Photosystems and Electron Flow

Photosynthesis

involves increases and decreases in the energy of an electron as it moves from PSII through PSI to NADPH.

H2O

is split by enzymes, and the electrons are transferred from the hydrogen atoms to P680+, thus reducing it to P680.

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.

Photosystem I

boosts the electron to an even higher energy level.

light harvesting complexes

The (pigment molecules bound to proteins) funnel the energy of photons to the reaction center.

photosystem

A(n) consists of a.

Solar

- powered transfer of an electron from a chlorophyll a molecule to the primary electron acceptor is the first step of the light reactions.

PSII

Light excites the electron in .

pigment molecules

A photon hits a pigment and its energy is passed among until it excites P680.

Cyclic electron flow

generates surplus ATP, satisfying the higher demand in the Calvin cycle.

Electron

on a nonexcited pigment molecule in PSII starts with the lowest energy.

Cyclic electron flow

produces ATP, but not NADPH.

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

uses only photosystem I  and produces ATP, but not NADPH

A photosystem consists of

reaction-center complex (a type of protein complex)

surrounded by light-harvesting complexes

Light-harvesting complexes

pigment molecules bound to proteins; funnel the energy of photons to the reaction center

First step of the light reactions

Solar-powered transfer of an electron from a chlorophyll a molecule to the primary electron acceptor

Z Scheme

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