Light-dependent reaction
From Wikipedia, the free encyclopedia
The first stage of the photosynthetic system is the light-dependent reaction, which converts solar energy into chemical energy.
The light dependent reaction produces oxygen gas and converts ADP and NADP+ into the energy carriers ATP and NADPH.
Contents |
[edit] Electron transport
The process of synthesizing ATP and NADPH is accomplished via the mechanism of an electron transport chain. This is a series of proteins embedded in a biological membrane that transfers high-energy electrons from one to another, accomplishing various activities along the way as the electron drops in energy level. When sunlight strikes a cluster of chlorophyll molecules, the molecule is dulled, and an electron is switched to a higher energy level of the molecule. The excitation is transferred as an exciton from one antenna chlorophyll to another until it is captured by a primary reaction center. It can be transferred from one molecule to another of the same kind of pigment, or from a carotenoid to chlorophyll, but not from chlorophyll to a carotenoid, because excitation of carotenoids carries more energy than that of chlorophyll. Only chlorophyll of the reaction center is capable of transferring an electron to an electron acceptor (an intermediate, e.g., pheophytin in photosystem II and another chlorophyll molecule in Photosystem I). Because the energy in light corresponds to its wavelength, the difference in excitation energy also allows the carotenoids to absorb light at wavelengths that chlorophyll does not absorb well. The P680 (photosystem II) and P700 (photosystem I) refer to reaction center molecules of the two photosystems that receive excitons from other chlorophyll and accessory pigment molecules; each number 680 or 700 refers to the preferred wavelength of light absorbed, in the region of the spectrum, by the chlorophyll pigments at the respective reaction centers. These P680 and P700 molecules are in very low concentrations ( 1 molecule each per about 600 other chlorophyll molecules).
The rate of this stage of the light-dependent reactions can be monitored with the dye DPIP, or ferricyanide or methyl viologen, which accepts some of the electrons that would normally go to NADPH and changes color as a result.
The chlorophyll's electron can follow either of two different pathways, cyclic or non-cyclic.
[edit] Cyclic photophosphorylation
- See also: photophosphorylation
In cyclic electron flow, the electron begins in a pigment complex called photosystem I, passes from the primary acceptor to ferredoxin, then to a complex of two cytochromes (similar to those found in mitochondria), and then to plastocyanin before returning to chlorophyll. This transport chain produces a proton-motive force, pumping H+ ions across the membrane; this produces a concentration gradient which can be used to power ATP synthase during chemiosmosis. This pathway is known as cyclic photophosphorylation, and it produces neither O2 nor NADPH. In bacterial photosynthesis, a single photosystem is used, and therefore is involved in cyclic photophosphorylation.
[edit] Noncyclic photophosphorylation
The other pathway, noncyclic photophosphorylation, is a two-stage process involving two different chlorophyll photosystems. First, a water molecule is broken down into 2H+ + 1/2O2 + 2e-. The two electrons from the water molecule are kept in photosystem II, while the 2H+ and 1/2O2 are left out for further use. Then a photon is absorbed by the chlorophyll core of photosystem II, exciting the two electrons which are transferred to the acceptor molecule. The deficit of electrons is replenished by taking electrons from another molecule of water. The electrons transfer from the primary acceptor to plastoquinone, then to plastocyanin, producing proton-motive force as with cyclic electron flow and driving ATP synthesis.
The photosystem II complex replaced its lost electrons from an external source, however, the two other electrons are not returned to photosystem II as they would in the analogous cyclic pathway. Instead, the still-excited electrons are transferred to a photosystem I complex, which boosts their energy level to a higher level using a second solar photon. The highly excited electrons are transferred to the acceptor molecule, but this time are passed on to an enzyme called Ferredoxin- NADP reductase|NADP+ reductase, for short FNR, which uses them to catalyst the reaction (as shown):
- NADP+ + 2H+ + 2e- → NADPH + H+
This consumes the H+ ions produced by the splitting of water, leading to a net production of 1/2O2, ATP, and NADPH+H+ with the consumption of solar photons and water.
The concentration of NADPH in the chloroplast may help regulate which pathway electrons take through the light reactions. When the chloroplast runs low on ATP for the Calvin cycle, NADPH will accumulate and the plant may shift from noncyclic to cyclic electron flow.
[edit] Steps
It is important to note that both photosystems are almost simultaneously excited; thus, both photosystems begin functioning at almost the same time.
- Light strikes photosystem II and the energy is absorbed and passed along until it reaches P680 chlorophyll.
- The excited electron is passed to the primary electron acceptor. Photolysis in the thylakoid takes the electrons from water and replaces the P680 electrons that were passed to the primary electron acceptor. ( O2 is released as a waste product)
- The electrons are passed to photosystem I via the electron transport chain (ETC) and in the process used to pump protons across the thylakoid membrane into the lumen.
- The stored energy in the proton gradient is used to produce ATP which is used later in the Calvin-Benson Cycle.
- P700 chlorophyll then uses light to excite the electron to its second primary acceptor.
- The electron is sent down another ETC and used to reduce NADP+ to NADPH.
- The NADPH is then used later in the Calvin-Benson Cycle.
