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Ch 21 Photosynthesis

Ch 21 Photosynthesis - chl a like(no Mg special...

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chl a – like (no Mg++) special plastoquinones membrane-assoc. ferrodoxins plastocyanin (Cu-contaning) chl a – like (no Mg++) special plastoquinones CHAPTER 21: PHOTOSYNTHESIS Introduction o 1% of sunlight from sun is absorbed by plants and converted to chemical energy chemicals formed are used by members of the food chain energy is used to fix CO 2 , with the evolution of O 2 (waste product) About 10 11 tons of CO 2 (20% in atmosphere) fixed per year process is reverse of glycolysis, and requires energy 6 CO 2 + 6 H 2 O C 6 H 12 O 6 (hexose) + 6 O 2 o Light energy also used to fix N 2 assimilate sulfur-containing compounds General aspects of photosynthesis o Can be done by simple prokaryotes to large trees – all have several common features. o occurs in membranes (which have a function to separate charges, esp H+) In photosynthetic bacteria, the membranes are the major components of the cytoplasm. In plants, the membranes are in large organelles called chloroplasts (a type of plastid, which are a group of related organelles) 1
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photosynthetic inner membrane system, called thylakoid membranes, are flattened discs or sacs. three membrane-bound aqueous regions o lumen (interior of thylakoid membranes) o stroma (outside thylakoid membrane, inside inner membrane) o intermembrane space (between inner and outer membrane) contain DNA, RNA, ribosomes (theory that chloroplasts used to be bacteria like cyanobacteria) ; but some functions encoded by nucleus o Light and dark reactions light rxn: Illuminated chloroplasts w/o CO 2 generates O 2 , NADPH, and ATP; requires lumen thylakoid membrane H 2 O is e - source for NADPH and ATP synthesis (in plants and cyanobacteria) o In bacteria, the e - source may also be H 2 S (photosynthetic green and purple sulfur bacteria), isopropanol, or other oxidizable (reduced) compounds (ex: short chain fatty acids) Note: o NADH – used for energy transduction o NADPH – used for biosyntheses o both have same energy association and more energy than ATP 2 H 2 O +2 NADP + +x ADP +x P i O 2 + 2 NADPH + 2 H + + x ATP + x H 2 O o stoichiometry is uncertain dark rxn: Once illuminated, they can be placed in dark and fix CO 2 ; occurs in the stroma; NADPH and ATP from light rxn are used for CO 2 fixation o Chlorophyll – light absorbing pigment; magnesium-containing substituted tetrapyrroles 2
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Properties Similar to heme, but differing with respect to the metal ion (Mg instead of Fe), a long-chain alcohol (phytol, which anchors everything to membrane), and a fifth five-membered ring. good light absorbers b/c of delocalized electrons above and below planar rings π o Visible light absorption promotes an e - to a higher orbital (this e - is stable this e - is actually only present in the reactive center), which can be transferred to another compound (reduction rxn) absorption (and absorption spectra) of chlorophylls a and b differ as R groups change o along with accessory light-harvesting reactions (like
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