Light-dependent reactions
The then-reduced PSI, absorbs another photon producing a more highly reducing electron, which converts NADP+ to NADPH.
In non-cyclic photophosphorylation, cytochrome b6f uses electrons from PSII and energy from PSI[citation needed] to pump protons from the stroma to the lumen.
In cyclic photophosphorylation, cytochrome b6f uses electrons and energy from PSI to create more ATP and to stop the production of NADPH.
Cyclic phosphorylation is important to create ATP and maintain NADPH in the right proportion for the light-independent reactions.
This reaction, called photoinduced charge separation, is the start of the electron flow and transforms light energy into chemical forms.
This article discusses a specific subset of these, the series of light-dependent reactions related to photosynthesis in living organisms.
It transfers absorbed light energy to a dimer of chlorophyll pigment molecules near the periplasmic (or thylakoid lumen) side of the membrane.
The extra energy can be converted into molecular motion and lost as heat, or re-emitted by the electron as light (fluorescence).
The loss of the electron gives the special pair a positive charge and, as an ionization process, further boosts its energy.
As the ionized pigment returns to the ground state, it takes up an electron and gives off energy to the oxygen evolving complex so it can split water into electrons, protons, and molecular oxygen (after receiving energy from the pigment four times).
Its return to the special pair would waste a valuable high-energy electron and simply convert the absorbed light energy into heat.
In the case of PSII, this backflow of electrons can produce reactive oxygen species leading to photoinhibition.
Activities of the electron transport chain, especially from cytochrome b6f, lead to pumping of protons from the stroma to the lumen.
The overall process of the photosynthetic electron transport chain in chloroplasts is: PSII is extremely complex, a highly organized transmembrane structure that contains a water splitting complex, chlorophylls and carotenoid pigments, a reaction center (P680), pheophytin (a pigment similar to chlorophyll), and two quinones.
The actual steps of the above reaction possibly occur in the following way (Kok's diagram of S-states): (I) 2H2O (monoxide) (II) OH.
[citation needed] (Dolai's mechanism) The electrons are transferred to special chlorophyll molecules (embedded in PSII) that are promoted to a higher-energy state by the energy of photons.
These special chlorophyll molecules embedded in PSII absorb the energy of photons, with maximal absorption at 680 nm.
This is one of two core processes in photosynthesis, and it occurs with astonishing efficiency (greater than 90%) because, in addition to direct excitation by light at 680 nm, the energy of light first harvested by antenna proteins at other wavelengths in the light-harvesting system is also transferred to these special chlorophyll molecules.
When the excited chlorophyll P680* passes the electron to pheophytin, it converts to high-energy P680+, which can oxidize the tyrosineZ (or YZ) molecule by ripping off one of its hydrogen atoms.
The high-energy oxidized tyrosine gives off its energy and returns to the ground state by taking up a proton and removing an electron from the oxygen-evolving complex and ultimately from water.
[4] Kok's S-state diagram shows the reactions of water splitting in the oxygen-evolving complex.
PSII and PSI are connected by a transmembrane proton pump, cytochrome b6f complex (plastoquinol—plastocyanin reductase; EC 1.10.99.1).
One imagines primitive eukaryotic cells taking up cyanobacteria as intracellular symbionts in a process known as endosymbiosis.
Purple bacteria contain a single photosystem that is structurally related to PSII in cyanobacteria and chloroplasts: This is a cyclic process in which electrons are removed from an excited chlorophyll molecule (bacteriochlorophyll; P870), passed through an electron transport chain to a proton pump (cytochrome bc1 complex; similar to the chloroplastic one), and then returned to the chlorophyll molecule.
As in cyanobacteria and chloroplasts, this is a solid-state process that depends on the precise orientation of various functional groups within a complex transmembrane macromolecular structure.
Green sulfur bacteria contain a photosystem that is analogous to PSI in chloroplasts: There are two pathways of electron transfer.
They are of interest because of their importance in precambrian ecologies, and because their methods of photosynthesis were the likely evolutionary precursors of those in modern plants.
[11] Then in 1939, Robin Hill demonstrated that isolated chloroplasts would make oxygen, but not fix CO2, showing the light and dark reactions occurred in different places.
