefore the difference in the rate of these processes.
Compensation point | Introduction
In plants, algae and cyanobacteria, long-term energy storage in the form of sugars is produced by a subsequent sequence of 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). 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.
Photosynthesis | Introduction
Primary production (plant growth) changes pH on a daily, seasonal, and annual basis. During photosynthesis, carbon dioxide is removed from the water, increasing pH. Organisms release carbon dioxide during respiration. This leads to a daily cycle of increased pH during daylight hours and a decrease in pH during the night, when respiration is dominant. Similarly, pH is higher during the winter when grazing is low compared to productivity.
Estuarine acidification | Photosynthesis and respiration
The compensation point is reached during early mornings and late evenings. Respiration is relatively constant, whereas photosynthesis depends on the intensity of sunlight. When the rate of photosynthesis equals the rate of respiration or photorespiration, the compensation point occurs.
Compensation point | Timing
Plant physiology encompasses all the internal chemical and physical activities of plants associated with life. Chemicals obtained from the air, soil and water form the basis of all plant metabolism. The energy of sunlight, captured by oxygenic photosynthesis and released by cellular respiration, is the basis of almost all life. Photoautotrophs, including all green plants, algae and cyanobacteria gather energy directly from sunlight by photosynthesis. Heterotrophs including all animals, all fungi, all completely parasitic plants, and non-photosynthetic bacteria take in organic molecules produced by photoautotrophs and respire them or use them in the construction of cells and tissues. Respiration is the oxidation of carbon compounds by breaking them down into simpler structures to release the energy they contain, essentially the opposite of photosynthesis.
Botany | Plant physiology
During this era, oxygenic photosynthesis first evolved, releasing an abundance of oxygen, that first reacted with minerals and afterward was free to react with greenhouse gases of the atmosphere, leaving the Earth's surface free to radiate its energy to space. This is known as the oxygen catastrophe which was to happen later in the Paleoproterozoic from a poisonous buildup of oxygen in the atmosphere, released by these oxygen producing photoautotrophs, which evolved earlier in the Neoarchean.
Neoarchean | Complex life
This makes excitation of O a less energetically restrictive process. In the first excited state of O, a 22 kcal/mol energy increase from the ground state, both electrons in the antibonding orbitals occupy a degenerate π* orbital, and oxygen is now in a singlet state (indicated as O). O is very reactive with a lifetime between 10-100µs.
Photooxygenation | States of molecular oxygen
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.
Photosynthesis | Calvin cycle
The oxygenation reaction of RuBisCO is a wasteful process because 3-phosphoglycerate is created at a reduced rate and higher metabolic cost compared with RuBP carboxylase activity. While photorespiratory carbon cycling results in the formation of G3P eventually, around 25% of carbon fixed by photorespiration is re-released as and nitrogen, as ammonia. Ammonia must then be detoxified at a substantial cost to the cell. Photorespiration also incurs a direct cost of one ATP and one NAD(P)H. While it is common to refer to the entire process as photorespiration, technically the term refers only to the metabolic network which acts to rescue the products of the oxygenation reaction (phosphoglycolate).
Photorespiration | Introduction
The presence of abundant 2-alpha-methylhopanes preserved in these shales was interpreted as evidence of oxygenic photosynthesis at least 2.7 Gya, unexpectedly suggesting a 400 million year gap between the evolution of oxygenic metabolism and when Earth’s atmosphere became oxidizing. This interpretation of the biomarker record was challenged when "Geobacter sulfurreducens" was demonstrated to synthesize diverse hopanols, although not 2-methyl-hopanols, when grown under strictly anaerobic conditions. The integrity of 2-methylhopanes as biomarkers for oxygenic photosynthesis was then further attenuated by evidence that the phototroph "Rhodopseudomonas palustris" produced 2-methyl-BHPs only under anoxic conditions. Concrete evidence came from studies showing that not all cyanobacteria make hopanoids and that genes coding for the methyltransferase HpnP are present in nonphotosynthetic alphaproteobacteria and acidobacteria.
Hopanoids | In paleobiology
In 1772, the chemist Joseph Priestley carried out a series of experiments relating to the gases involved in respiration and combustion. In his first experiment, he lit a candle and placed it under an upturned jar. After a short period of time, the candle burned out. He carried out a similar experiment with a mouse in the confined space of the burning candle. He found that the mouse died a short time after the candle had been extinguished. However, he could revivify the foul air by placing green plants in the area and exposing them to light. Priestley's observations were some of the first experiments that demonstrated the activity of a photosynthetic reaction center.
Photosynthetic reaction centre | Oxygenic photosynthesis
As light availability decreases deeper in the water column, photosynthesis rates also decrease, and less dissolved oxygen is produced. This means that dissolved oxygen concentrations generally decrease as you move deeper into the body of water because of photosynthesis is not replenishing dissolved oxygen that is being taken up through respiration. During periods of thermal stratification, water density gradients prevent oxygen-rich surface waters from mixing with deeper waters. Prolonged periods of stratification can result in the depletion of bottom-water dissolved oxygen; when dissolved oxygen concentrations are below 2 milligrams per liter, waters are considered hypoxic. When dissolved oxygen concentrations are approximately 0 milligrams per liter, conditions are anoxic.
Limnology | Oxygen and carbon dioxide
The final and most significant stage of this liberation was the development of oxygen-producing photosynthesis, since the main chemical inputs for this are carbon dioxide and water.
Microbial mat | Introduction
The transition to an oxygen-dominant atmosphere (the "Great Oxidation") did not begin until approximately 2.4–2.3 billion years ago, but photosynthetic processes started 0.3 to 1 billion years prior.
Ecology | Metabolism and the early atmosphere
Thus, oxygen is reduced to water and the original electron acceptors NAD and quinone are regenerated. This is why humans breathe in oxygen and breathe out carbon dioxide. The energy released from transferring the electrons from high-energy states in NADH and quinol is conserved first as proton gradient and converted to ATP via ATP synthase. This generates an additional "28" molecules of ATP (24 from the 8 NADH + 4 from the 2 quinols), totaling to 32 molecules of ATP conserved per degraded glucose (two from glycolysis + two from the citrate cycle). It is clear that using oxygen to completely oxidize glucose provides an organism with far more energy than any oxygen-independent metabolic feature, and this is thought to be the reason why complex life appeared only after Earth's atmosphere accumulated large amounts of oxygen.
Biochemistry | Aerobic
Photosynthesis is the synthesis of carbohydrates from sunlight and carbon dioxide (CO). In plants, cyanobacteria and algae, oxygenic photosynthesis splits water, with oxygen produced as a waste product. This process uses the ATP and NADPH produced by the photosynthetic reaction centres, as described above, to convert CO into glycerate 3-phosphate, which can then be converted into glucose. This carbon-fixation reaction is carried out by the enzyme RuBisCO as part of the Calvin – Benson cycle. Three types of photosynthesis occur in plants, C3 carbon fixation, C4 carbon fixation and CAM photosynthesis. These differ by the route that carbon dioxide takes to the Calvin cycle, with C3 plants fixing CO directly, while C4 and CAM photosynthesis incorporate the CO into other compounds first, as adaptations to deal with intense sunlight and dry conditions.
Metabolism | Carbon fixation
Pushing even further in the UV may also allow distinction between a biotic, high-O atmosphere from an abiotic, -rich atmosphere based on the ozone absorption of 0.3 μm. Molecular oxygen () can be produced by geophysical processes, as well as a byproduct of photosynthesis by life forms, so although encouraging, is not a reliable biosignature, and it should be considered in its environmental context.
Habitable Exoplanet Imaging Mission | Biosignatures
As photosynthesis has not taken place in the dark vessel, it provides a measure of ecosystem respiration. The light vessel permits both photosynthesis and respiration, so provides a measure of net photosynthesis (i.e. oxygen production via photosynthesis subtract oxygen consumption by respiration). Gross primary production is then obtained by adding oxygen consumption in the dark vessel to net oxygen production in the light vessel. The technique of using C incorporation (added as labelled NaCO) to infer primary production is most commonly used today because it is sensitive, and can be used in all ocean environments. As C is radioactive (via beta decay), it is relatively straightforward to measure its incorporation in organic material using devices such as scintillation counters.
Primary production | Aquatic
Molecular oxygen () can be produced by geophysical processes, as well as a byproduct of photosynthesis by life forms, so although encouraging, is not a reliable biosignature. In fact, planets with high concentration of in their atmosphere may be uninhabitable. Abiogenesis in the presence of massive amounts of atmospheric oxygen could be difficult because early organisms relied on the free energy available in redox reactions involving a variety of hydrogen compounds; on an -rich planet, organisms would have to compete with the oxygen for this free energy. Astrobiology mission concepts to water worlds in the outer Solar System:
Ocean world | Oxygen
In general, photosynthesis in cyanobacteria uses water as an electron donor and produces oxygen as a byproduct, though some may also use hydrogen sulfide a process which occurs among other photosynthetic bacteria such as the purple sulfur bacteria. Carbon dioxide is reduced to form carbohydrates via the Calvin cycle. The large amounts of oxygen in the atmosphere are considered to have been first created by the activities of ancient cyanobacteria. They are often found as symbionts with a number of other groups of organisms such as fungi (lichens), corals, pteridophytes ("Azolla"), angiosperms ("Gunnera"), etc. There are some groups capable of heterotrophic growth, while others are parasitic, causing diseases in invertebrates or algae (e.g., the black band disease).
Cyanobacteria | Metabolism
Oxygenic photosynthesis uses two multi-subunit photosystems (I and II) located in the cell membranes of cyanobacteria and in the thylakoid membranes of chloroplasts in plants and algae. Photosystem II (PSII) has a P680 reaction centre containing chlorophyll 'a' that uses light energy to carry out the oxidation (splitting) of water molecules, and to produce ATP via a proton pump. Photosystem I (PSI) has a P700 reaction centre containing chlorophyll that takes the electron and associated hydrogen donated from PSII to reduce NADP+ to NADPH. Both ATP and NADPH are subsequently used in the light-independent reactions to convert carbon dioxide to glucose using the hydrogen atom extracted from water by PSII, releasing oxygen as a by-product.
Ycf9 protein domain | Photosynthesis
Dissolved oxygen and dissolved carbon dioxide are often discussed together due their coupled role in respiration and photosynthesis. Dissolved oxygen concentrations can be altered by physical, chemical, and biological processes and reaction. Physical processes including wind mixing can increase dissolved oxygen concentrations, particularly in surface waters of aquatic ecosystems. Because dissolved oxygen solubility is linked to water temperatures, changes in temperature affect dissolved oxygen concentrations as warmer water has a lower capacity to "hold" oxygen as colder water. Biologically, both photosynthesis and aerobic respiration affect dissolved oxygen concentrations. Photosynthesis by autotrophic organisms, such as phytoplankton and aquatic algae, increases dissolved oxygen concentrations while simultaneously reducing carbon dioxide concentrations, since carbon dioxide is taken up during photosynthesis.
Limnology | Oxygen and carbon dioxide
The different components may be assembled in diverse ways, such as supramolecular complexes, compartmentalized cells, or linearly, covalently linked molecules. Research into finding catalysts that can convert water, carbon dioxide, and sunlight to carbohydrates or hydrogen is a current, active field. By studying the natural oxygen-evolving complex (OEC), researchers have developed catalysts such as the "blue dimer" to mimic its function . Photoelectrochemical cells that reduce carbon dioxide into carbon monoxide (CO), formic acid (HCOOH) and methanol (CHOH) are under development. However, these catalysts are still very inefficient.
Artificial photosynthesis | Current research
The evolution of photosynthesis refers to the origin and subsequent evolution of photosynthesis, the process by which light energy synthesizes sugars from carbon dioxide and water, 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: C photosynthesis, C photosynthesis, and CAM photosynthesis. C photosynthesis is the oldest and most common form.
Evolution of photosynthesis | Introduction