Light-dependent reactions
Light-dependent reactions are the chemical reactions involved in photosynthesis induced by light; all light-dependent reactions occur in thylakoids. There are two light-dependent reactions: the first occurs at photosystem II and the second occurs at photosystem I.
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 bf 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 bf 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 bf 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 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:
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.
Light-dependent reactions
In chemistry, many reactions depend on the absorption of photons to provide the energy needed to overcome the activation energy barrier and hence can be labelled light-dependent. Such reactions range from the silver halide reactions used in photographic film to the creation and destruction of ozone in the upper atmosphere. This article discusses a specific subset of these, the series of light-dependent reactions related to photosynthesis in living organisms.Reaction center
The reaction center is in the thylakoid membrane. It transfers absorbed light energy to a dimer of chlorophyll pigment molecules near the periplasmic side of the membrane. This dimer is called a special pair because of its fundamental role in photosynthesis. This special pair is slightly different in PSI and PSII reaction centers. In PSII, it absorbs photons with a wavelength of 680 nm, and is therefore called P680. In PSI, it absorbs photons at 700 nm and is called P700. In bacteria, the special pair is called P760, P840, P870, or P960. "P" here means pigment, and the number following it is the wavelength of light absorbed.Electrons in pigment molecules can exist at specific energy levels. Under normal circumstances, they are at the lowest possible energy level, the ground state. However, absorption of light of the right photon energy can lift them to a higher energy level. Any light that has too little or too much energy cannot be absorbed and is reflected. The electron in the higher energy level is unstable and will quickly return to its normal lower energy level. To do this, it must release the absorbed energy. This can happen in various ways. The extra energy can be converted into molecular motion and lost as heat, or re-emitted by the electron as light. The energy, but not the electron itself, may be passed onto another molecule; this is called resonance energy transfer. If an electron of the special pair in the reaction center becomes excited, it cannot transfer this energy to another pigment using resonance energy transfer. Under normal circumstances, the electron would return to the ground state, but because the reaction center is arranged so that a suitable electron acceptor is nearby, the excited electron is taken up by the acceptor. The loss of the electron gives the special pair a positive charge and, as an ionization process, further boosts its energy. The formation of a positive charge on the special pair and a negative charge on the acceptor is referred to as photoinduced charge separation. The electron can be transferred to another molecule. 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. Plant pigments usually utilize the last two of these reactions to convert the sun's energy into their own.
This initial charge separation occurs in less than 10 picoseconds. In their high-energy states, the special pigment and the acceptor could undergo charge recombination; that is, the electron on the acceptor could move back to neutralize the positive charge on the special pair. 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. Three factors in the structure of the reaction center work together to suppress charge recombination nearly completely:
- Another electron acceptor is less than 1 nanometer away from the first acceptor, and so the electron is rapidly transferred farther away from the special pair.
- An electron donor is less than 1 nm away from the special pair, and so the positive charge is neutralized by the transfer of another electron.
- The electron transfer back from the electron acceptor to the positively charged special pair is especially slow. The rate of an electron transfer reaction increases with its thermodynamic favorability up to a point and then decreases. The back transfer is so favorable that it takes place in the inverted region where electron-transfer rates become slower.
In chloroplasts
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.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, creating a non-cyclic electron flow. PSI releases FNR into the stroma, where it reduces 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:
Photosystem II
PSII is extremely complex, a highly organized transmembrane structure that contains a water splitting complex, chlorophylls and carotenoid pigments, a reaction center, pheophytin, and two quinones. It uses the energy of sunlight to transfer electrons from water to a mobile electron carrier in the membrane called plastoquinone:Plastoquinol, in turn, transfers electrons to cyt bf, which feeds them into PSI.
Water-splitting complex
The step → P680 is performed by an imperfectly understood structure embedded within PSII called the water-splitting complex or oxygen-evolving complex. It catalyzes a reaction that splits water into electrons, protons and oxygen,using energy from P680. The actual steps of the above reaction possibly occur in the following way :
2 OH. .
The electrons are transferred to special chlorophyll molecules that are promoted to a higher-energy state by the energy of photons.
Reaction center
The excitation P680 → P680 of the reaction center pigment P680 occurs here. These special chlorophyll molecules embedded in PSII absorb the energy of photons, with maximal absorption at 680 nm. Electrons within these molecules are promoted to a higher-energy state. This is one of two core processes in photosynthesis, and it occurs with astonishing efficiency 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.This is followed by the electron transfer P680 → pheophytin, and then on to plastoquinol, which occurs within the reaction center of PSII. The electrons are transferred to plastoquinone and two protons, generating plastoquinol, which released into the membrane as a mobile electron carrier. This is the second core process in photosynthesis. The initial stages occur within picoseconds, with an efficiency of 100%. The seemingly impossible efficiency is due to the precise positioning of molecules within the reaction center. This is a solid-state process, not a typical chemical reaction. It occurs within an essentially crystalline environment created by the macromolecular structure of PSII. The usual rules of chemistry do not apply in solid-state environments.