BBMB 405 1st Edition Lecture 2 Outline of Last Lecture I Organic reactions II Use of Food III Human Carbohydrate metabolism IV Synthesis of Major Body Constituents Outline of Current Lecture V Summary of Photosynthesis VI Chapter 19 Light Reactions of Photosynthesis A Photosynthesis takes place in chloroplast B Light absorption by chlorophyll induces electron transfer C Two photosystems generate a proton gradient and NADPH in oxygenic photosynthesis D A proton gradient across the thylakoid membrane drives ATP synthesis E Accessory pigments funnel energy into reaction centers F The ability to convert light into chemical energy is ancient Current Lecture V Summary of Photosynthesis A Summary reaction CO2 H2O light ATP NADPH CH2O O2 B Occurs in chloroplasts C Chlorophyll captures light energies D Electrons exited to higher energies E Reducing potential O2 and proton gradients are generated F NADPH and ATP photophosphorylation are generated light reactions NABPH and ATP are used to drive CO2 reduction to form CH2O Calvin cycle dark reactions G About 1010 tons of carbon is converted to CH2O 1017 kcal in the world each year H Photosynthesis feeds animals carbon containing compounds and provides O2 VI Chapter 19 Light Reactions of Photosynthesis A Photosynthesis takes place in chloroplast 1 Photosynthesis converts light energy into chemical energy 2 Chloroplast outer membrane leaky and inner membrane impermeable to most molecules and ions with intervening intermembrane space 3 Inner membrane surrounds stroma site of dark reactions of photosynthesis 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 4 Z scheme 5 Simple Photosynthesis a CO2 H2O Light CH2O O2 this equation represents the simplist reaction in photosynthesis if you multiply it by 6 that is the overall reaction of photosysnthesis which creates one molecule of glucose C6H12O6 b Light reactions B 6 Primary events of photosynthesis takes place in thylakoid membranes a Process generates proton gradient and eventually results in production of ATP b Thylakoids like mitochondrial cristae are site of coupled oxidationreduction reactions of light reactions that generate proton motive force 7 Chloroplasts arose from endosymbiotic event a Chloroplasts have own DNA like mitochondria in animal cells b Chloroplasts are not autonomous in that they need cell s DNA as well as their own to carry out function c Derived from ancient symbiotic relationship between eukaryotic host and cyanobacterium many generations past and they exchanged DNA Light absorption by chlorophyll induces electron transfer 1 Trapping light energy a Absorption of light by photoreceptor molecule pigement molecule chlorophyll a b Unlike heme chlorophyll has reduced pyrrole ring and additional 5carbon ring c Chlorophylls have strong absorption bands on visible region of spectrum absorption colors Blue first peak green lowest points in middle red second peak 2 What happens to light absorbed a Light excites electron from ground level to excited energy level b Energy can either be converted to heat or excited electron can move to acceptor Photoinduced charge separation Note the charges on the final state of the donor and acceptor molecules c Takes place in reaction center photosynthetic apparatus is arranged to maximize photoinduced charge separation and minimize unproductive return of electron to ground state d Now electron can reduce other molecules to store obtained energy 3 Special pair of chlorophylls initiate charge separation pair of bacteriochlorophyll b called special pair are fundamental in photosynthesis absorbs light at 960 nm P960 after absorbing light the exiceted special pair ejects electron transfter through another BChl b to bacteriopheophytin creates positive charge on special pair See Electron chain in reaction center diagram 4 Reaction Center a Involves different pigments and proteins bacteriochlorophyll b and bacteriopheophytin b Electron chain in photosynthetic bacterial reaction center memorize this process p 569 or 19 2 1 for further explanation 5 Cyclic electron flow reduces cytochrome of reaction center a Cytochrome subunit needs to regain electron to complete cycle takes two electrons from reduced quinone b QH2 enters Q pool and is reoxidized to Q by complex bc1 like complex III of respiratory electron transport chain c Complex bc1 transfters electrons to cytochrome c2 protein in periplasm and pumps protons into periplasmic space d Electrons on cytochrome c2 flow to cytochrome subunit of reaction center e Diagram C Two photosystems generate a proton gradient and NADPH in oxygenic photosynthesis 1 Photosynthesis in green plants and photosynthetic bacteria is different in in plants electron flow is not cyclic 2 Diagram Note that Photosystem PS I absorbs light at wavelength less than 700nm and PS II absorbs light at wavelength less than 680 nm 3 Photosystem II transfers electrons from water to plastiquinone closely resembles ubiquinone in mitochondrial electrons transport chain and generates a proton gradient 4 PS II 2Q 2H2O Light O2 2QH2 5 PS II drives reaction thermodynamically uphill by using energy of light 6 Electron flow through photosystem II a Excitation of special pair of chlorophyll molecules P680 b P680 rapidly transfers electron to pheophytin c Electrons transferred to tightly bound plastoquinone at QA then to mobile plastoquinone at QB d Analogous to bacterial system e With second electron and uptake of two protons plastoquinone is reduced to QH2 energy is stored in reducing potential of QH2 7 Manganese center water molecules bind in PS II and their electrons taken by P680 can exist in multiple oxidation state when photon kicks electron out of P680 positively charged special pair takes electron from 8 Manganese center where O2 is produced from H2O walk through but don t need to know 9 Proton gradient a Put more protons in lumen and decrease pH make gradient stroma give H to further the gradient b Two protons are taken from stroma to reduce Q four protons liberated when water oxidized and released into luman 10 Cytochrome bf links photosystem II to photosystem I a QH2 2Pc Cu2 Q 2Pc Cu 2H thylakoid lumen b Cytochrome bf catalyzes reactions Q cycle i Plastoquinol QH2 oxidized to plastoquinone Q ii Reduce molecule of plastoquinone from Q pool to plastoquinol by taking two protons and reoxidizes plastoquinol to release protons in thylakoid