BISC110: Series 3 Experiment 9 Hill Reaction: Difference between revisions

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Central to life on Earth is the photochemical process carried out by plants and cyanobacteria, wherein quanta of light are converted into chemical energy.  In plants, light is absorbed by chlorophyll and other antenna pigments of the light-harvesting complexes of Photosystems I and II.  The energy from these photons is then passed via resonance energy transfer to a special pair of chlorophyll a molecules located in the reaction center, leading to the excitation and loss of electrons from these molecules.  In each photosystem the excited electron from one reaction center chlorophyll is passed to the quinone primary electron acceptor thereby reducing it.  The primary acceptor immediately donates its electrons to a neighboring molecule and so on through an electron transport chain to ultimately reduce NADP+ to NADPH.  The resulting oxidized reaction center of photosystem II is able to split water molecules into protons, electrons and O2.  The electrons extracted from water replace the electrons lost by the reaction center II chlorophyll.  The reaction center chlorophyll of photosystem I is reduced by electrons coming from photosystem II.  

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This electron transport is coupled in two steps to the formation of ATP through the mechanism of chemiosmosis.  First, during the light reactions, the transport of electrons is coupled to the movement of protons from the stroma to the thylakoid lumen, forming a pH gradient across the thylakoid membrane.  The sources of these protons are the splitting of water, which occurs on the lumenal side of the thylakoid membrane, and the transport of protons from the stromal side across the membrane into the lumen by the electron transport chain components plastoquinone and cytochrome b/f complex.  In the second step, this gradient of protons is released when the protons diffuse through the membrane-spanning ATP-synthase molecule, which couples proton movement to the synthesis of ATP from ADP and Pi.