BIOLOGY 111 1st Edition Lecture 13 Outline of Last Lecture I Photosynthesis II Photosynthesis in Plants III Light Reactions and Calvin Cycle IV Nature of Sunlight V Photosystems Outline of Current Lecture I Light Energy Reactions II Calvin Cycle III Photorespiration IV C4 Plants V CAM plants Current Lecture Light Energy Reactions Z scheme linear electron flow A photon of light enters the chlorophyll and strikes a pigment molecule in a lightharvesting complex of Photosystem 2 called p680 As the electron in an excited state comes back into ground state a nearby pigment molecule is stimulated to an excited state continuing this process until it reaches the p680 pair of electrons inside the photosystem 2 reaction center complex As this pair of electrons get excited they are transferred to the primary electron acceptor A water molecule gets split releasing two H and one oxygen atom which immediately combines with another oxygen molecule to form O2 Each photoexcited electron passes from Photosystem 2 to Photosystem 1 by an electron transport chain Plastoquinone Pq is a transport electron carrier that delivers the electron from These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute Photosystem 2 to the cytochrome complex In the cytochrome complex H are pumped into the thylakoid lumen contributing to the proton gradient From the cytochrome complex plastocyanin Pc transports and delivers the electron to Photosystem 1 Photosystem 1 p700 now containing the photoexcited electron passes this electron through a series of redox reactions down a second electron transport chain In this second transport chain is a protein called Ferredoxin Fd which transports the electron without creating a proton gradient nor ATP to NADP reductase NADP reductase is responsible for catalyzing the transfer of electrons from Fd to NADP which allows the reduction to NADPH and is now ready for reactions in the Calvin cycle Calvin Cycle dark reactions This reaction occurs in the stroma uses ATP and NADPH available in the stroma created from light reactions synthesizes a sugar molecule by reducing CO2 first time to be used in this reaction For every three molecules of CO2 a 3 carbon sugar molecule called G3P is created C3 plant most of the plants we know Calvin cycle steps very expensive process requires 9 ATP 6 NADPH and 3 CO2 to create ONLY 1 G3P o carbon fixation each CO2 molecule that enters is attached to a 5 carbon sugar RuBP which is catalyzed by rubisco the product of the reaction is a six carbon intermediate that is so unstable its splits in half forming two 3 carbon molecules that move on to reduction step o reduction each of the 3 carbon molecules created in the carbon fixation step receives an additional phosphate group from ATP NADPH donates electrons to reduces the 3 carbon molecule by losing a phosphate group This creates G3P for every 3 molecules of CO 2 6 molecules of G3P are formed but only one molecule can be counted as a net gain of carbohydrate We started with 15 carbons worth and ended with 18 carbons worth of carbohydrate in the form of 6 G3P molecules One molecule exits the cycle to be used by the plant cell The other five molecules get recycled to regenerate the three molecules of RuBP o regeneration of CO2 acceptor RuBP the carbon skeletons of five molecules of G3P are rearranged into three molecules of RuBP this requires the cycle to spend three more molecules of ATP RuBP is now ready to receive CO2 again to continue the cycle Photorespiration Occurs in the light photo and consumes O2 while producing CO2 respiration Does NOT produce ATP unlike normal cellular respiration it actually consumes ATP Does NOT produce sugar unlike photosynthesis it actually decreases photosynthetic output by stealing organic material from Calvin cycle and releasing CO2 caused by hot conditions plants run short of CO2 because worried about water conservation RuBP is very specific but not very because of evolution so it still binds with O2 o gives 3 carbon molecule a lot of 2 carbons molecules o use a lot of ATP costly for essentially nothing at all o gives off a lot of CO2 as a by product this helps the plant protect itself BUT it also makes the stored oxygen very reactive to become free radical dangerous so it gets fixed as to not harm the plant constant exposure to heat caused evolution for C4 photosynthesis and CAM C4 Plant Photosynthesis in C3 plants the first organic product of carbon fixation is a 3 carbon compound in C4 plants the product of carbon fixation from the Calvin cycle is a four carbon compound incorporates an extra set of cells o in addition to mesophyll cells there are bundle sheath cells adaption o bundle sheath cells take in the four carbon compound and then release CO2 it is also where ATP is generated via cyclic electron flow in photosystem 1 When it is hot and dry stromata are partially closed causing CO2 concentration in the leaf to decrease and O2 concentration to increase Steps PEP carboxylase inside the mesophyll cells adds CO2 to PEP forming a four carbon compound PEP carboxylase is an adaption for this type of plant The mesophyll cells export the four carbon compound to bundle sheath cells via plasmodesmata In the bundle sheath cells the four carbon compound releases CO2 which is reassimilated into organic molecule by rubisco catalyst to be used in the Calvin cycle To generate extra ATP bundle sheath cells carry out cyclic electron flow which contain ONLY photosystem 1 and NOT photosystem 2 CAM Plants adaption works between day and night o Day closing the stromata during the day helps the plant conserve water and it also prevents the CO2 from entering the leaves This is when plants use ATP and NADPH from the light reaction for the Calvin Cycle to work on the organic acids made at night o Night when the stromata are open at night the plant takes in CO2 fixes them into organic acids The mesophyll cells store these organic acids in their vacuoles until the morning when the plants can do work on it The difference between CAM plants and C4 plants is in the initial steps of carbon fixation which are separated structurally in C4 plants from the Calvin cycle and in CAM plants the two steps occur at separate times but within the same cell