Photosynthesis: The Details

Photosynthesis is a two stage process

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

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 + )

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

The Light Reactions Water-splitting enzyme holds H 2 0 in place P680 + pulls e - from water P680 + pulls e - from water Enzyme splits Water into O 2 gas (released to atmosphere) H + (released to lumen, to be used later) e - (used to replace the e - from PS 680)

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STEP 2 Energized e - continue through the ETC At the b 6 -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 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

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

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]

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

Light Reactions **The ATP and NADPH produced in the light reactions go to fuel the Calvin Cycle. non- cyclic photophosphorylation This series of reactions is called non- cyclic photophosphorylation hill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/s ites/dl/free/ /120072/bio13.swf::Photosynt hetic%20Electron%20Transport%20and%20ATP%2 0Synthesis

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

Occurs when plants are low in water or NADP + e - in PS I, pass backwards to cytochrome b 6 -F

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 O 2 nor NADPH are produced. BUT neither O 2 nor NADPH are produced.

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