1. The Light Reactions Non-cyclic Electron Flow
Packet #31
Chapter #10

2. 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.

3. Another View
A photon with a wavelength of 680 nm enters photosystem II.
The entry of the photon causes an electron, found within a chlorophyll molecule of the photosytem, to become excited.
The electron moves across multiple chlorophyll molecules until it reaches the reaction center of the photosytem.
Once in the reaction center, the electron seeks and finds the primary electron acceptor (bus stop).
The “bus”, called the mobile electron carrier, plastoquinone, picks up the first electron and waits for the second electron to arrive.
The second electron originates from water.
Once water is broken apart, that new electron replaces the one that has already left.
This electron, seeing the “bus” is waiting, runs towards the bus using the same route that the first electron took.
However, despite all of the current events, there is still an electron void in photosytem II.
In order to fill the void, another water molecule is broken.
Special note: -The hydrogen ions produced by the breaking of water, helps establish the hydrogen ion gradient.
The mobile carrier transports the electrons to the cytochrome B6F.
Once the electrons arrive at the cytochrome, they cause hydrogen ions to move from the stroma into thylakoid space (lumen)…allow the gradient to become stronger.
The electrons then transfer onto another “bus” (mobile carrier) plastocyanin and are transported to photosytem I.
The electrons, once they have arrived at photosytem I are “tired” and need to be reenergized again. Another photon, with a wavelength of 700nm reenergizes the electrons, they become excited, and work their way through a series of chlorophyll molecules to the reaction center and to another “bus stop.” (primary electron acceptor) The electrons board the “bus” (mobile electron carrier) called ferrodoxin.
The electrons reach their final destination called Ferrodoxin NADP reductase. Their “homie” NADP+ picks them up and forms NADPH. The electrons are then moved in the car to the Calvin Cycle.
But wait, there is more…
The hydrogen ions were forgotten and they move from the thylakoid lumen, into the stroma, via ATP synthase, and allows the production of ATP. The ATP, once produced in the stroma, enters the Calvin Cycle.

4. 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
Error on the picture.
Ferredoxin-NADP+ reductase is located just outside the membrane of the thylakoid in the stroma.

5. Introduction III
To effectively understand the non-cyclic electron flow, one must remember the “key.”
Electrons are moved and ultimately stored in NADPH.
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6. The Photosystems

7. 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.

8. Photosystems—Part II Photosystem II Photosystem I
Absorbs light with wavelengths of 680 nM
Yellow-green light
Photosystem I
Absorbs light with wavelengths of 700 nM
Red light

9. Non-Cyclic Electron Flow
The Process

10. 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.

11. Non-cyclic Electron Flow Step by Step II
The excited electron moves along multiple chlorophyll molecules until it reaches the reaction center.

12. 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.
Ignore the ATP shown on the picture.

13. Non-cyclic Electron Flow Step by Step IV
Plastoquinone has the ability to carry two electrons and waits until a second electron becomes available.
Ignore the ATP shown on the picture.

14. 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.
Remember plastoquinone is still waiting for another electron.

15. 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.
Remember plastoquinone is still waiting for another electron.

16. 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.
Ignore the ATP shown on the picture.

17. Non-cyclic Electron Flow Step by Step VIII
The two electrons, and four hydrogen ions, in plastoquinone are transported to the stationary electron acceptor cytochrome B6F.

18. Non-cyclic Electron Flow Step by Step IX
The two electrons and four hydrogen ions enter the stationary electron acceptor cytochrome B6F.

19. Non-cyclic Electron Flow Step by Step X
Once both the electrons and the four 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

20. Non-cyclic Electron Flow Step by Step XI
Meanwhile, as theH+ ions enter into the thylakoid space, the two electrons board the second mobile electron acceptor called plastocyanin.

21. Non-cyclic Electron Flow Step by Step XII
Plastocyanin takes the two electrons to photosystem I.

22. 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.

23. Non-cyclic Electron Flow Step by Step XIV
The two electrons leave photosystem I and board the third mobile electron acceptor ferredoxin.

24. 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.

25. 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.

26. Overall Review

27. “Overall” Inputs & Outputs Non-Cyclic Electron Flow
Light
NADP+
ADP P
H2O
Outputs
ATP
NADPH O2
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