Chapter 10-OverviewPhotosynthesis: converts solar energy into chemical energy.Autotrophs: sustain themselves without eating other organismsPhotoautotrophs: Use sunlight for energyHeterotrophs: Obtain organic material from other organism.-Concept 10.1Chloroplasts are structurally similar to and likely evolved from photosynthetic bacteria.-ChloroplastsChlorophyll: Green pigment within chloroplastsMesophyll: Interior tissue of the leaf.Location of chloroplast30-40 ChloroplastsThylakoids: Connected sacs in the chloroplastsGrana: Stacked columns of ThylakoidsStomata: Microscopic pores where CO2 enters and O2 exits.Stroma: Dense interior fluid located in chloroplasts.-Photosynthesis as a redox processH2O is oxidized, CO2 is reducedEndergonic, +Delta G, Non-Spontaneous-Two Stages of PhotosynthesisLight Reaction (In the thylakoids)Split H2ORelease O2Reduce NADP+ to NADPHGenerate ATP from ADP by PhotophosphorylationCalvin Cycle (In the Stroma)Uses ATP and NADPHBegins with Carbon fixation, incorporating CO2 into organic molecules.-Concept 10.2Chloroplasts are solar-powered chemical factories.Their Thylakoids transform light energy into the chemical energy of ATP and NADPH-Nature of SunlightElectromagnetic Radiation: Light is a form of electromagnetic energy.Wavelength: The distance between crests of waves.Visible Light: Wavelengths that produce colors that we can see.Photons: Light also behaves as thought is consists of discrete particles.Spectrophotometer: Measures a pigment’s ability to absorb various wavelengths.Absorption Spectrum: Graph plotting a pigment vs. wavelength.Chlorophyll (a): Violet-blue and red light work best for photosynthesis, main photosynthetic pigment.Action Spectrum: Profiles the relative effectiveness of different wavelengths of radiation in driving a process.First demonstrated in 1883 by Theodor W.Exposed different segment of a filamentous alga to different wavelengthsAreas receiving wavelengths favorable to photosynthesis produced excess O2.Used growth of aerobic bacteria clustered along the alga as a measure of O2 production.Chlorophyll (b): Broaden the spectrum used for photosynthesis, accessory pigment.Carotenoids: Accessory pigments that absorb excessive light that would damage chlorophyll (photoprotection)-Excitation of Chlorophyll by lightWhen electrons go from unstable to a ground state, photons are given off causing fluorescence.-PhotosystemConsists of a reaction-center complex (protein couples) surrounded by light harvesting complexes (pigment molecules bound to proteins) transfer the energy of photons to the reaction center.Primary electron acceptor: In the reaction center, accepts excited electronsfrom chlorophyll (a) and is reduced as a result. Two types of photosystems in the thylakoid membranePhotosystem II (PS II): functions first, (#’s reflect order of discovery) and isbest at absorbing a wavelength of 680 nm.Reaction Center Chlorophyll is called P680Photosystem I (PS I): Best at absorbing wavelength of 700.Reaction Center Chlorophyll is called P700-Linear Electron Flow2 Possible routes; Cyclic and LinearLinear Electron Flow: Primary pathways, involves both photosystems and produces ATP and NADPH using light energy.A photo hits a pigment and its energy is passed among pigment molecules until it excites P680.An excited electron from P680 is transferred to the primary electron acceptor (We call it P680+)P680+ is a strong oxidizing agentEnzymes split H2O, and the electrons are transferred from the H atoms to P680+, thus reducing it to P680Each electron “falls” down electron transport chain from the primary electron acceptor of PS II to PS IEnergy released by the fall drives the creation of a proton gradient across the thylakoid membrane.Diffusion of H+ (Protons) across the membrane (from thylakoid to Stroma)drives ATP synthesisIn PS I (like PS II) transferred light energy excites P700, which loses an electron to an electron acceptor.P700+ accepts an electron passed down from PS II via the ETCEach electron “falls down” an ETC form the primary electron acceptor of PS I to the protein ferredoxin (Fd)The electrons are transferred to NADP+ and reduce it to NADPH (Removes H+ from Stroma)Electrons of NADPH are available for the reactions of the Calvin cycle.-Cyclic Electron FlowUses ONLY photosystem I and produces ATP, but NOT NADPH, no oxygen is released.Cyclic electron flow generates surplus ATP, satisfying the higher demand in the Calvin cycle.Some organism such as purple sulfur bacteria have PS I but not PS II.-Comparison of Chemiosmosis in Chloroplasts and MitochondriaChloroplasts and mitochondria generate ATP via Chemiosmosis.In mitochondria, protons are pumped to the intermembrane space and drive ATP synthesis as they diffuse back into the mitochondrial space.In Chloroplasts, protons are pumped into the thylakoid space and drive ATP.ATP and NADPH are produced on the side facing the Stroma (outside the thylakoid) where the Calvin cycle takes place.Reactions generate ATP and increase potential energy of electrons by moving them from water to NADPH.-Concept 10.3Calvin Cycle, like the citric acid cycle, regenerates its staring material after molecules enter and leave the cycle.Cycle builds sugar from smaller molecules by using ATP and the reducing power of electrons carried by NADPHCarbon enters the cycle as CO2 and leaves as a sugar named glyceraldehyde3-phosphate (G3P)For net synthesis of 1 G3P, the cycle must take place 3 times, fixing 3 molecules of CO23 Phases of the Calvin Cycle1) Carbon fixation (Catalyzed by rubisco-ribulose bisphosphate (RuBP) carboxylase, the most abundant protein in the world)2) Reduction (By NADPH) – 6 ATP and 6 NADPH used.3) Regeneration of CO2 acceptor (RuBP) – 3 ATP used.-Concept 10.4Dehydration is a problem for plants, sometimes requiring trade offs with other metabolic processes especially photosynthesis.On hot, dry days, plants close stomata, which conserves water but also limits photosynthesis.Closing of stomata reduces access to CO2 and causes oxygen to buildup.Favor wasteful process photorespiration-PhotorespirationIn most plants (C3 Plants) initial fixation of CO2 via Rubisco forms a 3-carbon compound (3-phophoglycerate)Examples of important agricultural C3
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