Photosynthesis - Anoka-Hennepin School District 11

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PhotosynthesisChapter 8Photosynthesis OverviewEnergy for all life on Earth ultimately comesfrom photosynthesis.6CO2 12H2OC6H12O6 6H2O 6O2Oxygenic photosynthesis is carried out by:cyanobacteria, 7 groups of algae,all land plants2Photosynthesis OverviewPhotosynthesis is divided into:light-dependent reactions-capture energy from sunlight-make ATP and reduce NADP to NADPHcarbon fixation reactions (lightindependent)-use ATP and NADPH to synthesize organicmolecules from CO231

4Photosynthesis OverviewPhotosynthesis takes place in chloroplasts.thylakoid membrane – internal membranearranged in flattened sacs-contain chlorophyll and other pigmentsgrana – stacks of thylakoid membranesstroma – semiliquid substance surroundingthylakoid membranes562

Pigmentsphoton: a particle of light-acts as a discrete bundle of energy-energy content of a photon is inverselyproportional to the wavelength of the lightphotoelectric effect: removal of an electronfrom a molecule by light-occurs when photons transfer energy toelectrons78PigmentsPigments: molecules that absorb visiblelightEach pigment has a characteristicabsorption spectrum, the range andefficiency of photons it is capable ofabsorbing.93

10Pigmentschlorophyll a – primary pigment in plantsand cyanobacteria-absorbs violet-blue and red lightchlorophyll b – secondary pigmentabsorbing light wavelengths thatchlorophyll a does not absorb11PigmentsStructure of pigments:porphyrin ring: complex ring structure withalternating double and single bonds-magnesium ion at the center of the ring-photons excite electrons in the ring-electrons are shuttled away from the ring124

13Pigmentsaccessory pigments: secondary pigmentsabsorbing light wavelengths other thanthose absorbed by chlorophyll a-increase the range of light wavelengths thatcan be used in photosynthesis-include: chlorophyll b, carotenoids,phycobiloproteins-carotenoids also act as antioxidants14Photosystem OrganizationA photosystem consists of1. an antenna complex of hundreds ofaccessory pigment molecules2. a reaction center of one or morechlorophyll a moleculesEnergy of electrons is transferred throughthe antenna complex to the reactioncenter.155

16Photosystem OrganizationAt the reaction center, the energy from theantenna complex is transferred tochlorophyll a.This energy causes an electron fromchlorophyll to become excited.The excited electron is transferred fromchlorophyll a to an electron acceptor.Water donates an electron to chlorophyll ato replace the excited electron.17186

Light-Dependent ReactionsLight-dependent reactions occur in 4 stages:1. primary photoevent – a photon of light iscaptured by a pigment molecule2. charge separation – energy is transferredto the reaction center; an excited electronis transferred to an acceptor molecule3. electron transport – electrons movethrough carriers to reduce NADP 4. chemiosmosis – produces ATP19Light-Dependent ReactionsIn chloroplasts, two linked photosystems areused in noncyclic photophosphorylation1. photosystem I-reaction center pigment (P700) with a peakabsorption at 700nm2. photosystem II-reaction center pigment (P680) has a peakabsorption at 680nm20217

Carbon Fixation ReactionsTo build carbohydrates, cells need:1. energy-ATP from light-dependent reactions2. reduction potential-NADPH from photosystem I22Carbon Fixation ReactionsCalvin cycle-biochemical pathway that allows for carbonfixation-occurs in the stroma-uses ATP and NADPH as energy sources-incorporates CO2 into organic molecules23Carbon Fixation Reactionscarbon fixation – the incorporation of CO2into organic molecules-occurs in the first step of the Calvin cycleribulose-bis-phosphate CO25 carbons1 carboncarbons2(PGA)3The reaction is catalyzed by rubisco.248

Carbon Fixation ReactionsThe Calvin cycle has 3 phases:1. carbon fixationRuBP CO22 molecules PGA2. reductionPGA is reduced to G3P3. regeneration of RuBPG3P is used to regenerate RuBP2526Carbon Fixation ReactionsGlucose is not a direct product of the Calvincycle.-2 molecules of G3P leave the cycle-each G3P contains 3 carbons-2 G3P are used to produce 1 glucose inreactions in the cytoplasm279

Carbon Fixation ReactionsDuring the Calvin cycle, energy is needed.The energy is supplied from:- 18 ATP molecules- 12 NADPH molecules28Carbon Fixation ReactionsThe energy cycle:-photosynthesis uses the products ofrespiration as starting substrates-respiration uses the products ofphotosynthesis as starting substrates293010

PhotorespirationRubisco has 2 enzymatic activities:1. carboxylation – the addition of CO2 toRuBP-favored under normal conditions2. photorespiration – the oxidation ofRuBP by the addition of O2-favored in hot conditionsCO2 and O2 compete for the active site onRuBP.31323311

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Photosynthesis Chapter 8 2 Photosynthesis Overview Energy for all life on Earth ultimately comes from photosynthesis. 6CO 2 12H 2O C 6H 12O 6 6H 2O 6O 2 Oxygenic photosynthesis is carried out by: cyanobacteria, 7 groups of algae, all land plants 3 Photosynthesis Overview Photosynt

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