12-14: Light Dependent Reactions

photosynthesis equation

6CO2 + 6H2O → C6H12O6 + 6O2

* takes in carbon dioxide and water
* takes out carbohydrates and oxygen

photosystem 2 & photoexcitation

* light dependent rxns
* light antenna complex absorbs a photon and transfers it to p680 chlorophyll molecule → excites an electron
* p680 loses an electron bc it gets positively charged and gets it back from water splitting
* each water molecule donates 2 electrons so that the process happens twice to produce oxygen
* acceptor molecules taking p680’s electron passes it to a molecule of PQ

electron transport shuttles

* PQ accepts electron from PS 2 and protons from the stroma
* the electron then travels through the thylakoid membrane → donates electron to cytochrome complex
* simultaneously it releases the proton into the lumen → increases the proton complex
* cytochrome complex passes the electron to another shuttle molecule PC which carries it PS 1

photosystem 1 and photo excitation

* electron gets re-excited
* photon strikes PS 1 and electron is excited and transferred to another acceptor
* p700 is left with a positive charge
* electron transferred from PS 2 is delivered by PQ to neutralize P700 (electron comes from water)

ferredoxin and NADPH

* electron from PS 1 is transferred to ferredoxin
* oxidation of ferredoxin transfers the electron to NADP+ and makes NADP
* a second ferredoxin (or the same one twice) adds another electron along w protons from the stroma to make NADPH
* all of this happens with an enzyme called NADP+ reductase

proton gradient

* activity increases proton concentration in thylakoid lumen & decreased it in the stroma
* splitting of water
* PQ & cytochrome complex transport protons from stroma to lumen
* NADPH production removes protons form the stroma

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This proton gradient can be used for chemiosmosis

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ATP synthase

* produced by phosphorylation - light energy, no oxidation occurred to produce the proton gradient

electron energy

* the boost in energy provided by the photons in sunlight gradually decreases as the electrons move through ETCs of PS 1 and 2

cyclic electron transport

* PS 1 works independently of PS 2 if the electron on ferredoxin is transferred to PQ rather than NADP+ reductase
* this continuously pumps protons into the lumen → produces more ATP
* more ATP is needed more than NADPH in the Calvin cycle which makes this process very useful
* 9 ATP and 6 NADPH are needed to make 1 G3P (half of a glucose)