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Lecture 6

BIO204 CH10 - Lecture 6

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Department
Biology
Course
BIO203H5
Professor
Sanja Hinic- Frlog
Semester
Fall

Description
6 (CH10) October-09-13 7:43 PM Review Nutrient Uptake: Proton pumps (H -ATPase)establish a proton (or electrochemical)gradient, that supports entry of ions Photosynthesis: Electrons link two sets of reactions Thylakoid membrane - light reactions Absorption of light excites electrons in pigments • Photon strikes chlorophyll  transfer of energy to an electron in chlorophyll head  excitation  raise of electron to higher energy shell with greater potential energy • Chlorophyll absorbs red and blue photons  electrons are raised to different states  red photons raise electrons to state 1  higher-energy blue photons raise electrons to state 2 • Green photons are of an intermediate energy level and are not easily absorbed by chlorophyll Energy is transferred and electrons are effected when photons strike chlorophyll Absorption -> transition from ground state to excited state Chl + hv -> Chl* (excited) Chl + hv -> Chl* (excited) Chl = chlorophyll molecule What happens to the potential energy in an excited electron? *Fluorescence: Absorbed energy is released as electromagneticradiation Photosynthetic pigments absorb light energy that powers photosynthesis Absorption of photon -> transition from ground state to excited state: Chl + hν -> Chl* Only 2% of red and blue photons absorbed by chlorophylls produce fluorescence • What happens to 98% percent of excited electrons? • Excitation energy is transferred: Chl1* + Chl2 -> Chl1 + Chl2*  RESONANCE = Chlorophyll moleculestransmit resonance energy Pigments are arranged in photosystems to facilitate transfer of excitation energy The Antenna complex • Absorption of red or blue photon by pigment molecules -> Excitation of electrons • Transfer of energy, NOT ELECTRONS, to nearby pigment molecule, where another electron is excited in response • Transfer continues inside the antenna complex and excitation energy is directed to reaction center A photosystem:Chlorophyll moleculesin antenna complexes“surround” a reaction center The Reaction Center • Eventually - Excitation energy is transferred to a reaction center • In reaction centers, excited e- are transferred to an electron acceptor Photochemical energy conversion occurs in the reaction center Reduction of electron acceptor/Oxidationof electron donor in reaction centers Transformationof electromagneticenergy to chemical energy Summary - Three fates for excited electrons in photosynthesis (1) Drop back down to a low energy level -> fluorescence (be given off as heat and electromagneticradiation) (2) Excite an electron in a nearby pigment and induce resonance (3) Transfer to an electron acceptor in a redox reaction Discoveryof PhotosystemsI and II • In oxygen evolving organisms: photosystemII & photosystemI operate in series • produce an enhancementeffect (Emersoneffect, late 1950s) -> photosynthesis morethan doubles upon exposure of cells to both red and far-red light If you gave the plant both red and orange light, the plants just goes crazy with photosynthesis The enhancement effect In which he concluded that there were 2 photosystems One in charge of red One in charge of orange work together A very simple model of two photosystems • ~ 300- 500 chlorophylls a a
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