Photosynthesis

Photosynthesis is a <a href="/facts/Biological_system/yKqVbaxu">system</a> of <a href="/facts/Biological_process/jpHldXz2">biological processes</a> by which <a href="/facts/Photoautotrophism/I1KGoVhw">photosynthetic organisms</a>, such as most plants, <a href="/facts/Algae/U1ftVKU3">algae</a>, and <a href="/facts/Cyanobacteria/S8bKxIU9">cyanobacteria</a>, convert <a href="/facts/Light_energy/8QHpm6KY">light energy</a>, typically from sunlight, into the <a href="/facts/Chemical_energy/Y3VP3Ifq">chemical energy</a> necessary to fuel their <a href="/facts/Metabolism/WxDrm8k5">metabolism</a>. 
Photosynthesis usually refers to oxygenic photosynthesis, a process that produces oxygen. Photosynthetic organisms store the chemical energy so produced within intracellular <a href="/facts/Organic_compound/gqCNaN45">organic compounds</a> (compounds containing carbon) like sugars, <a href="/facts/Glycogen/kWa8fPFE">glycogen</a>, <a href="/facts/Cellulose/wYPafTU0">cellulose</a> and <a href="/facts/Starch/gHAqGfJb">starches</a>. To use this stored chemical energy, an organism's cells metabolize the organic compounds through <a href="/facts/Cellular_respiration/kb6gpsvL">cellular respiration</a>. Photosynthesis plays a critical role in producing and maintaining the <a href="/facts/Atmospheric_oxygen/EbmISm33">oxygen content</a> of the Earth's atmosphere, and it supplies most of the <a href="/facts/Biological_energy/uQG6zfcK">biological energy</a> necessary for <a href="/facts/Complex_life/rk8QBF7o">complex life</a> on Earth.
Some <a href="/facts/Bacteria/wC8xSCsU">bacteria</a> also perform <a href="/facts/Anoxygenic_photosynthesis/vALSo4pU">anoxygenic photosynthesis</a>, which uses <a href="/facts/Bacteriochlorophyll/Dqu1Hua2">bacteriochlorophyll</a> to split <a href="/facts/Hydrogen_sulfide/UbCqjMMk">hydrogen sulfide</a> as a <a href="/facts/Reductant/CtfSGVXC">reductant</a> instead of water, producing <a href="/facts/Sulfur/IK0amTfM">sulfur</a> instead of oxygen. <a href="/facts/Archaea/Adxh0aHw">Archaea</a> such as <a href="/facts/Halobacterium/bQf51sw3">Halobacterium</a> also perform a type of non-<a href="/facts/Carbon_fixation/JqvfmCdH">carbon-fixing</a> anoxygenic photosynthesis, where the simpler <a href="/facts/Photopigment/GcoRGDda">photopigment</a> <a href="/facts/Retinal/h20jN8FC">retinal</a> and its <a href="/facts/Microbial_rhodopsin/c1jDXUUH">microbial rhodopsin</a> <a href="/facts/Derivative_(chemistry)/JYcbLkbm">derivatives</a> are used to absorb green light and power <a href="/facts/Proton_pump/mMzXv8P3">proton pumps</a> to directly synthesize <a href="/facts/Adenosine_triphosphate/nB9bwgcU">adenosine triphosphate</a> (ATP), the "energy currency" of cells. Such archaeal photosynthesis might have been the earliest form of photosynthesis that evolved on Earth, as far back as the <a href="/facts/Paleoarchean/5cldXjwo">Paleoarchean</a>, preceding that of <a href="/facts/Cyanobacteria/S8bKxIU9">cyanobacteria</a> (see <a href="/facts/Purple_Earth_hypothesis/SfE7fFm9">Purple Earth hypothesis</a>).
While the details may differ between <a href="/facts/Species/oDg6b96r">species</a>, the process always begins when light energy is absorbed by the <a href="/facts/Reaction_center/SNdpK0Dp">reaction centers</a>, proteins that contain <a href="/facts/Photosynthetic_pigment/BHJeRbId">photosynthetic pigments</a> or <a href="/facts/Chromophore/9RykVkek">chromophores</a>. In plants, these pigments are <a href="/facts/Chlorophyll/1Sb0NyyM">chlorophylls</a> (a <a href="/facts/Porphyrin/lKNtdXp1">porphyrin</a> derivative that absorbs the red and blue <a href="/facts/Optical_spectrum/frPBh8o5">spectrums</a> of light, thus reflecting green) held inside <a href="/facts/Chloroplast/fo8U3yTx">chloroplasts</a>, abundant in <a href="/facts/Leaf/1m6wCNTg">leaf</a> cells. In bacteria, they are embedded in the <a href="/facts/Plasma_membrane/vAXSD0DT">plasma membrane</a>. In these light-dependent reactions, some energy is used to strip <a href="/facts/Electron/L0KBo5cW">electrons</a> from suitable substances, such as water, producing oxygen gas. The <a href="/facts/Hydrogen/BoYHjl0J">hydrogen</a> freed by the splitting of water is used in the creation of two important molecules that participate in energetic processes: reduced <a href="/facts/Nicotinamide_adenine_dinucleotide_phosphate/XMgmPQ5t">nicotinamide adenine dinucleotide phosphate</a> (NADPH) and ATP.
In plants, <a href="/facts/Algae/U1ftVKU3">algae</a>, and <a href="/facts/Cyanobacteria/S8bKxIU9">cyanobacteria</a>, sugars are synthesized by a subsequent sequence of light-independent reactions called the <a href="/facts/Calvin_cycle/M1bIgQb9">Calvin cycle</a>. In this process, atmospheric carbon dioxide is incorporated into already existing organic compounds, such as <a href="/facts/Ribulose_bisphosphate/1DwB2uHE">ribulose bisphosphate</a> (RuBP). Using the ATP and NADPH produced by the light-dependent reactions, the resulting compounds are then <a href="/facts/Biological_reductant/Pyd5RVcj">reduced</a> and removed to form further carbohydrates, such as <a href="/facts/Glucose/oUSkoLCD">glucose</a>. In other bacteria, different mechanisms like the <a href="/facts/Reverse_Krebs_cycle/AYbVnH3c">reverse Krebs cycle</a> are used to achieve the same end.
The first photosynthetic organisms probably <a href="/facts/Evolved/TuWap5IK">evolved</a> early in the <a href="/facts/Evolutionary_history_of_life/Ss6eB0EX">evolutionary history of life</a> using <a href="/facts/Reducing_agent/CtfSGVXC">reducing agents</a> such as hydrogen or hydrogen sulfide, rather than water, as sources of electrons. Cyanobacteria appeared later; the <a href="/facts/Oxygen_cycle/btlq2mXr">excess oxygen</a> they produced contributed directly to the <a href="/facts/Oxygenation_of_the_Earth/LmTyM8jT">oxygenation of the Earth</a>, which rendered the evolution of complex life possible. The average rate of energy captured by global photosynthesis is approximately 130 <a href="/facts/Terawatts/VK8oqRrm">terawatts</a>, which is about eight times the total <a href="/facts/Power_consumption_of_human_civilization/Yujj9NrF">power consumption of human civilization</a>. Photosynthetic organisms also convert around 100–115 billion <a href="/facts/Ton/HDTb0fQo">tons</a> (91–104 Pg <a href="/facts/Orders_of_magnitude_(mass)/2jIXpAFv">petagrams</a>, or billions of metric tons), of carbon into <a href="/facts/Biomass_(ecology)/A5rDsUax">biomass</a> per year. Photosynthesis was discovered in 1779 by <a href="/facts/Jan_Ingenhousz/L0oQdN70">Jan Ingenhousz</a> who showed that plants need light, not just soil and water.

Photosynthesis open-in-new

Photosynthesis