Light-dependent reaction
From Freepedia
The first stage of the photosynthetic system is the light-dependent reaction, which converts solar energy into chemical energy. Light absorbed by chlorophyll or other photosynthetic pigments such as carotene is used to drive a transfer of electrons and hydrogen from water (or some other donor molecule) to an acceptor called NADP+, reducing it to the form of NADPH by adding a pair of electrons and a single proton (hydrogen nucleus). The water or some other donor molecule is split in the process; it is the light reaction which produces waste oxygen.
The light reaction also generates ATP by powering the addition of a phosphate group to ADP, a process called photophosphorylation. ATP is a versatile source of chemical energy used in most biological processes. Note, however, that the light reaction produces no carbohydrates such as sugars.
Both of these processes are 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. The electrons originate when a photon of sunlight strikes a pigment molecule contained within a photosystem (cluster of associated pigment molecules) and excites one of its electrons. The excitation is transferred from one pigment molecule to another until it is captured by a primary acceptor protein. 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 is capable of transfer to an acceptor protein. 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 chlorophyll's electron can follow either of two different pathways, cyclic or non-cyclic.
Cyclic photophosphorylation
In cyclic electron flow, the electron originates 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.
Noncyclic photophosphorylation
The other pathway, noncyclic photophosphorylation, is a two-stage process involving two different chlorophyll photosystems. First, a photon is absorbed by the chlorophyll core of photosystem II, exciting two electrons which are transferred to the primary acceptor protein. The deficit of electrons is replenished by taking electrons from a molecule of water, splitting it into O2 and H+ (hydrogen ions). 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.
Since the photosystem II complex replaced its lost electrons from an external source, however, these 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 primary acceptor protein, but this time are passed on to ferredoxin, and then to an enzyme called NADP+ reductase which uses them to drive the reaction
- NADP+ + H+ + 2e- → NADPH
This consumes the H+ ions produced by the splitting of water, leading to a net production of O2, ATP, and NADPH with the consumption of solar photons and water.
