Photosystems and Electron Flow
A photosystem consists of a
reaction-center complex (a type of protein complex)
surrounded by light-harvesting complexes
The light-harvesting complexes (pigment molecules bound to proteins) funnel the energy of photons to the reaction center
A primary electron acceptor in the reaction center accepts an excited electron from chlorophyll
Solar-powered transfer of an electron from a chlorophyll a molecule to the primary electron acceptor is the first step of the light reactions
During the light reactions, there are two possible routes for electron flow: cyclic and linear
Linear electron flow: the primary pathway; involves both photosystems and produces ATP and NADPH using light energy
Cyclic electron flow produces ATP, but not NADPH
A photon hits a pigment and its energy is passed among pigment molecules until it excites P680
An excited electron from P680 is transferred to the primary electron acceptor
P680+ (P680 that is missing an electron) is a very strong oxidizing agent
H2O is split by enzymes, and the electrons are transferred from the hydrogen atoms to P680+, thus reducing it to P680
O2 is released as a by-product of this reaction
Each electron “falls” down an electron transport chain from the primary electron acceptor of PS II to PS I
Energy released by the fall drives the creation of a proton gradient across the thylakoid membrane
Diffusion of H+ (protons) across the membrane drives ATP synthesis
Zigzag shape of energy curve
Photosynthesis involves increases and decreases in the energy of an electron as it moves from PSII through PSI to NADPH
Electron on a nonexcited pigment molecule in PSII starts with the lowest energy
Light excites the electron in PSII
Photosystem I boosts the electron to an even higher energy level
Each electron “falls” down an electron transport chain from the primary electron acceptor of PS I to the protein ferredoxin (Fd)
The electrons are then transferred to NADP+ and reduce it to NADPH
The electrons of NADPH are available for the reactions of the Calvin cycle
Cyclic electron flow: uses only photosystem I and produces ATP, but not NADPH
Cyclic electron flow generates surplus ATP, satisfying the higher demand in the Calvin cycle
A photosystem consists of a
reaction-center complex (a type of protein complex)
surrounded by light-harvesting complexes
The light-harvesting complexes (pigment molecules bound to proteins) funnel the energy of photons to the reaction center
A primary electron acceptor in the reaction center accepts an excited electron from chlorophyll
Solar-powered transfer of an electron from a chlorophyll a molecule to the primary electron acceptor is the first step of the light reactions
During the light reactions, there are two possible routes for electron flow: cyclic and linear
Linear electron flow: the primary pathway; involves both photosystems and produces ATP and NADPH using light energy
Cyclic electron flow produces ATP, but not NADPH
A photon hits a pigment and its energy is passed among pigment molecules until it excites P680
An excited electron from P680 is transferred to the primary electron acceptor
P680+ (P680 that is missing an electron) is a very strong oxidizing agent
H2O is split by enzymes, and the electrons are transferred from the hydrogen atoms to P680+, thus reducing it to P680
O2 is released as a by-product of this reaction
Each electron “falls” down an electron transport chain from the primary electron acceptor of PS II to PS I
Energy released by the fall drives the creation of a proton gradient across the thylakoid membrane
Diffusion of H+ (protons) across the membrane drives ATP synthesis
Zigzag shape of energy curve
Photosynthesis involves increases and decreases in the energy of an electron as it moves from PSII through PSI to NADPH
Electron on a nonexcited pigment molecule in PSII starts with the lowest energy
Light excites the electron in PSII
Photosystem I boosts the electron to an even higher energy level
Each electron “falls” down an electron transport chain from the primary electron acceptor of PS I to the protein ferredoxin (Fd)
The electrons are then transferred to NADP+ and reduce it to NADPH
The electrons of NADPH are available for the reactions of the Calvin cycle
Cyclic electron flow: uses only photosystem I and produces ATP, but not NADPH
Cyclic electron flow generates surplus ATP, satisfying the higher demand in the Calvin cycle