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Bioinorganic Chemistry Photosynthesis Photosynthesis can be carried out by Bacteria Algae Plants The detailed chemistry of photosynthesis is not the same in all organisms having the capability. Proton pumping across membranes together with electron transfer between membrane proteins is used to synthesize ATP, as in respiration. Light absorption is carried out by pigments, combinations of chromophores with proteins. Bioinorganic Chemistry Photosynthesis Chloroplast 1. outer membrane 2. intermembrane space 3. inner membrane 4. stroma (aqueous fluid) 5. thylakoid lumen 6. thylakoid membrane 7. granum (stack of thylakoids) 8. thylakoid (lamella) 9. starch 10. ribosome 11. plastidial DNA 12. plastoglobule Bioinorganic Chemistry Photosynthesis Bioinorganic Chemistry Photosynthesis Thylakoid redox systems Bioinorganic Chemistry Photosynthesis Chromophores are chlorophylls, carotenes, pycoerythrinin and others. In anglo-saxon texts, chromophores themselves are often called pigments. Pigments are organized in light harvesting complexes. The multiple chromophores collect photon energy efficiently by creating excitons through FRET (Förster resonance energy transfer). Finally, the exciton is transferred to a chlorphyll a dimer, the special pair, in the reaction center. Bioinorganic Chemistry Photosynthesis The special pair undergoes charge separation by transferring an electron to pheophytin, leaving a radical cation. This separation is irreversible which is crucial for efficiency. Charge recombination would produce only unwanted heat. The oxidized special pair is reduced with electrons coming from the water oxidation unit. This leaves protons behind inside the thylakoid. Bioinorganic Chemistry Photosynthesis The electrons stored initially on the pheophytin are passed on to quinones which are reduced to quinols at the exterior of the thylakoid. Protons are extracted from the stroma. The quinols move freely in the membrane and are oxidized on the inside of the thylakoid to quinonones again. Protons are released into the lumen. This process reminds us of the Q-cycle.
