Photosystems

• photoexitation: photon absorption by an electron of chlorophyll
• ETC: transfer of excited electrons through a sequence of membrane-bound electron carriers creating H+ reservoir
• Chemiosmosis: use of proton motive force and ATPase to drive ADP-ATP
• photoexcitation: photon of light strikes chlorophyll molecule in the thylakoid membrane and 1 electrons move from ground state to excited. they are then captured by PEA (repeated for 2nd e-)
• redox reaction: chlorophyll is oxidized, pea is reduced
• photosystems antenna complex: consists of a number of chlorophyll molecules and accessory pigments and a reaction centre (chlorophyll a)
• differ in the light wavelengths the RC chlorophyll a is best at absorbing : photosystem I: 700 nm , photosystem II: 680 nm
• as electron is transfered to PEA: 1. Z protein in the thylakoid space splits a H2O [2 e- fill the “hole” in P680, o2 released, H + remain in thylakoid space]. 2. 4 H+ ions from the stroma to the thylakoid space
• the electron from chlorophyll P700 passes through Fd to the enzyme NADP reductase which uses the 2 electrons and 2 H+ ions from stroma to reduce NADP+ to NADPH
• the 2 electrons ejected by P680 replace every 2 electrons ejected by P700
• ATP is generated by ATPase as H+ ions move from thylakoid to stroma by proton motive force.
• as light is required for ADP→ATP conversion, this is called photophosphorylation
• 1 ATP is generated for every 4 + that pass through ATPase
• non-cyclic because once an electron is lost by a RC chlorophyll, It ends up as part of an NADPH molecule.
• cyclid if only photosystem I is used: an electron from P700 travels Fd → Q cycle → b6-f complex → chlorophyll P700. NADPH is not produced and only ATP is generated
• 6 turns of the Calvin cycle for 1 glucose molecule