BBMB 405 1st Edition Lecture 2Outline of Last Lecture I. Organic reactionsII. Use of FoodIII. Human Carbohydrate metabolismIV. Synthesis of Major Body ConstituentsOutline of Current Lecture V. Summary of PhotosynthesisVI. Chapter 19: Light Reactions of PhotosynthesisA. Photosynthesis takes place in chloroplastB. Light absorption by chlorophyll induces electron transferC. Two photosystems generate a proton gradient and NADPH in oxygenic photosynthesisD. A proton gradient across the thylakoid membrane drives ATP synthesisE. Accessory pigments funnel energy into reaction centersF. The ability to convert light into chemical energy is ancientCurrent LectureV. Summary of PhotosynthesisA. Summary reaction: CO2 + H2O –light, ATP, NADPH (CH2O) + O2B. Occurs in chloroplastsC. Chlorophyll captures light energiesD. Electrons exited to higher energiesE. Reducing potential, O2, and proton gradients are generatedF. 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 yearH. Photosynthesis “feeds” animals carbon-containing compounds and provides O2VI. Chapter 19: Light Reactions of PhotosynthesisA. Photosynthesis takes place in chloroplast1. Photosynthesis converts light energy into chemical energy2. 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 scheme5. 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 reactions6. Primary events of photosynthesis takes place in thylakoid membranesa. Process generates proton gradient and eventually results in production of ATPb. Thylakoids (like mitochondrial cristae) are site of coupled oxidation- reduction reactions of light reactions that generate proton-motive force7. Chloroplasts arose from endosymbiotic eventa. Chloroplasts have own DNA like mitochondria in animal cellsb. Chloroplasts are not autonomous in that they need cell’s DNA as well as their own to carry out functionc. Derived from ancient symbiotic relationship between eukaryotic host and cyanobacterium; many generations past and they exchanged DNAB. Light absorption by chlorophyll induces electron transfer1. Trapping light energya. Absorption of light by photoreceptor molecule (pigement molecule chlorophyll a)b. Unlike heme, chlorophyll has reduced pyrrole ring and additional 5-carbon ringc. 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 absorbeda. Light excites electron from ground level to excited energy levelb. 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 moleculesc. Takes place in reaction center; photosynthetic apparatus is arranged to maximize photoinduced charge separation and minimize unproductive return of electron to ground stated. Now electron can reduce other molecules to store obtained energy3. 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 Centera. Involves different pigments and proteins; bacteriochlorophyll b and bacteriopheophytinb. 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 centera. Cytochrome subunit needs to regain electron to complete cycle, takes two electrons from reduced quinoneb. 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 spaced. Electrons on cytochrome c2 flow to cytochrome subunit of reaction centere. DiagramC. Two photosystems generate a proton gradient and NADPH in oxygenic photosynthesis1. Photosynthesis in green plants and photosynthetic bacteria is different in in plants electron flow is not cyclic2. Diagram: Note that Photosystem (PS) I absorbs light at wavelength less than 700nm and PS II absorbs light at wavelength less than 680 nm3. Photosystem II transfers electrons from water to plastiquinone (closely resembles ubiquinone in mitochondrial electrons transport chain) and generates a proton gradient4. PS II: 2Q + 2H2O –Light-> O2 + 2QH25. PS II drives reaction thermodynamically uphill by using energy of light6. Electron flow through photosystem IIa. Excitation of special pair of chlorophyll molecules (P680)b. P680 rapidly transfers electron to pheophytinc. Electrons transferred to tightly bound plastoquinone at QA then to mobile plastoquinone at QBd. Analogous to bacterial systeme. With second electron and uptake of two protons plastoquinone is reduced to QH2, energy is stored in reducing potential of QH27. 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 from8. Manganese center where O2 is produced from H2O ( walk through, but don’t need to know)9. Proton gradienta. Put more protons in lumen and decrease pH, make gradient, stroma give H+ to further the gradientb. Two protons are taken from stroma to reduce Q, four protons liberated when water oxidized and released into luman10. Cytochrome bf links photosystem II to photosystem Ia. QH2 + 2Pc(Cu2+)  Q + 2Pc(Cu+) + 2H+thylakoid lumenb. Cytochrome bf catalyzes reactions (Q cycle)i. Plastoquinol (QH2) oxidized to