Light reaction or Primary photochemical reaction or Hill’s reaction

Light reaction or Primary photochemical reaction or Hill’s reaction
    • In light reaction, ATP and NADPH2 are produced and in the dark reaction, CO2 is reduced with the help of ATP and NADPH2 to produce glucose.
    • The light reaction is called primary photochemical reaction as it is induced by light.
    • Light reaction is also called as Hill’s reaction as Hill proved that chloroplast produce O2 from water in the presence of light.
    • It is also called as Arnon’s cycle because Arnon showed that the H+ ions released by the break down of water are used to reduce the coenzyme NADP to NADPH.
    • Light reaction includes photophosphorylation as ATP is synthesised in the presence of light.
    • The reaction takes place only in the presence of light in grana portion of the chloroplast and it is faster than dark reaction.
    • The chlorophyll absorbs the light energy and hence the chlorophyll is called as photosystem or pigment system.
    • Chlorophylls are of different types and they absorb different wavelengths of light.
    • Accordingly, chlorophylls exist in two photosystems, Photosystem I (PSI) and Photosystem II (PS II).
    • Both photosystems are affected by light with wavelengths shorter than 680nm, while PS I is affected by light with wavelengths longer than 680nm.
    The components of photosystems
    Photosystem I
    Photosystem II

    Chlorophyll a 670
    Chlorophyll a 680
    Chlorophyll a 695
    Chlorophyll a 700 or P700
    Chlorophyll b
    Carotenoids
    P700 form of Chlorophyll a is the active reaction centre

    Chlorophyll a 660
    Chlorophyll a 670
    Chlorophyll a 680 or P680
    Chlorophyll b
    Phycobilins
    Xanthophylls
    P680 form of Chlorophyll a is the active reaction centre

    The light reaction can be studied under the following headings
    Absorption of light energy by chloroplast pigments
    • Different chloroplast pigments absorb light in different regions of the visible part of the spectrum.
    Transfer of light energy from accessory pigments to chlorophyll a
    • All the photosynthetic pigments except chlorophyll a are called as accessory or antenna pigments.
    • The light energy absorbed by the accessory pigments is transferred by resonance to chlorophyll a which alone can take part in photochemical reaction.
    • Chlorophyll a molecule can also absorb the light energy directly.
    • In pigment system I, the photoreaction centre is P700 and in pigment system II, it is P680.
    Activation of chlorophyll molecule by photon of light
    • When P700 or P680 forms of chlorophyll a receives a photon (quantum) of light, becomes an excited molecule having more energy than the ground state energy.
    • After passing through the unstable second singlet state, and first singlet stage the chlorophyll molecules comes to the metastable triplet state.
    • This excited state of chlorophyll molecule takes part further in primary photochemical reaction i.e. the electron is expelled from the chlorophyll a molecule.
    Activation
    Photolysis of water and O2 evolution (oxidation of water)
    • These processes are associated with pigment system II and are catalyzed by Mn++ and Cl- ions.
    • When pigment system II is active i.e it receives the light, the water molecules split into OH- and H+ ions (Photolysis of water).
    • The OH- ions unite to form some water molecules again and release O2 and electrons.
    4H2O —>4H+ + 4 (OH-)
    4(OH-) —>2H2O + O2 + 4e-
    2H2O —>4H+ + O2 + 4e-
    Electron transport and production of assimilatory powers (NADPH2 and ATP)
    • It has already been observed that when chlorophyll molecule receives the photon of light, an electron is expelled from the chlorophyll a molecule along with extra energy.
    • This electron after traveling through a number of electron carriers is utilized for the production of NADPH2 from NADP and also utilized for the formation of ATP molecules from ADP and inorganic phosphate (Pi).
    • The transfer of electrons through a series of coenzymes is called electron transport and the process of formation of ATP from ADP and Pi using the energy of electron transport is called as photosynthetic phosphorylation or photophosphoryration.
    • The types of Phosphorylation include cyclic and non- cyclic.


Last modified: Wednesday, 6 June 2012, 5:41 PM