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. obstacles in the form of destructive interference cause the particle to lose its wave properties for an instant before it regains them once again after it 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. evolution life timeline−4500 —–—–−4000 —–—–−3500 —–—–−3000 —–—–−2500 —–—–−2000 —–—–−1500 —–—–−1000 —–—–−500 —–—–0 — water single-celled life photosynthesis eukaryotes multicellular life plants arthropods molluscsflowersdinosaurs mammalsbirdsprimateshadeanarcheanproterozoicphanerozoic ←earth formed←earliest water←luca←earliest fossils←lhb meteorites←earliest oxygen←pongola glaciation*←atmospheric oxygen←huronian glaciation*←sexual reproduction←earliest multicellular life←earliest fungi←earliest plants←earliest animals←cryogenian ice age*←ediacaran biota←cambrian explosion←andean glaciation*←earliest tetrapods←karoo ice age*←earliest apes / humans←quaternary ice age*(million years ago)*ice ages 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 than water as electron donors. 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 electron donors. 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. fossils of what are thought to be filamentous photosynthetic organisms have been dated at 3.4 billion years old. more recent studies also suggest that photosynthesis may have begun about 3.4 billion years ago. the main source of oxygen in the earth's atmosphere derives from oxygenic photosynthesis, and its 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, using water as an electron donor, which is oxidized to molecular oxygen in the photosynthetic reaction center. symbiosis and the origin of chloroplasts 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. 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. 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. 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. 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. 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. photosynthetic eukaryotic lineages symbiotic and kleptoplastic organisms excluded: the glaucophytes and the red and green algae—clade archaeplastida (uni- and multicellular) the cryptophytes—clade cryptista (unicellular) the haptophytes—clade haptista (unicellular) the dinoflagellates and chromerids in the superphylum myzozoa, and pseudoblepharisma in the phylum ciliophora—clade alveolata (unicellular) the ochrophytes—clade heterokonta (uni- and multicellular) the chlorarachniophytes and 3 species of paulinella in the phylum cercozoa—clade rhizaria (unicellular) the euglenids—clade excavata (unicellular) except for the euglenids, which are found within the excavata, all of these belong to the diaphoretickes. archaeplastida and the photosynthetic paulinella got their plastids, which are surrounded by two membranes, through primary endosymbiosis in two separate events, by engulfing a cyanobacterium. the plastids in all the other groups have either a red or green algal origin, and are referred to as the "red lineages" and the "green lineages". the only known exception is the ciliate pseudoblepharisma tenue, which in addition to its plastids that originated from green algae also has a purple sulfur bacterium as symbiont. in dinoflagellates and euglenids the plastids are surrounded by three membranes, and in the remaining lines by four. a nucleomorph, remnants of the original algal nucleus located between the inner and outer membranes of the plastid, is present in the cryptophytes (from a red alga) and chlorarachniophytes (from a green alga). some dinoflagellates that lost their photosynthetic ability later regained it again through new endosymbiotic events with different algae. while able to perform photosynthesis, many of these eukaryotic groups are mixotrophs and practice heterotrophy to various degrees. 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), 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. 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 cyanobacteria as the major primary producers of oxygen on continental shelves near the end of the proterozoic, but 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. experimental history discovery although some of the steps in photosynthesis are still not completely understood, the overall photosynthetic equation has been known since the 19th century. portrait of jan baptist van helmont by mary beale, c. 1674 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 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 1779, 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. refinements 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 atoms as electrons and protons 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 pigments. 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 is equal in both psii and psi, assuring that input energy from the antenna complex is divided between the psi and psii systems, which in turn powers the photochemistry. robert hill thought that a complex of reactions consisted of an intermediate to cytochrome b6 (now a plastoquinone), and that another was from cytochrome f to a step in the carbohydrate-generating mechanisms. these are linked by plastoquinone, which does require energy to reduce cytochrome f. 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. in the hill reaction: 2 h2o + 2 a + (light, chloroplasts) → 2 ah2 + o2 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 works in his photosynthesis laboratory. 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, but many scientists refer to it as the calvin-benson, benson-calvin, or even calvin-benson-bassham (or cbb) cycle. nobel prize–winning scientist rudolph a. marcus was later 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 co2, activated by the respiration. 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. in 1954, daniel i. arnon et al. discovered photophosphorylation in vitro in isolated chloroplasts with the help of p32. louis n. m. duysens and jan amesz discovered that chlorophyll "a" will absorb one light, oxidize cytochrome f, while 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