Photosynthesis: Light Reaction, The Electron Transport, and  Water Splitting

Light Reaction

The photochemical phase is also known as the light reaction. It involves the absorption of light, the splitting of water, the release of oxygen, and the production of high-energy chemical intermediates (ATP and NADPH). The process involves a large number of complexes.

Within Photosystem I (PS I) and Photosystem II (PS II), the pigments are arranged into two distinct photochemical light-harvesting complexes (LHC). The photosystems are called according to the order in which they were discovered, not their function during the light reaction.

Except for one molecule of chlorophyll a, each photosystem contains all of the pigments. The reaction centre is formed by a single chlorophyll molecule. PS I is named after the reaction centre chlorophyll a, which has an absorption peak at 700 nm. PS II has an absorption maximum at 680 nm, hence the name PS680.

Light energy transfer from accessory pigment to chlorophyll:

Resonance of photons transfers light energy collected by pigments other than chlorophyll a to chlorophyll -a. A photochemical reaction takes place in the chlorophyll. With p-690 and p-700, chlorophyll has two pigment systems.

Photoexcitation and activation of the chlorophyll-a molecule:

When a pigment molecule in a photosystem absorbs a photon of light, it is excited and releases the excess energy level known as the excited second singlet state; after that, it transitions to the Meta stable state, which is known as the triplet state. It receives electrons from the outside source and then returns to the ground state.

Photochemical reaction centres and light-harvesting antennas:

The light energy is absorbed by a collaboration of numerous chlorophyll and carotenoid molecules. The majority of the pigments function as an antenna complex, receiving light and transferring it to the reaction centre complex, which performs chemical oxidation and reduction reactions that lead to long-term energy storage. A single chlorophyll molecule absorbs only a few photons per second, even under strong sunshine. If each chlorophyll molecule had a reaction centre, the reaction centre enzymes would be inactive most of the time, only being activated by photon absorption on rare occasions. When a reaction centre receives energy from a high number of pigments at the same time, the system is kept active for a longer period. Each reaction centre has several hundred pigments connected with it, and each reaction centre must operate four times to create one molecule of oxygen – hence the 2500 chlorophylls per O2 value. The photosynthetic membrane contains the reaction centres and the majority of the antenna complexes. These membranes are present within the chloroplast in eukaryotic photosynthetic organisms, while the plasma membrane or membranes generated from it is the location of photosynthesis in photosynthetic prokaryotes.

Photochemistry has a quantum yield of about 1.0, but each molecule of O2 requires the actions of roughly 10 photons, therefore the overall maximal quantum yield of O2 generation is about 0.1. Any photon absorbed by chlorophyll or other pigments is equally effective in causing photosynthesis like any other photon. However, in the far-red area of chlorophyll absorption, the output declines substantially (greater than 680 nm). The enhancing effect was found by Emerson. He employed two different wavelengths of light to determine the rate of photosynthesis and then combined the two beams. The rate of photosynthesis was greater than the sum of the individual rates when red and far-red light were combined


Red light with a wavelength of 680 nm is absorbed by the reaction centre in PS II. This causes electrons to become excited and move away from the atomic nucleus onto a different orbit. Electron acceptors collect these electrons and send them to cytochromes, which serve as an electron transport mechanism.

This electron flow is downhill on an oxidation-reduction potential scale. As electrons go through the electron transport chain, they are not consumed. They are handed down to the PS I pigments. When electrons in PS I’s reaction centre are exposed to red light with a wavelength of 700 nm, they are also excited. They are subsequently transferred to a molecule with a higher redox potential as an acceptor.

 Z Scheme: The electrons are subsequently transferred downhill to a molecule of energy-rich NADP+. NADP+ is reduced to NADPH+ H+ by adding these electrons. The Z scheme is named after the unique shape of the electron transport from PS II to the acceptor, PS I, another acceptor, and ultimately NADP+.

Water Splitting

H+, [O], and electrons are formed when water is split. PS II is linked to water splitting. This releases oxygen into the atmosphere. Electrons that have been eliminated from Photosystem I are replaced by Photosystem II.

 2H2O——- 4H+ 6O2 + 4e

 PSII is linked to the water-splitting complex.

1. Manganese, chlorine, and other elements have a significant impact.

2. Photolysis is the light-dependent splitting of water.

3. The electrons produced are used to replace those lost by P680.

4. P680 absorbs light and converts to a powerful oxidising agent, splitting a water molecule to liberate oxygen. Photosynthesis produces oxygen as a byproduct.

5. Protons are involved in the conversion of NADP to NADPH+.

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