Photosystem

Reaction centers are multi-protein complexes found within the thylakoid membrane.

At the heart of a photosystem lies the reaction center, which is an enzyme that uses light to reduce and oxidize molecules (give off and take up electrons). This reaction center is surrounded by light-harvesting complexes. The pigments that absorb the highest energy light are furthest from the reaction center. The energy they acquire is transferred - or funneled - to the inner part. At the reaction center, the special chlorophyll molecule will be excited, inducing electron transfer. In rare cases, ET does not occur, so the excited state relaxes (loses energy) by fluorescence back to the ground state.^[3] that can then be used by the chloroplast.^[2]

Two families of reaction centers in photosystems can be distinguished: type I reaction centers (such as photosystem I (P700) in chloroplasts and in green-sulfur bacteria) and type II reaction centers (such as photosystem II (P680) in chloroplasts and in non-sulfur purple bacteria). The two photosystems evolved from a common ancestor.^[4][5]

Each of the photosystem can be distinguished by the wavelength of light to which it is most reactive (700 nanometers for PSI and 680 nanometers for PSII in chloroplasts), the amount and type of light-harvesting complex present, and the type of terminal electron acceptor used.

Type I photosystems use ferredoxin-like iron-sulfur cluster proteins as terminal electron acceptors, while type II photosystems ultimately shuttle electrons to a quinone terminal electron acceptor. Both reaction center types are present in chloroplasts and cyanobacteria, and work together to form a unique photosynthetic chain able to extract electrons from water, creating oxygen as a byproduct.

Each photosystem has two main subunits: an antenna complex (a light harvesting complex or LHC) and a reaction center. The antenna complex is where light is captured, while the reaction center is where this light energy is transformed into chemical energy. At the reaction center, there are many polypeptides that are surrounded by pigment proteins. At the center of the reaction center is a special pair of chlorophyll molecules. A reaction center comprises several (about 25-30)^[6] protein subunits, which provide a scaffold for a series of cofactors. The cofactors can be pigments (like chlorophyll, pheophytin, carotenoids), quinones, or iron-sulfur clusters.^[7]

                        plastocyanin → P700 → P700* → FNR → NADPH
                             ↑                ↓
                           b6f        ←    phylloquinone

Both photosystem I and II are required for oxygenic photosynthesis. Oxygenic photosynthesis can be performed by plants and cyanobacteria; cyanobacteria are believed to be the progenitors of the photosystem-containing chloroplasts of eukaryotes. Photosynthetic bacteria that cannot produce oxygen have only one photosystem, which is similar to either PSI or PSII.

At the core of photosystem II is P680, a special chlorophyll to which incoming excitation energy from the antenna complex is funneled. One of the electrons of excited P680* will be transferred to a non-fluorescent molecule, which ionizes the chlorophyll and boosts its energy further, enough that it can split water in the oxygen evolving complex (OEC) of PSII and recover its electron.^{[citation needed]} At the heart of the OEC are 4 Mn atoms, each of which can trap one electron. The electrons harvested from the splitting of two waters fill the OEC complex in its highest-energy state, which holds 4 excess electrons.^[2]

Electrons travel through the cytochrome b6f complex to photosystem I via an electron transport chain within the thylakoid membrane. Energy from PSI drives this process^{[citation needed]} and is harnessed (the whole process is termed chemiosmosis) to pump protons across the membrane, into the thylakoid lumen space from the chloroplast stroma. This will provide a potential energy difference between lumen and stroma, which amounts to a proton-motive force that can be utilized by the proton-driven ATP synthase to generate ATP. If electrons only pass through once, the process is termed noncyclic photophosphorylation, but if they pass through PSI and the proton pump multiple times it is called cyclic photophosphorylation.

When the electron reaches photosystem I, it fills the electron deficit of light-excited reaction-center chlorophyll P700⁺ of PSI. The electron may either continue to go through cyclic electron transport around PSI or pass, via ferredoxin, to the enzyme NADP⁺ reductase. Electrons and protons are added to NADP⁺ to form NADPH. This reducing (hydrogenation) agent is transported to the Calvin cycle to react with glycerate 3-phosphate, along with ATP to form glyceraldehyde 3-phosphate, the basic building block from which plants can make a variety of substances.

In intense light, plants use various mechanisms to prevent damage to their photosystems. They are able to release some light energy as heat, but the excess light can also produce reactive oxygen species. While some of these can be detoxified by antioxidants, the remaining oxygen species will be detrimental to the photosystems of the plant. More specifically, the D1 subunit in the reaction center of PSII can be damaged. Studies have found that deg1 proteins are involved in the degradation of these damaged D1 subunits. New D1 subunits can then replace these damaged D1 subunits in order to allow PSII to function properly again.^[8]

In non-cyclic photophosphorylation, PSII absorbs a photon to produce a so-called high energy electron which transfers via an electron transport chain to cytochrome b₆f and then to PSI. The then-reduced PSI absorbs another photon producing a more highly reducing electron, which converts NADP⁺ to NADPH. In the cyclic form, only PSI is involved, and the electron is moved through cytochrome b₆f before returning to PSI; NADPH is not produced. Thus, the balance between the two types of photophosphorylation is important to maintain the concentrations of ATP and NADPH in the right proportion for the light-independent reactions. For both cyclic and non-cyclic photophosphorylation, cytochrome b₆f produces a proton gradient across the thylakoid membrane that creates a proton-motive force, which is then used by ATP synthase to form ATP.

In oxygenic photosynthesis, the first electron donor is water, creating oxygen (O₂) as a by-product. In anoxygenic photosynthesis, various electron donors are used.

The net-reaction of the light-dependent reactions using non-cyclic photophosphorylation in oxygenic photosynthesis is:

2H₂O + 2NADP⁺ + 3ADP + 3P_i → O₂ + 2H⁺ + 2NADPH + 3ATP

PSI and PSII are light-harvesting complexes. If a special pigment molecule in a photosynthetic reaction center absorbs a photon, an electron in this pigment attains the excited state and then is transferred to another molecule in the reaction center. This reaction, called photoinduced charge separation, is the start of the electron flow and transforms light energy into chemical forms.

The photosynthesis process in chloroplasts begins when an electron of P680 of PSII attains a higher-energy level. This energy is used to reduce a chain of electron acceptors that have subsequently higher redox potentials. This chain of electron acceptors is known as an electron transport chain. When this chain reaches PSI, an electron is again excited, creating a high redox-potential. The electron transport chain of photosynthesis is often put in a diagram called the Z-scheme, because the redox diagram from P680 to P700 resembles the letter Z.^[9]

The final product of PSII is plastoquinol, a mobile electron carrier in the membrane. Plastoquinol transfers the electron from PSII to the proton pump, cytochrome b6f. The ultimate electron donor of PSII is water. Cytochrome b₆f transfers the electron chain to PSI through plastocyanin molecules. PSI can continue the electron transfer in two different ways. It can transfer the electrons either to plastoquinol again, creating a cyclic electron flow, or to an enzyme called FNR (Ferredoxin—NADP(+) reductase), creating a non-cyclic electron flow. PSI releases FNR into the stroma, where it reduces NADP⁺
to NADPH.

Activities of the electron transport chain, especially from cytochrome b₆f, lead to pumping of protons from the stroma to the lumen. The resulting transmembrane proton gradient is used to make ATP via ATP synthase.

The overall process of the photosynthetic electron transport chain in chloroplasts is:

PSII, PSI, and cytochrome b₆f are found in chloroplasts. All plants and all photosynthetic algae contain chloroplasts, which produce NADPH and ATP by the mechanisms described above. In essence, the same transmembrane structures are also found in cyanobacteria.

Unlike plants and algae, cyanobacteria are prokaryotes. They do not contain chloroplasts; rather, they bear a striking resemblance to chloroplasts themselves. This suggests that organisms resembling cyanobacteria were the evolutionary precursors of chloroplasts. One imagines primitive eukaryotic cells taking up cyanobacteria as intracellular symbionts in a process known as endosymbiosis.

         H
2O → PSII → plastoquinol → b6f → cytochrome c6 → PSI → ferredoxin  →  NADPH
                                                  ↑                     ↓
                                                 b6f       ←     plastoquinol

           NADH dehydrogenase → plastoquinol → b6f → cyt c6 → cyt aa3 → O
2

                            P840 → P840* → ferredoxin → NADH
                              ↑                ↓
                           cyt c553 ← bc1 ← menaquinol

The first ideas about light being used in photosynthesis were proposed by Jan IngenHousz in 1779^[15] who recognized it was sunlight falling on plants that was required, although Joseph Priestley had noted the production of oxygen without the association with light in 1772.^[16] Cornelis Van Niel proposed in 1931 that photosynthesis is a case of general mechanism where a photon of light is used to photo decompose a hydrogen donor and the hydrogen being used to reduce CO
₂.^[17] Then in 1939, Robin Hill demonstrated that isolated chloroplasts would make oxygen, but not fix CO
₂, showing the light and dark reactions occurred in different places. Although they are referred to as light and dark reactions, both of them take place only in the presence of light.^[18] This led later to the discovery of photosystems I and II.

• Light reaction
• Photoinhibition
• Photosynthetic reaction centre