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