Photophosphorylation: Cyclic and Noncyclic Processes

Noncyclic Photophosphorylation

This process is similar to what occurs after the electron transport chain in the mitochondrial membrane.

• With the stroma, protons are transported to the lumen through the fixed plastoquinone.
• This generates a potential gradient that moves to an enzyme, ATPase, located in F particles, similar to those of mitochondria.
• ATPase uses four protons to phosphorylate ADP to ATP.

Cyclic Photophosphorylation

This occurs when:

• Light striking the plant is between 681 and 700 nm, exciting only Photosystem I (PSI).
• The plant urgently needs ATP, as this process is faster than noncyclic photophosphorylation and does not waste energy reducing NADP when not needed.

The process unfolds as follows:

1. PSI donates electrons to the acceptor chain, leading to ferredoxin.
2. Instead of reducing NADP, ferredoxin returns the electrons to cytochrome b6f.
3. Cytochrome b6f assigns the electrons to the fixed plastoquinone, facilitating proton transport from the stroma to the lumen.
4. The cycle continues back to the cytochrome, plastocyanin, PSI, the acceptor chain, and ferredoxin, which will reassign the electrons to the cytochrome.

This process produces more ATP and maintains proton and electron transport but does not reduce NADP, perform water photolysis, or release oxygen into the atmosphere.

Differences from Oxidative Phosphorylation

• Photophosphorylation occurs in the lamellae and is independent of any other system or organelle.
• No substrate, beyond photons, can serve as an energy source for this process.
• When only PSI is active, oxygen is neither required nor consumed.
• Electrons in photophosphorylation do not originate from outside the system, unlike in oxidative phosphorylation.

Electron Transport (Z Scheme)

The light-dependent phase takes place in the thylakoid membrane, which houses two photosystems and a series of electron acceptors.

• PSII is located below the water potential.
• PSI is located between the potential level of the water and NADP.

The process begins when light strikes the plant, causing a photon to excite PSII.

1. PSII donates two electrons to pheophytin. To return to its original state, it receives two electrons from a protein complex called the oxygen-evolving complex.
2. The oxygen-evolving complex performs the photolysis of one H₂O molecule, releasing two H⁺ (which will replenish PSII with electrons and protons that go to the lumen) and ½ O₂, which is released into the atmosphere.
3. Pheophytin transfers the electrons to a known acceptor, plastoquinone (first mobile and then fixed).
4. Upon receiving electrons, the fixed plastoquinone gains the ability to become hydrogenated, taking H⁺ from the stroma and releasing it into the lumen (crucial for photophosphorylation).
5. Finally, plastoquinone transfers the two electrons to cytochrome b6f.
6. Cytochrome b6f assigns the electrons to plastocyanin, which donates them to PSI.
7. PSI transfers the electrons to another acceptor chain, which passes them to ferredoxin.
8. Ferredoxin uses these electrons to reduce NADP⁺ to NADPH.
9. Alternatively, ferredoxin may not reduce NADP⁺ but transfer the electrons to cytochrome b6f, forming a cycle as explained above.