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Photosynthesis: using light energy to reduce CO 2 to make CH 2 O (carbohydrate)

Photosynthesis: using light energy to reduce CO 2 to make CH 2 O (carbohydrate). Photosynthesis light reactions. How do photosynthetic organisms capture light energy? What reactions convert light energy to chemical energy ?. Light reaction take-home points.

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Photosynthesis: using light energy to reduce CO 2 to make CH 2 O (carbohydrate)

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  1. Photosynthesis: using light energy to reduce CO2 to make CH2O (carbohydrate)

  2. Photosynthesis light reactions • How do photosynthetic organisms capture light energy? • What reactions convert light energy to chemical energy?

  3. Light reaction take-home points • Light energy is used to generate a proton gradient across a membrane for chemiosmotic ATP synthesis. • Light energy is used to reduce electron carriers that are used, in turn, to reduce inorganic carbon (CO2)to make carbohydrate (CH2O). • A key innovation enabled cyanobacteria to take electrons from water molecules instead of organic molecules or sulfur and changed the surface of the planet forever.

  4. Oxygenic Photosynthesis • 6 CO2 + 12 H2O C6H12O6 + 6 H2O + 6 O2 • Reverse of aerobic respiration: C6H12O6 + 6 H2O + 6 O2 6 CO2 + 12 H2O

  5. Evolution of chloroplasts from cyanobacterialendosymbiont? http://evolution.berkeley.edu/evosite/history/endosym.shtml

  6. Visible light is a portion of the EM spectrum Light is energy: E = h

  7. Bacteria and chloroplasts use chlorophyll to capture light energy Xiong et al. 2000 Science 289:1724-1730

  8. Chlorophylls and other pigments form a light-harvesting complex in photosynthetic membranes

  9. Absorption spectra for chlorophylls match photosynthetic action spectrum Campbell & Reece, 7th ed.

  10. Light-harvesting complexes funnel light energy to a photosystem reaction center Light-activated reaction center chlorophyll molecules give up electrons (are oxidized)

  11. Light + H2O + NADP+ + ADP + Pi  NADPH + H+ + ATP + 1/2O2 Noncyclicphotophosphorylation (Z-scheme) uses two photosystems: PSI and PSII

  12. Energy diagram of non-cyclic electron flow

  13. Protons are pumped into the thylakoid interior Campbell & Reece, Biology 8th ed.

  14. Electron transport chains are located on the thylakoid membrane and inner mitochondrial membranes. Chemiosmotic ATP synthesis in chloroplasts resembles that in mitochondria Campbell & Reece, Biology 8th ed.

  15. PS I P700 Gets electrons from: Gives electrons to: Products: Summary of Z-scheme (non-cyclic) • PS II • P680 • Gets electrons from: • Gives electrons to: • Products:

  16. Fd(red) can reduce NADP+, or PQ (electron transport chain) Cyclic photophosphorylation by PSI makes ATP but not NADPH Campbell & Reece, Biology 8th ed.

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