JoMan2 wrote: ↑Mon Nov 22, 2021 12:17 pm
Communism (from Latin communis, 'common, universal')[1][2] is a philosophical, social, political, and economic ideology and movement whose goal is the establishment of a communist society, namely a socioeconomic order structured upon the ideas of common ownership of the means of production and the absence of social classes, money,[3][4] and the state.[5][6] Communism is a specific, yet distinct, form of socialism. Communists agree on the withering away of the state but disagree on the means to this end, reflecting a distinction between a more libertarian approach of communization, revolutionary spontaneity, and workers' self-management, and a more vanguardist or Communist party-driven approach through the development of a constitutional socialist state.[7]
Variants of communism have been developed throughout history, including anarcho-communism, Marxism, Leninism, Marxism–Leninism, Trotskyism, and Maoism. Communism includes a variety of schools of thought which broadly include Marxism and libertarian communism as well as the political ideologies grouped around both, all of which share the analysis that the current order of society stems from capitalism, its economic system and mode of production, namely that in this system there are two major social classes, the relationship between these two classes is exploitative, and that this situation can only ultimately be resolved through a social revolution.[8][nb 1] The two classes are the proletariat (the working class), who make up the majority of the population within society and must work to survive, and the bourgeoisie (the capitalist class), a small minority who derives profit from employing the working class through private ownership of the means of production. According to this analysis, revolution would put the working class in power and in turn establish social ownership of the means of production which is the primary element in the transformation of society towards a communist mode of production.[8]
In the 20th century, Communist governments espousing Marxism–Leninism and its variants came into power in parts of the world,[10] first in the Soviet Union with the Russian Revolution of 1917, and then in portions of Eastern Europe, Asia, and a few other regions after World War II.[11][nb 2] Along with social democracy, communism became the dominant political tendency within the international socialist movement by the 1920s.[17] Criticism of communism can be divided into two broad categories, namely that which concerns itself with the practical aspects of 20th century Communist states[18] and that which concerns itself with communist principles and theory.[19] Several academics and economists, among other scholars,[20][21] posit that the Soviet model under which these nominally Communist states in practice operated was not an actual communist economic model in accordance with most accepted definitions of communism as an economic theory but in fact a form of state capitalism,[22][23][24] or non-planned administrative-command system.
Chlorophyll was first isolated and named by Joseph Bienaimé Caventou and Pierre Joseph Pelletier in 1817.[6] The presence of magnesium in chlorophyll was discovered in 1906,[7] and was that element's first detection in living tissue.[8]
After initial work done by German chemist Richard Willstätter spanning from 1905 to 1915, the general structure of chlorophyll a was elucidated by Hans Fischer in 1940. By 1960, when most of the stereochemistry of chlorophyll a was known, Robert Burns Woodward published a total synthesis of the molecule.[8][9] In 1967, the last remaining stereochemical elucidation was completed by Ian Fleming,[10] and in 1990 Woodward and co-authors published an updated synthesis.[11] Chlorophyll f was announced to be present in cyanobacteria and other oxygenic microorganisms that form stromatolites in 2010;[12][13] a molecular formula of C55H70O6N4Mg and a structure of (2-formyl)-chlorophyll a were deduced based on NMR, optical and mass spectra.[14]
Photosynthesis
Absorbance spectra of free chlorophyll a (blue) and b (red) in a solvent. The spectra of chlorophyll molecules are slightly modified in vivo depending on specific pigment-protein interactions.
Chlorophyll A
Chlorophyll B
Chlorophyll is vital for photosynthesis, which allows plants to absorb energy from light.[15]
Chlorophyll molecules are arranged in and around photosystems that are embedded in the thylakoid membranes of chloroplasts.[16] In these complexes, chlorophyll serves three functions. The function of the vast majority of chlorophyll (up to several hundred molecules per photosystem) is to absorb light. Having done so, these same centers execute their second function: the transfer of that light energy by resonance energy transfer to a specific chlorophyll pair in the reaction center of the photosystems. This pair effects the final function of chlorophylls, charge separation, leading to biosynthesis. The two currently accepted photosystem units are photosystem II and photosystem I, which have their own distinct reaction centres, named P680 and P700, respectively. These centres are named after the wavelength (in nanometers) of their red-peak absorption maximum. The identity, function and spectral properties of the types of chlorophyll in each photosystem are distinct and determined by each other and the protein structure surrounding them. Once extracted from the protein into a solvent (such as acetone or methanol),[17][18][19] these chlorophyll pigments can be separated into chlorophyll a and chlorophyll b.
The function of the reaction center of chlorophyll is to absorb light energy and transfer it to other parts of the photosystem. The absorbed energy of the photon is transferred to an electron in a process called charge separation. The removal of the electron from the chlorophyll is an oxidation reaction. The chlorophyll donates the high energy electron to a series of molecular intermediates called an electron transport chain. The charged reaction center of chlorophyll (P680+) is then reduced back to its ground state by accepting an electron stripped from water. The electron that reduces P680+ ultimately comes from the oxidation of water into O2 and H+ through several intermediates. This reaction is how photosynthetic organisms such as plants produce O2 gas, and is the source for practically all the O2 in Earth's atmosphere. Photosystem I typically works in series with Photosystem II; thus the P700+ of Photosystem I is usually reduced as it accepts the electron, via many intermediates in the thylakoid membrane, by electrons coming, ultimately, from Photosystem II. Electron transfer reactions in the thylakoid membranes are complex, however, and the source of electrons used to reduce P700+ can vary.
The electron flow produced by the reaction center chlorophyll pigments is used to pump H+ ions across the thylakoid membrane, setting up a chemiosmotic potential used mainly in the production of ATP (stored chemical energy) or to reduce NADP+ to NADPH. NADPH is a universal agent used to reduce CO2 into sugars as well as other biosynthetic reactions.
Reaction center chlorophyll–protein complexes are capable of directly absorbing light and performing charge separation events without the assistance of other chlorophyll pigments, but the probability of that happening under a given light intensity is small. Thus, the other chlorophylls in the photosystem and antenna pigment proteins all cooperatively absorb and funnel light energy to the reaction center. Besides chlorophyll a, there are other pigments, called accessory pigments, which occur in these pigment–protein antenna complexes.
Chemical structure
Space-filling model of the chlorophyll a molecule
Chlorophylls are numerous in types, but all are defined by the presence of a fifth ring beyond the four pyrrole-like rings. Most chlorophylls are classified as chlorins, which are reduced relatives of porphyrins (found in hemoglobin). They share a common biosynthetic pathway with porphyrins, including the precursor uroporphyrinogen III. Unlike hemes, which feature iron at the center of the porphyrin based tetrapyrrole ring, in chlorophylls central magnesium atom coordinates with chlorin, a partially reduced porphyrin. For the structures depicted in this article, some of the ligands attached to the Mg2+ center are omitted for clarity. The chlorin ring can have various side chains, usually including a long phytol chain. The most widely distributed form in terrestrial plants is chlorophyll a.
The structures of chlorophylls are summarized below:[20][14]
Chlorophyll a Chlorophyll b Chlorophyll c1 Chlorophyll c2 Clorophyll d Chlorophyll f[14]
Molecular formula C55H72O5N4Mg C55H70O6N4Mg C35H30O5N4Mg C35H28O5N4Mg C54H70O6N4Mg C55H70O6N4Mg
C2 group −CH3 −CH3 −CH3 −CH3 −CH3 −CHO
C3 group −CH=CH2 −CH=CH2 −CH=CH2 −CH=CH2 −CHO −CH=CH2
C7 group −CH3 −CHO −CH3 −CH3 −CH3 −CH3
C8 group −CH2CH3 −CH2CH3 −CH2CH3 −CH=CH2 −CH2CH3 −CH2CH3
C17 group −CH2CH2COO−Phytyl −CH2CH2COO−Phytyl −CH=CHCOOH −CH=CHCOOH −CH2CH2COO−Phytyl −CH2CH2COO−Phytyl
C17−C18 bond Single
(chlorin) Single
(chlorin) Double
(porphyrin) Double
(porphyrin) Single
(chlorin) Single
(chlorin)
Occurrence Universal Mostly plants Various algae Various algae Cyanobacteria Cyanobacteria
Structures of chlorophylls
chlorophyll a
chlorophyll b
chlorophyll c1
chlorophyll c2
chlorophyll d
chlorophyll f
Measurement of chlorophyll content
Chlorophyll forms deep green solutions in organic solvents.
Measurement of the absorption of light[how?] is complicated by the solvent used to extract the chlorophyll from plant material, which affects the values obtained,
In diethyl ether, chlorophyll a has approximate absorbance maxima of 430 nm and 662 nm, while chlorophyll b has approximate maxima of 453 nm and 642 nm.[21]
The absorption peaks of chlorophyll a are at 465 nm and 665 nm. Chlorophyll a fluoresces at 673 nm (maximum) and 726 nm. The peak molar absorption coefficient of chlorophyll a exceeds 105 M−1 cm−1, which is among the highest for small-molecule organic compounds.[22]
In 90% acetone-water, the peak absorption wavelengths of chlorophyll a are 430 nm and 664 nm; peaks for chlorophyll b are 460 nm and 647 nm; peaks for chlorophyll c1 are 442 nm and 630 nm; peaks for chlorophyll c2 are 444 nm and 630 nm; peaks for chlorophyll d are 401 nm, 455 nm and 696 nm.[23]
By measuring the absorption of light in the red and far red regions, it is possible to estimate the concentration of chlorophyll within a leaf.[24]
Ratio fluorescence emission can be used to measure chlorophyll content. By exciting chlorophyll a fluorescence at a lower wavelength, the ratio of chlorophyll fluorescence emission at 705±10 nm and 735±10 nm can provide a linear relationship of chlorophyll content when compared with chemical testing. The ratio F735/F700 provided a correlation value of r2 0.96 compared with chemical testing in the range from 41 mg m−2 up to 675 mg m−2. Gitelson also developed a formula for direct readout of chlorophyll content in mg m−2. The formula provided a reliable method of measuring chlorophyll content from 41 mg m−2 up to 675 mg m−2 with a correlation r2 value of 0.95.[25]
Biosynthesis
Main article: Chlorophyllide
In some plants, chlorophyll is derived from glutamate and is synthesised along a branched biosynthetic pathway that is shared with heme and siroheme.[26][27][28] Chlorophyll synthase[29] is the enzyme that completes the biosynthesis of chlorophyll a[30][31] by catalysing the reaction EC 2.5.1.62
chlorophyllide a + phytyl diphosphate {\displaystyle \rightleftharpoons }\rightleftharpoons chlorophyll a + diphosphate
This forms an ester of the carboxylic acid group in chlorophyllide a with the 20-carbon diterpene alcohol phytol. Chlorophyll b is made by the same enzyme acting on chlorophyllide b.
In Angiosperm plants, the later steps in the biosynthetic pathway are light-dependent and such plants are pale (etiolated) if grown in darkness. Non-vascular plants and green algae have an additional light-independent enzyme and grow green even in darkness.[citation needed]
Chlorophyll itself is bound to proteins and can transfer the absorbed energy in the required direction. Protochlorophyllide, one of the biosynthetic intermediates, occurs mostly in the free form and, under light conditions, acts as a photosensitizer, forming highly toxic free radicals. Hence, plants need an efficient mechanism of regulating the amount of this chlorophyll precursor. In angiosperms, this is done at the step of aminolevulinic acid (ALA), one of the intermediate compounds in the biosynthesis pathway. Plants that are fed by ALA accumulate high and toxic levels of protochlorophyllide; so do the mutants with a damaged regulatory system.[32]
Senescence and the chlorophyll cycle
The process of plant senescence involves the degradation of chlorophyll: for example the enzyme chlorophyllase (EC 3.1.1.14) hydrolyses the phytyl sidechain to reverse the reaction in which chlorophylls are biosynthesised from chlorophyllide a or b. Since chlorophyllide a can be converted to chlorophyllide b and the latter can be re-esterified to chlorophyll b, these processes allow cycling between chlorophylls a and b. Moreover, chlorophyll b can be directly reduced (via 71-hydroxychlorophyll a) back to chlorophyll a, completing the cycle.[33][34] In later stages of senescence, chlorophyllides are converted to a group of colourless tetrapyrroles known as nonfluorescent chlorophyll catabolites (NCC's) with the general structure:
hese compounds have also been identified in ripening fruits and they give characteristic autumn colours to deciduous plants.[34][35]
Defective environments can cause chlorosis
Further information: Chlorosis
Chlorosis is a condition in which leaves produce insufficient chlorophyll, turning them yellow. Chlorosis can be caused by a nutrient deficiency of iron — called iron chlorosis — or by a shortage of magnesium or nitrogen. Soil pH sometimes plays a role in nutrient-caused chlorosis; many plants are adapted to grow in soils with specific pH levels and their ability to absorb nutrients from the soil can be dependent on this.[36] Chlorosis can also be caused by pathogens including viruses, bacteria and fungal infections, or sap-sucking insects.[37]
Complementary light absorbance of anthocyanins
Superposition of spectra of chlorophyll a and b with oenin (malvidin 3O glucoside), a typical anthocyanidin, showing that, while chlorophylls absorb in the blue and yellow/red parts of the visible spectrum, oenin absorbs mainly in the green part of the spectrum, where chlorophylls don't absorb at all.
Anthocyanins are other plant pigments. The absorbance pattern responsible for the red color of anthocyanins may be complementary to that of green chlorophyll in photosynthetically active tissues such as young Quercus coccifera leaves. It may protect the leaves from attacks by plant eaters that may be attracted by green color.[38]
Distribution
The chlorophyll maps show milligrams of chlorophyll per cubic meter of seawater each month. Places where chlorophyll amounts were very low, indicating very low numbers of phytoplankton, are blue. Places where chlorophyll concentrations were high, meaning many phytoplankton were growing, are yellow. The observations come from the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA's Aqua satellite. Land is dark gray, and places where MODIS could not collect data because of sea ice, polar darkness, or clouds are light gray. The highest chlorophyll concentrations, where tiny surface-dwelling ocean plants are thriving, are in cold polar waters or in places where ocean currents bring cold water to the surface, such as around the equator and along the shores of continents. It is not the cold water itself that stimulates the phytoplankton. Instead, the cool temperatures are often a sign that the water has welled up to the surface from deeper in the ocean, carrying nutrients that have built up over time. In polar waters, nutrients accumulate in surface waters during the dark winter months when plants cannot grow. When sunlight returns in the spring and summer, the plants flourish in high concentrations.[39]
Culinary use
Synthetic chlorophyll is registered as a food additive colorant, and its E number is E140. Chefs use chlorophyll to color a variety of foods and beverages green, such as pasta and spirits. Absinthe gains its green color naturally from the chlorophyll introduced through the large variety of herbs used in its production.[40] Chlorophyll is not soluble in water, and it is first mixed with a small quantity of vegetable oil to obtain the desired solution.[citation needed]
Biological use
A 2002 study found that "leaves exposed to strong light contained degraded major antenna proteins, unlike those kept in the dark, which is consistent with studies on the illumination of isolated proteins". This appeared to the authors as support for the hypothesis that "active oxygen species play a role in vivo" in the short-term behaviour of plants.[41]
See also
Wikimedia Commons has media related to Chlorophyll.
Bacteriochlorophyll, related compounds in phototrophic bacteria
Chlorophyllin, a semi-synthetic derivative of chlorophyll
Deep chlorophyll maximum
Grow light, a lamp that promotes photosynthesis
Chlorophyll fluorescence, to measure plant stress
according to WIKIPEDIA
Communism (from Latin communis, 'common, universal')[1][2] is a philosophical, social, political, and economic ideology and movement whose goal is the establishment of a communist society, namely a socioeconomic order structured upon the ideas of common ownership of the means of production and the absence of social classes, money,[3][4] and the state.[5][6] Communism is a specific, yet distinct, form of socialism. Communists agree on the withering away of the state but disagree on the means to this end, reflecting a distinction between a more libertarian approach of communization, revolutionary spontaneity, and workers' self-management, and a more vanguardist or Communist party-driven approach through the development of a constitutional socialist state.[7]
