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Photosynthesis: The Details

Photosynthesis: The Details. Photosynthesis is a two stage process. Stage one :. The Light Reactions. The Light Reactions. Aka light-dependent reactions Occur in the thylakoid membrane involve photosystems and protein complexes. The Players...take out some colours.

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Photosynthesis: The Details

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  1. Photosynthesis: The Details

  2. Photosynthesis is a two stage process

  3. Stage one: The Light Reactions

  4. The Light Reactions Aka light-dependent reactions Occur in the thylakoid membrane involve photosystems and protein complexes

  5. The Players...take out some colours

  6. The Light Reactions STEP 1 Photon of light hits PS II and excites an e- of chlorophyll P680 (e- goes to the e- acceptor) P680 is now missing an e- and has a positive charge (P680+ )

  7. Pg 160

  8. The Light Reactions Note: this e- will end up in PS I further down the chain (in order to replace PS I’s e-)

  9. The Light Reactions Water-splitting enzyme holds H20 in place • P680+ pulls e- from water Enzyme splits Water into • O2 gas (released to atmosphere) • H+ (released to lumen, to be used later) • e- (used to replace the e- from PS 680)

  10. Pg 161

  11. Light Reactions STEP 2 • Energized e- continue through the ETC • At the b6-F complex, the E from the e- is used to pump H+ from the stroma into the thylakoid lumen. • H+ accumulate in the thylakoid lumen & contribute to the electrochemical gradient

  12. Light Reactions STEP 3(occurs while steps 1 & 2 are taking place) • A photon strikes PSI, and excites an e- of chlorophyll P700 • e- is passed to a high-E e- acceptor • lost e- from PSI are replaced by those that have reached the end of the ETC from PSII

  13. Light Reactions STEP 4 (occurs while steps 1 & 2 are taking place) • NADP reductase uses the e- from PSI and H+ ions from the stroma to reduce NADP+ to NADPH

  14. Light Reactions Chemiosmosis • H+ in thylakoid space cannot diffuse back across the membrane (bc the membrane is impermeable to ions) • ATP synthase is embedded in the membrane and provides the only pathway for hydrogen ions to move down their [gradient]

  15. Light Reactions Chemiosmosis • H+ moving through the ATP synthase complex drives the phosphorylation of ADP to ATP called Photophosphorylation

  16. Light Reactions • **The ATP and NADPH produced in the light reactions go to fuel the Calvin Cycle. • This series of reactions is called non-cyclic photophosphorylation http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120072/bio13.swf::Photosynthetic%20Electron%20Transport%20and%20ATP%20Synthesis

  17. Soooo…what about when we are low on water or there is not enough NADP+???

  18. Cyclic Photophosphorylation • Occurs when plants are low in water or NADP+ • e- in PS I, pass backwards to cytochrome b6 -F

  19. Cyclic Photophosphorylation • Still produces a PMF, pumping H+ ions across the membrane • produces a [gradient] that can be used to power ATP synthase during chemiosmosis • BUT neither O2 nor NADPH are produced.

  20. Pg 162

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