Light reaction

Light reaction, also called as "light-dependent reaction" convert visible light into chemical energy in the form of NADPH and ATP.
So-called light reaction is complex process that completes in a series of steps in the thylakoid membrane. This includes absorption of light by photosynthetic pigments, photo-oxidation of water molecule, transfer of electrons from water to NADP⁺ through a number of carriers, establishment of proton gradient across the thylakoid membrane and formation of high-energy bond between ADP and P_i to form ATP.
As the complete process take place in the presence of light and terminate into the phosphorylation of ADP to form ATP, this is also known as photophosphorylation.

Photophosphorylation

Schematic representation of photophosphorylation / light reaction / Z-scheme

Light reaction starts with light absorption by photosynthetic pigments

Light energy is absorbed by the antenna pigments of photosystem II (PSII) and transferred to the reaction center chlorophyll molecule (P680) by inductive resonance.

The excited reaction center returns to its ground energy state by emitting the e^- to the e^--carrier present in the thylakoid membrane.

PSII has Oxygen Evolving Complex (OEC) for photolysis of water

OEC attached to PSII towards the lumen side of thylakoid breaks the water molecule.

2H₂O → O₂ + 4e^- + 4H⁺

e^- released here is accepted by the reaction center of PSII and H⁺ released in the lumen establish H⁺ gradient across the membrane. O₂ is released in the atmosphere.

e^- emitted by PSII reaches to the reaction center of photosystem I (PSI)

e^- released from reaction center of PSII through a number of mobile carrier including phyeophytin, quinone cycle, cytochrome b₆f complex and plastocyanin reaches to the reaction center of PSI.

In quinone cycle plastoquinone (PQ) is reduced to plastohydroquinone (PQH₂). During this reduction, e^- comes from oxidation of pheophytin, while H⁺ comes from the stroma. PQH₂ is then oxidized to PQ releasing the e^- to cytochrome b₆f complex and plastocyanin, while H⁺ is released to the lumen side again establishing the H⁺ gradient across the membrane.

e^- released from PSI reduces NADP⁺ to NADPH

Light energy is absorbed by the antenna pigments of PSI and transferred to the reaction center chlorophyll molecule (P700) by inductive resonance.

The excited P700 returns to its ground energy state by emitting the e^- to the e^--carriers.

This e^- finally reaches to the ferridoxin, where an enzyme so-called ferridoxin-NADP reductase (FNR) reduces NADP⁺ to NADPH.

FNR is located towards the stromal side of thylakoid membrane, which takes H⁺ from stroma during reduction, again increasing the H⁺ gradient across the membrane.

Chemiosmotic mechanism of ATP synthesis

Electrochemical potential gradient across the thylakoid membrane drives ATP synthesis

Pumping of H⁺ inside the lumen during the electron transport chain establishes electrochemical potential gradient across the thylakoid membrane.

These H⁺ passes back to the stroma side through the H⁺ ATP synthase complexes.

H⁺ ATP synthase complex has a trans-membrane protein (CF₀), that provides the passage of H⁺ back to the stroma.

The energy stored in H⁺ in the form of electrochemical potential is utilized by the protruding part (CF₁) of the H⁺ ATP synthase complexes in the formation of high-energy phosphoanhydride bond between ADP and P_i to form ATP.

In the above described light-dependent reaction, e^- released from water is finally accepted by NADP⁺ and never returns back. Hence the process is also known as non-cyclic photophosphorylation.

Cyclic photophosphorylation

In certain conditions, e.g., in C4 cycle more ATP is needed to fix the CO₂. To fulfill the need of more ATP, some of the e^- from excited reaction center of PSI return back to it via the cytochrome b₆f complex and plastocyanin. During this cyclic flow of e^- quinone cycle operate in association with cytochrome b₆f complex increasing the concentration of H⁺ gradient across the membrane. This ultimately increases the synthesis of ATP. This is known as cyclic photophosphorylation.
As in cyclic photophosphorylation, PSII and OEC are not involved, hence no water is oxidized, no O₂ is evolved and no NADPH is produced.

"Z-scheme"

The process of photophosphorylation takes place in the thylakoid membrane. All the components of light reactions are spatially arranged according to their function. Some are fixed towards the stroma or lumen side, while others are readily mobile in the hydrophilic core of the membrane.
If all these components are arranged in a sequence on an scale according to their redox potential it form a zig-zag or Z-like appearance. This is what we know as "Z-scheme". "Z-scheme" contains both non-cyclic and cyclic photophosphorylation as well.

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First published on 13-04-2021
Last updated on 08-11-2021