PACKET #31CHAPTER #10

The Light ReactionsNon-cyclic Electron Flow

Introduction I

The goal of the light reactions is to produce ATP and NADPH via the movement of electrons Solar energy is converted

into chemical energy. The ATP, and NADPH,

produced is used in the Calvin Cycle.

The movement of electrons is similar to that of oxidative phosphorylation Cellular respiration.

Another View

Introduction II

Non-cyclic flow requires the use of Photosystems

Photosystem II Photosystem I

Mobile electron acceptors Plastoquinone Plastocyanin Ferrodoxin

Stationary electron acceptor Cytochrome B6F

Enzymes Ferrodoxin-NADP+

reductase ATP synthase

Introduction III

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5To effectively

understand the non-cyclic electron flow, one must remember the “key.” Electrons are moved

and ultimately stored in NADPH.

The Photosystems

Photosystems—Part I

The light reactions involve the use of photosystems.

The photosystems, absorb different wavelengths of light within their chlorophyll molecules.

This absorption allows for the excitation and movement of electrons.

Photosystems—Part II

Photosystem II Absorbs light with

wavelengths of 680 nM Yellow-green light

Photosystem I Absorbs light with

wavelengths of 700 nM Red light

THE PROCESS

Non-Cyclic Electron Flow

Non-cyclic Electron Flow Step by Step I

Photon, of 680nm, hits ONE of the many chlorophyll molecules found in photosystem II.

This causes an electron to become excited.

Non-cyclic Electron Flow Step by Step II

The excited electron moves along multiple chlorophyll molecules until it reaches the reaction center.

Non-cyclic Electron Flow Step by Step III

The excited electron arrives and waits at the primary electron acceptor until it is picked up by the mobile electron carrier plastoquinone.

Non-cyclic Electron Flow Step by Step IV

Plastoquinone has the ability to carry two electrons and waits until a second electron becomes available.

Non-cyclic Electron Flow Step by Step V

The second electron originates from H2O. Once an electron leaves

photosystem II, it has to be replaced.

Water is broken into H+

The hydrogen ion is used to help establish the hydrogen gradient inside the thylakoid space.

O2 Eventually leaves the leaf

through the stomata. An electron

This electron fills in the gap made by the electron that has boarded plastoquinone.

Non-cyclic Electron Flow Step by Step VI

The second electron, now residing in a chlorophyll molecule of photosystem II, becomes excited by a another photon of 680nm.

Non-cyclic Electron Flow Step by Step VII

The second electron eventually boards plastoquinone to join the electron that previously boarded.

The gap produced again by a missing electron is replaced when another molecule of water is broken.

Non-cyclic Electron Flow Step by Step VIII

The two electrons, and two hydrogen ions, in plastoquinone are transported to the stationary electron acceptor cytochrome B6F.

Non-cyclic Electron Flow Step by Step IX

The two electrons and two hydrogen ions enter the stationary electron acceptor cytochrome B6F.

Non-cyclic Electron Flow Step by Step X

Once both the electrons and the two H+ are inside cytochrome B6F, the cytochrome opens it’s H+ channel gate and allows the H+ to leave, through the channel, freely into the thylakoid space. The H+, once in the

thylakoid space, helps strengthen the hydrogen gradient. Chemiosmosis

Non-cyclic Electron Flow Step by Step XI

Meanwhile, as the H+ ions enter into the thylakoid space, the two electrons board the second mobile electron acceptor called plastocyanin.

Non-cyclic Electron Flow Step by Step XII

Plastocyanin takes the two electrons to photosystem I.

Non-cyclic Electron Flow Step by Step XIII

The electrons, once in photosystem I, are re-excited with a photon of wavelength 700nm. Experience similar

process as seen in photosystem II.

Non-cyclic Electron Flow Step by Step XIV

The two electrons leave photosystem I and board the third mobile electron acceptor ferredoxin.

Non-cyclic Electron Flow Step by Step XV

Ferredoxin transports the two electrons to the enzyme ferrodoxin-NADP+ reductase.

The enzyme transfers the electrons to the electron acceptor NADP+ for transport to the Calvin Cycle. NADP+ is changed into

NADPH.

Non-cyclic Electron Flow Step by Step XVI

But wait…there is more…

The hydrogen ions, that were used to produce the hydrogen gradient in the thylakoid space, are pumped into the stroma via ATP synthase. Each hydrogen ion that

passes through ATP synthase produces 1 ATP.

The ATP is then used in the Calvin Cycle.

Overall Review

“Overall” Inputs & OutputsNon-Cyclic Electron Flow

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Inputs Light NADP+ ADP P H2O

Outputs ATP NADPH O2