Light-dependent reaction

The first stage of the photosynthetic system is the light-dependent reaction, which converts solar energy into chemical energy.

Overview

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. The light reaction occurs in the stacked membranes of the grana in the thylakoid membrane. Oxygen is a byproduct.

Electron transport


The "Z scheme"

The process of synthesizing ATP and NADPH 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. When sunlight strikes a chlorophyll complex (cluster of chlorophyll), the molecule is excited, and an electron is transferred to a higher energy level of the molecule. The excitation is transferred as 'excitation energy' (also called 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 'excitation energy' from other chlorophyll and accessory pigment molecules; each number 680 or 700 refers to the preferred wavelength of light absorbed, in the red 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.

Cyclic 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 photophosporylation.

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 . 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 Ferredoxin- NADP+ reductase, for short FNR, which uses them to drive the reaction (as shown):

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.

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 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 a 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.

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