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