Detailed Notes on Linear Photosynthesis and Photosystem II Mechanics

Linear photosynthesis is the process occurring in plants where light energy is converted into chemical energy via a specific pathway of electron movement.
The general scheme involves electrons originating from water molecules ( $H_2O$ ).
These electrons are passed to a reaction center known as $p\,680$ .
The designation $680$ refers to the wavelength of light ( $680\,nm$ ) required to excite the molecule.
Upon excitation, the molecule becomes $p\,680^*$ (excited state).
The excited electron is passed through a collection of molecules, including a molecule called pheophyton a.
- Pheophyton a is essentially a chlorophyll molecule that does not contain magnesium ( $Mg$ ).
- During the transfer, pheophyton a is converted into $pheophyton^-$ , while $p\,680$ becomes $p\,680^+$ .
The electron then moves through the cytoker v\,six f complex.
From the cytoker v\,six f complex, the electron is transferred to class of cyanide (also referred to as plastic cyanin or clastocyanin).
The electron then reaches a second photosystem, known as photosystem one ( $PSI$ ), which contains a reaction center designated as $t\,700$ (or $P_{700}$ , requiring $700\,nm$ of light for excitation).
Once the electron is excited at $PSI$ , it follows a similar process, ultimately reducing $NADP^+$ to form NABPH (also referred to as NADPH).
Each excitation of an individual electron requires exactly one photon of light.

The movement of electrons can be quantified in two ways:
- Per oxygen molecule ( $O_2$ ) generated: This requires the movement of $4$ electrons.
- Per NABPH generated: This involves $2$ electrons.
To generate a single molecule of oxygen ( $O_2$ ), two molecules of water ( $H_2O$ ) are oxidized.
- Each molecule of water contributes $2$ electrons, totaling $4$ electrons for the production of one $O_2$ .
The reduction of $NADP^+$ to NABPH is a $2$ -electron reduction process.
Consequently, for every one oxygen molecule ( $O_2$ ) generated, two NABPH molecules are produced.

The photosynthetic pathway begins with photosystem two ( $PSII$ ) and ends with photosystem one ( $PSI$ ).
This counterintuitive naming (two before one) is a result of the order in which they were discovered; researchers were initially unsure of the electron flow direction and named them in the "wrong" order.
Photosystem two is the specific site of oxygen generation.
Photosystem one is the site that ultimately leads to the production of NABPH.
Generally, photosystem one is slightly more prevalent within the system than photosystem two.

The cytoker v\,six f complex (analogous to complex three in the mitochondria) is located in the middle of the electron transport chain.
It facilitates the movement of electrons while simultaneously performing proton pumping.
Proton pumping is critical as it establishes the gradient used for the production of ATP.
The complex utilizes a quinone known as plastoquinone, which is functionally similar to coenzyme q.
Photosystem two also takes up protons, much of which enters a q cycle.

The components of the photosynthetic process are embedded in the thylakoid membrane (or bilacoid membrane) of the chloroplast.
Localization of reactions:
    - Water is oxidized at the photosystem two complex, releasing protons into the bilacoid lumen.
    - Plastic cyanin (an electron carrier similar to cytochrome c, but using copper instead of pinene) moves electrons to photosystem one.
    - Ferredoxin, a small electron carrier similar in size to clastocyanin and cytochrome c, supplies electrons for the reduction of $NADP^+$ to NABPH.
ATP Synthesis:
    - The proton gradient is generated inside the bilacoid lumen.
    - Protons flow from the lumen out into the stroma through ATP synthase.
    - ATP is produced in the stroma.
NABPH Generation:
- NABPH is also generated in the stroma.
Efficiency:
- Because both ATP and NABPH are produced directly in the stroma (where they are needed for subsequent reactions), the chloroplast does not require transport mechanisms for these products.

Photosystem II is characterized as a very large protein complex.
The cytoker v\,six f complex is also relatively large, comparable in size to complex III of the mitochondria.
Photosystem one is a very large complex, though it is not quite as large as photosystem two.
Ferredoxin and clastocyanin (plastic cyanin) are relatively small electron carriers.

The electron transport process moves electrons from water ( $H_2O$ ) to NABPH.
Redox Potentials for $p\,680$ :
- The voltage potential for $p\,680^+ + e^- \rightarrow p\,680$ is approximately $1.2\,V$ .
- The difference in potential between the excited state ( $p\,680^*$ ) and the ground state ( $p\,680$ ) is approximately $1.83\,V$ .
Energy Calculations:
    - Using the delta g ( $\Delta G$ ) equation, the energy difference for one electron is calculated to be $176\,kJ\,mol^{-1}$ .
    - Light with a wavelength of $680\,nm$ provides exactly $176\,kJ/Einstein$ .
    - This confirms that the energy of the photon matches the energy required to excite $p\,680$ from the ground state to the excited state.
Chemical Roles:
- $p\,680^+$ serves as a powerful oxidizing agent.
- Pheophyton (pheobitin) serves as an effective reducing agent to drive the electron transport process.

Reaction centers are located within the photosystem two complex.
Antenna molecules collect light photons and transfer the energy toward these reaction centers through a process involving the movements of excited electrons (hopping).
Photosystems act as exciton phones (or funnels), transferring energy from various points of photon impact to the "special pair" of core molecules in the structure.
The excitation of this special pair is what facilitates the $p\,680 \rightarrow p\,680^*$ conversion.
Oxygen Evolving Complex (OEC):
    - Located in close proximity to the special pair.
    - It is a manganese calcium oxide complex.
    - This specific complex is responsible for the actual generation of oxygen.