Photosystems and Electron Flow
Introduction
- 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
Electron Flow
- 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
Linear Electron Flow
- 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
Z Scheme
- 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
- 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