Non-Cyclic Photophosphorylation: Definition, Significance, and Reactions

Non-Cyclic Photophosphorylation Overview

Using light energy from photosynthesis, the process of photophosphorylation turns ADP into ATP. By transferring the phosphate group into the ADP molecule when it is illuminated, energy-dense ATP molecules are created. Two forms of photophosphorylation exist- Cyclic Photophosphorylation and Non-cyclic Photophosphorylation.

What is Non-Cyclic Photophosphorylation?

Non-cyclic photophosphorylation is the name given to the photophosphorylation procedure that causes the electrons to travel non-cyclically in order to synthesize ATP molecules using the energy from excited electrons supplied by photosystem II. Because P700 of Photosystem I takes up the lost electrons from P680 of Photosystem II and prevents them from being converted back to P680, this process is known as non-cyclic photophosphorylation. In this case, the whole movement of the electrons is unidirectional or non-cyclic.

The electrons generated by P700 during non-cyclic photophosphorylation are first transported by the main acceptor before being transferred to NADP. One ATP and two NADPH2 molecules are produced by non-cyclic photophosphorylation. Here, the water molecules' splitting produces protons (H+), which are coupled with the electrons to convert NADP into NADPH. The response is displayed below-

NADP⁺+ 2H⁺+ 2e⁻→ NADPH + H⁺

Significance of Non-Cyclic Photophosphorylation

Non-cyclic electron transport/photophosphorylation is the electron transport sequence in which NADP+ is reduced by PSI, PSI is reduced by PSII, and lastly, PSII is reduced by electrons derived from photo-oxidation of water. As a result of the cycle being disrupted during this electrotransfer, it is referred to as non-cyclic. This kind of electron transport is linked to the photooxidation of water and the generation of molecular oxygen. It released a significant amount of protons, which eventually generated a proton motive force to manufacture ATP. Non-cyclic electron transport is critical in photosynthesis because it provides assimilatory power in the form of NADPH and ATP for CO2 assimilation while also purifying the ambient air.

Non-Cyclic Photophosphorylation Reactions

Reason for Photosystem I

A P700 (chlorophyll molecule) reaction center and a number of auxiliary pigments make up PS I. The PS I, or photosystem I, is linked to the following responses-

1. Photon hits PS I: One photon striking the photosystem I trigger the light response of photosynthesis. The accessory pigments in the PS I receive the photon's energy.
2. Photon → P700: The auxiliary pigments begin vibrating after absorbing the photon's energy. The energy contained in these vibrations is transferred from one accessory pigment molecule to another until it reaches the P700 reaction center. The P700's outermost e- absorbs energy and raises its degree of excitement.
3. P700 → Ferredoxin (Fd): The core of the P700 reaction has an unstable electron at a higher energy level. As a result, the electron leaves the P700 reaction center's higher energy orbital and is taken up by chlorophyll A0, a different modified chlorophyll molecule. It releases its electron to the A1 molecule, which has a greater potential. The electron formerly in molecule A1 will now go to an iron-sulfur complex (Fe-S). The reduction potential of the Fe-S complex is lower than that of ferredoxin, the next protein. As a result, Fd steals the electron that Fe-S gave up. 

Ferredoxin → Ferredoxin NADP+ oxidoreductase (FNR)

Read more about the Difference Between Cyclic and Non-Cyclic Photophosphorylation.

Reason for Photosystem II join the light reaction

The two P700 reaction centers, each of which lost one electron, are unstable because of this. They are unable to absorb photon energy through the accessory pigments and are unable to release further electrons until the initially lost electrons have been replaced. Photosystem II now participates in the light response. 

Read more about the Gemmules.

Reactions of Photosystem II

NADPH was created by the first known photosystem, photosystem I. The later-discovered new photosystem was given the name PS II. The following is the list of reactions of photosystem II-

1. Photon hits PS II: The accessory pigments absorb one photon when it strikes photosystem II.
2. Photon → P680: The P680 reaction center receives the photon's energy through the auxiliary pigments. The P680 reaction center's outermost electron takes up the energy and is stimulated to a higher energy level.
3. P680 → Pheophytin (Pheo): The accessory pigments absorb one photon when
4. Comparing P680 to the next protein in the cascade, pheophytin, reveals that P680 has a larger redox potential due to the excited electron. Therefore, pheophytin receives the electron that was lost by the P680 reaction center.
5. Pheophytin → Plastoquinone (PQ): Plastoquinone, sometimes known as PQ, is the next electron acceptor. Inside the thylakoid membrane, it is found. A portable electron carrier, that is. It has two electrons in it. The release of a second electron from a second P680 reaction center is triggered by a second photon, and this second electron is then transferred to the PQ protein. Plastoquinone, or PQ, is converted toPQH2 by taking two electrons from the two pheophytins and two hydrogen ions from the stroma.
6. Plastoquinone → Cytochrome b6f: PQH2 crosses the thylakoid membrane from the exterior to the inside, where it interacts with cytochrome b6f, the following protein complex. The thylakoid membrane is covered with cyt B6F. Two H+ are released into the lumen, and two electrons are transferred to Cyt B6f once PQH2 reaches there.
7. Cytochrome b6f → Phycocyanin: The next protein receives the electrons, which are now transmitted once again one at a time. The potential of cytot b6f is greater than that of phycocyanin. PC receives an electron, whereas Cyt b6f loses an electron. A mobile electron carrier found inside the lumen is called PC, or phycocyanin. The unstable P700 is ultimately given an electron by the PC. The P700 is now stable. To fill the two-electron gap in the two P700s, two electrons from the Cyt b6f are utilized.