PHOTOSYNTHESIS
Light-dependent reaction
Non-cyclic phosphorylation
(1) A photon of light strikes a pigment molecule in a light harvesting complex (LHC) and energy is
relayed via resonance energy transfer until it reaches one of the two special chlorophyll a
molecules (P680) in the PSII reaction centre
(a) An electron in P680 is excited to a higher energy state
(2) This photoexcited electron is captured by the primary electron acceptor in the reaction
centre
(3) Photolysis of water produces two electrons, two hydrogen ions and an oxygen atom per
water molecule; an electrons released is used to replenish the electron deficit in P680, while
the oxygen atom combines with another oxygen atom to release O2 as a by-product
(4) From the primary electron acceptor, the energised electron passes (unidirectionally through a
series of oxidation-reduction reactions) from PSII to PSI via a first electron transport chain
(ETC) consisting of plastoquinone (Pq), cytochrome (b-f) complex and plastocyanin (Pc)
(5) Free energy released from the exergonic flow of electrons is used to pump protons against
concentration gradient from the stroma into the thylakoid space, generating a proton
gradient used to drive ATP synthesis (photophosphorylation)
(a) Chemiosmosis:
Thylakoid membrane is impermeable to H+; H+ ions accumulate in thylakoid space as
light reaction proceeds (pumping of H+ by cytochrome complex, photolysis of water)
H+ diffuse down the gradient from thylakoid space across thylakoid membrane into
the stroma, through the ATP synthase complex; ATP synthase catalyses the formation
of ATP (chemiosmosis)
(6) Meanwhile, another photon of light strikes a pigment molecule in a LHC of PSI, exciting an
electron of one of the two special chlorophyll a molecules in the PSI reaction centre (P700)
(7) This excited electron is captured by the primary electron acceptor in PSI
(8) The electron deficit in P700 is replenished by the electron from PSII
(9) The excited electron is passed from primary electron acceptor in PSI down a second ETC
through ferredoxin (Fd)
(10) NADP reductase transfers electrons from Fd to NADP; two electrons are required to produce
NADPH
Non-cyclic phosphorylation
(1) A photon of light strikes a pigment molecule in a LHC of PSI, exciting an electron of one of the
two special chlorophyll a molecules in the PSI reaction centre (P700)
(2) This excited electron is captured by the primary electron acceptor in PSI
(3) The excited electron is passed to ferredoxin (Fd), cycled back to cytochrome (b-f) complex on
first ETC and eventually back to PSI
(4) As these electrons are passed along first ETC, enough energy is released to synthesise ATP
(chemiosmosis) for the light-independent reaction
Copyright © 2019 tonyndr
Light-dependent reaction
Non-cyclic phosphorylation
(1) A photon of light strikes a pigment molecule in a light harvesting complex (LHC) and energy is
relayed via resonance energy transfer until it reaches one of the two special chlorophyll a
molecules (P680) in the PSII reaction centre
(a) An electron in P680 is excited to a higher energy state
(2) This photoexcited electron is captured by the primary electron acceptor in the reaction
centre
(3) Photolysis of water produces two electrons, two hydrogen ions and an oxygen atom per
water molecule; an electrons released is used to replenish the electron deficit in P680, while
the oxygen atom combines with another oxygen atom to release O2 as a by-product
(4) From the primary electron acceptor, the energised electron passes (unidirectionally through a
series of oxidation-reduction reactions) from PSII to PSI via a first electron transport chain
(ETC) consisting of plastoquinone (Pq), cytochrome (b-f) complex and plastocyanin (Pc)
(5) Free energy released from the exergonic flow of electrons is used to pump protons against
concentration gradient from the stroma into the thylakoid space, generating a proton
gradient used to drive ATP synthesis (photophosphorylation)
(a) Chemiosmosis:
Thylakoid membrane is impermeable to H+; H+ ions accumulate in thylakoid space as
light reaction proceeds (pumping of H+ by cytochrome complex, photolysis of water)
H+ diffuse down the gradient from thylakoid space across thylakoid membrane into
the stroma, through the ATP synthase complex; ATP synthase catalyses the formation
of ATP (chemiosmosis)
(6) Meanwhile, another photon of light strikes a pigment molecule in a LHC of PSI, exciting an
electron of one of the two special chlorophyll a molecules in the PSI reaction centre (P700)
(7) This excited electron is captured by the primary electron acceptor in PSI
(8) The electron deficit in P700 is replenished by the electron from PSII
(9) The excited electron is passed from primary electron acceptor in PSI down a second ETC
through ferredoxin (Fd)
(10) NADP reductase transfers electrons from Fd to NADP; two electrons are required to produce
NADPH
Non-cyclic phosphorylation
(1) A photon of light strikes a pigment molecule in a LHC of PSI, exciting an electron of one of the
two special chlorophyll a molecules in the PSI reaction centre (P700)
(2) This excited electron is captured by the primary electron acceptor in PSI
(3) The excited electron is passed to ferredoxin (Fd), cycled back to cytochrome (b-f) complex on
first ETC and eventually back to PSI
(4) As these electrons are passed along first ETC, enough energy is released to synthesise ATP
(chemiosmosis) for the light-independent reaction
Copyright © 2019 tonyndr
