12) Photosynthesis summary_9744_ 2018

Prepared by: Mrs Selvamani Nair (2018) Raffles Institution (Yr 5-6) 1 Photosynthesis ( 6CO2 + 6H2O  C6H12O6 + 6O2) Light Dependent Reactions (on thylakoid membrane) * Primary pigment: special chl a mlc, P680 & P700. P680 is found in the reaction centre (RC) of Photosystem II (PSII) & P700 in Photosystem I (PS I) * Accessory pigments: other chl a, chl b mlcs & carotenoids (fd in the light harvesting complex (LHC) Non-cyclic photophosphorylation * When a photon of light is absorbed by an accessory pigment molecule in the light harvesting com plex ( LHC) of PS II, one of its electrons is excited to a higher energy level. As the excited electron drops to its ground state, the energy released is passed on to the next pigment molecule. This resonance transfer of energy continues until P680, the special chlorophyll a molecule in the reaction centre (RC) is reached. * When P680 absorbs the energy from the accessory pigments of the light harvesting apparatus , it loses an electron, leaving an electron hole in PSII. The displaced electron is accepted by a primary electron acceptor (X) in the reaction centre. * The electron hole in PSII is filled by an electron released from the splitting of water in an enzyme-catalysed reaction in the thylakoid space. During the splitting of water, the H + released contributes to a high concentration of H + in the thylakoid space while the O atom combines with another O atom, forming molecular oxygen (O2) as a by-product * The electron from the primary e - acceptor (X) is then passed down a series of increasingly electr onegative electron carriers (of the 1st ETC) losing energy during the transfer. The energy lost during this electron flow is used to actively pump H + from the stroma to the thylakoid space , generating a proton gradient across the membrane. Chemiosmosis occurs when H+ diffuse down the proton gradient back into the stroma via ATP synthase, & ADP is phosphorylated to ATP. * Meanwhile, PSI loses an electron in a manner similar to PSII . When P700 absorbs the energy from the accessory pigments in the light har vesting apparatus, it loses an electron, leaving an electron hole in PSI. The displaced electron is accepted by a primary electron acceptor (Y) in the reaction centre. The electron hole in PSI is filled by the displaced electron from PSII when it reaches the end of the first electron transport chain. * The electron from the primary electron acceptor (Y) is then is passed down a series of electron carriers of a 2nd ETC. (Energy is not released during electron transfer down this 2 nd ETC.). The electron is finally accepted by NA