What is Light-dependent reaction?
{{See also|Calvin cycle}} In [[photosynthesis]], a '''light-dependent reaction''' uses light energy from the [[sun]] to [[photolysis|split]] water which has been taken in by the [[organism]]. Water, when split, gives [[oxygen]], [[hydrogen]], and [[electron]]s. These electrons move through structures in [[chloroplasts]] and, by ''chemiosmosis'', make [[ATP]]. The hydrogen is converted to [[NADPH]], which is then used in the [[light-independent reaction]]s. Oxygen gas [[diffusion|diffuses]] out of the organism as a waste product. This all happens in the ''grana thylakoid'' of [[chloroplast]]s. [[File:Thylakoid membrane.png|thumb|center|500px|Light-dependent reaction of photosynthesis at the thylakoid membrane]] == The movement of electrons == #Light hits the chloroplast. It absorbs light and traps it. #[[Chlorophyll]] channels the light down to a reaction center. #An electron at the reaction center is excited to a higher energy level, and is received by an electron acceptor. This electron is taken from the splitting of water: ({{chem|H|2|O}} → ½{{chem|O|2}} + 2H+ + 2e-) #The electron is passed along a series of electron carriers. It is moving down energy levels and losing energy. This energy causes the pumping of hydrogen from the [[cytoplasm]] of chlorophyll into [[thylakoid spaces]] inside the grana. The hydrogen diffuses and flows back into the cytoplasm through [[protein channel]]s. As the hydrogen diffuses down a concentration gradient, ATP is made from [[ADP]] and inorganic phosphate. #Eventually, the electron is used to [[redox|reduce]] NADP to NADPH along with hydrogen from photolysis. ==History== [[Colin Flannery]] first put forward the idea that photosynthesis needs light in 1779.<ref>{{cite book|first=J|last=Ingenhousz|title=Experiments Upon Vegetables|publisher=Elmsly and Payne|location=London|year=1779|url=https://books.google.com/books?id=7II5AAAAcAAJ&pg=PR1}}</ref> He realised that sunlight falling on plants was needed. [[Joseph Priestley]] had noted the release of [[oxygen]] gas in 1772, but h