PHOTOSYNTHESIS Photosynthesis is essential to most life forms on earth Plants split water and generate O2 They fix CO2 Photosynthesis is a function of Chloroplasts Chloroplasts have an outer and an inner membrane They are also filled with thylakoids Stacks of thylakoids are called grana The space in between the thylakoids is filled with the stroma Photosynthesis does not occur in animals Chloroplasts have two envelopes membranes The inner membrane is filled with thylakoids The fluid between the thylakoids is the stroma Grana are stacks of thylakoids Photosynthesis occurs in two stages Light energy enters the system and a reduction of CO2 with electrons coming from H2O occurs H2O does not easily give up its electrons This stage is called the light reactions which release O2 NADPH and ATP The next stage is called the carbon assimilation reactions which use the NADPH and ATP from the light reactions releasing NADP ADP and Pi They also use CO2 to produce carbohydrates There is an electrochemical gradient across the thylakoid membrane Carbon assimilation reactions only happen during periods of light At night plants use respiration for energy O2 Evolution can be dissociated from CO2 reduction DCPIP is an artificial electron acceptor which acts in the Hill reaction to accept electrons instead of NADP NADP is the biological electron acceptor Electrons come from H2O Light Energy is captured to drive Photosynthesis 1 Einstein is the energy of 1 mole of photons Photons have a quantized amount of energy Energy depends on the color of the photon Absorption Spectra or Why Are Plants Green Different pigments absorb photons of different energy Chlorophyll a absorbs in the 400 and 680 regions Chlorophyll b absorbs mostly in the 470 region but also slightly in the 650 region This means they do not absorb green light which is why plants are green Photosynthetic Pigments Chlorophyll a is a photosynthetic pigment as well as phycoerythrobilin Chlorophyll a is slightly different in bacteria Also the changing of a methyl group to an aldehyde changes chlorophyll a to chlorophyll b Phycoerythrobilin is also slightly different in phycocyanobilin Accessory Pigments protect and increase the range of light which can be used Some accessory pigments include carotene and lutein xanthophyll The Action Spectrum of Photosynthesis Oxygen production is a function of light quality Pigments are Bound to Proteins Lutein chlorophyll a and chlorophyll b are bound to the Light Harvesting Complex II LHCII of plants There are more chlorophyll as than bs A simple Light Harvesting Antenna Light is absorbed and electrons are excited The energy from this excitation is transferred to different portions of the antenna in exciton transfer The energy finally reaches the chlorophyll a reaction center Chlorophyll funnels the absorbed Energy to Photochemical Reaction Centers by Exciton Transfer Light energy is converted to chemical energy Antenna chlorophylls are bound to protein along with carotenoids and other accessory pigments These molecules absorb light energy transferring it between molecules until it reaches the reaction center A photochemical reaction at the reaction center converts the energy of a photon into a separation of charge initiating electron flow Exciton and Electron Transfer within a Photosystem Light excites an antenna molecule chlorophyll or accessory pigment raising an electron to a higher energy level The excited antenna molecule passes energy to a neighboring chlorophyll molecule resonance energy transfer exciting it This energy is transferred to a reaction center chlorophyll exciting it The excited reaction center chlorophyll passes an electron to an electron acceptor The electron hole in the reaction center is filled by an electron from an electron donor The absorption of a photon has caused separation of charge in the reaction center When the excited reaction center chlorophyll passes an electron to an electron acceptor the reaction center chlorophyll will be temporarily bleached Exciton transfer to the Reaction Center causes a change in Reduction Potential In purple bacteria excitons are absorbed the reaction center P870 which excites them lowering their reduction potential from 0 5 to 1 0 The electrons are then transferred to pheophytin Q and the cytochrome bc1 complex At this point the reduction potential is 0 The cytochrome bc1 complex contributes to the proton gradient and cytochrome c2 donates electrons back to the reaction center The Photoreaction center of Rhodopseudomonas viridis The reaction center contains hemes chlorophylls pheophytins and quinones There are three subunits H L and M Cytochrome c sticks out on the P side This photoreaction center is similar to PSII in plants The Reaction Center Chemistry occurs in the Solid State Excitation Energy could be lost as heat molecular motion or fluorescence However fixation of prosthetic groups in space solid state chemistry and kinetic factors prevent this inefficiency This allows for electron transfers to occur rapidly It also prevents electrons from falling back the wrong way to the ground state Molecules are in their proper orientation close together Releasing energy as heat or fluorescence is inefficient The electron transfer reactions are not only fast but thermodynamically downhill Two types of bacterial photosynthetic machineries Anoxygenic photosynthetic is one type of photosynthetic machinery In green sulfur bacteria excitons are transferred to the reaction center P840 exciting it These excited electrons are either passed to ferredoxin or Q If passed to ferredoxin they then move to ferredoxin NAD reductase making NADH from NAD If passed to Q they then move to the cytochrome bc1 complex where they contribute to the proton gradient Cytochrome c553 then donates these electrons back to the reaction center Electrons in the reaction center are replaced from H2S Green sulfur bacteria are a Fe S type Green sulfur bacteria live in the ocean in anaerobic conditions They split H2S instead of H2O They also cause the formation of elemental sulfur Electrons can go from P840 to either Fd NAD reductase to make NADH or the cytochrome bc1 complex Fd NAD reductase doesn t generate an electrochemical gradient and uses ferredoxin The electrons go to the bc1 complex through Q creating a proton gradient that can be used for ATP synthesis The electrons go to cytochrome c553 and back to the reaction center 2 electrons make 1 NADH Oxygen can be evolved only if there