12-14: Light Dependent Reactions

photosynthesis equation

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

    This proton gradient can be used for chemiosmosis

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)