PhotosynthesisPhotosynthesis Lecture OutlineThe overall chemical reaction of photosynthesis isTwo linked processes of photosynthesisThe Structure of the ChloroplastPhotosynthesis is dividedHow Does Chlorophyll Capture Light Energy?Absorption Spectrum and PigmentsThe structure of ChlorophyllPhotosystemsWhen light is absorbed, electrons are excitedThe Three Fates of Excited Electrons in Photosynthetic PigmentsWhat happens in the reaction center?PowerPoint PresentationElectron transport chainSlide 16Slide 17Slide 18Slide 19The ETC in PS II Pump ProtonsPhotophosphorylationPhotosystem I Produces NADPHThe Z Scheme Model Links Photosystem I and IICyclic phosphorylationLight Independent Reactions (the Calvin Cycle)Calvin Cycle Fixes CarbonThe Three Phases of the Calvin CyclePhotorespirationPhotorespiration and Its ConsequencesSlide 30Slide 31Photosynthesis Lecture OutlineOverviewChloroplastsPigment moleculesThe light reactions Calvin cycle and carbon fixationC4 and CAM metabolism•CO2 + 2 H2O + light energy (CH2O)n + H2O + 2S•Photosynthesis is made by two linked set of reactions:–Light dependent reactions•It produces oxygen, ATP and NADPH–Light independent reactions•It produces sugar from carbon dioxide.•The two set of reactions are linked by electrons.The overall chemical reaction of photosynthesis isTwo linked processes of photosynthesis•During the Calvin Cycle, the electrons from NADPH and the potential energy of ATP is used to reduce CO2 to carbohydrate.•The resulting sugar is used in cellular respiration.•Plants oxidize sugars in their mitochondriaThe Structure of the Chloroplast•Thylakoids are vesicle like structures.•Often the thylakoid are interconnected in stacks called grana (singular granum).•Lumen is the internal space in the thylakoid.•Stroma is the fluid-filled space between the thylakoids and the inner membrane.•Pigments are molecules capable of absorbing certain wavelengths of light.Photosynthesis is divided•Light dependent reactionsThylakoid•Light independent reactionsStromaHow Does Chlorophyll Capture Light Energy?•Light is a type of electromagnetic radiation (energy).•Photosynthesis converts this energy into the C-C and C-H bonds of sugar.Absorption Spectrum and Pigments•Shorter wavelengths of electromagnetic radiation have more energy.The structure of Chlorophyll•Both chlorophyll a & b have the same structures:–A long tail of isoprene subunits–A head which is a large structure with magnesium atom in is center.–The tail keeps the molecule embedded in the thylakoid membrane– The head is where the light is absorbed.Photosystems•In the thylakoid membrane between 200-300 chlorophyll molecules and accessory pigments are grouped together in an array of proteins.–This complex is known as a photosystem.–Photosystems have two major elements:•An antenna complex•The reaction centerWhen light is absorbed, electrons are excitedThe Three Fates of Excited Electrons in Photosynthetic PigmentsWhat happens in the reaction center?•When the energy of the reaction reaches the reaction center of a photosystem, excited electrons are transferred to a molecule that acts as an electron acceptor.–When this molecule becomes reduced.•absorption of light becomes permanent.–At this point we can say that electromagnetic energy has become chemical energy•Pheophytin is similar to chlorophyll molecules but they lack the central magnesium atom.–This molecule acts as an electron acceptor.–When the electrons are excited in the reaction center, pheophytin gets reduced and the reaction center is oxidized.Electron transport chain•Once pheophytin is reduced, the electron is transferred to an electron transfer chain.•This ETC is similar in structure and function to the ECT studied in cellular respiration.H+H+H+H+H+e-e-Antenna complexReactioncenterElectronTransportChainElectronCarrierMoleculePlastocyaninATP SynthasePhotosystem IPhotosystem IIPhotosystems in the thylakoid membraneLumenStromaH+H+H+H+H+e-e-H+H+H+H+H+H+H+H+H+H+H+H+H+H+H+ADP+PiATPADP+PiADP+PiATPATPe-e-e-e-H+H+H+H+OO++H2OH2Oe-e-SunLightLumenStromaPhotosystem IIH+H+H+H+H+e-e-e-e-00e-e-e-e-NADP+NADPHSunLightLumenStromaPhotosystem IH+H+H+H+H+e-e-e-e-00e-e-e-e-NADP+NADPHATPATPATPATPATPATPATPSunLightOOLumenStromaPhotosystem IIPhotosystem IThe ETC in PS II Pump ProtonsPhotophosphorylation•Production of ATP molecules using the energy released as light-excited electrons flow through an electron transport chain during photosynthesis. Involves the generation of a proton-motive force during electron transport and is used to drive ATP synthesis.Photosystem I Produces NADPHThe Z Scheme Model Links Photosystem I and IIPlastocyaninConnects bothsystems PlastoquinineCyclic phosphorylation•Instead of Photosystem I transfers electrons back to the electron transport chain in photosynthesis II–This will increase ATP generation •Cyclic phosphorylation coexists with the Z scheme and produces additional ATP.Light Independent Reactions (the Calvin Cycle)Calvin Cycle Fixes Carbon•RuBP- ribulose bisphosphate (RuBP)•Glyceraldehyde-3-phosphate (G3P)The Three Phases of the Calvin Cycle•Fixation- CO2 reacts with RuBP.–This fixes carbon into a more complex molecule.–Carbon fixation is the addition of a carbon dioxide to an organic compound.•Reduction- 3-phosphoglycerate is phosphorylated by ATP and then reduced by an electron from NADP, producing gluceraldehyde-3-phosphate (G3P).Photorespiration•Rubisco is a carboxylase. This means that it adds CO2 to RuBP. However, It can also be an oxygenase, adding O2 to RuBP. –These two reactions compete with each other. –When RuBP reacts with O2, it cannot react with CO2, which reduces the rate of CO2 fixation.Photorespiration and Its Consequences•Rubisco acts as an oxygenase if;– the CO2 levels are very low and– the O2 levels are very high. •O2 levels become very high when stomata are closed to prevent water loss (when the weather is hot and dry).In some plants, CO2 can be fixed by rubisco or by pep carboxylate.They are located in different cell