Hill reaction or Light reaction of photosynthesis

The below article deals you with the mechanism of photosynthesis. You will find the detail explaination of light reaction or Hill reaction. You will also find the cyclic and non-cyclic photophosphorylation and differences between them. To know about the mechanism of phtosynthesis, please read this article.


Modern researches have proved that at least three types of reactions take place in photosynthesis. These reactions are as follows:

  1. Trapping of light energy through pigment system or photosystem or quantasomes.

  2. Conversion of trapped light energy into chemical energy.

  3. Formation of carbohydrates

First two reactions take place in the presence of light, therefore, these are known as light reactions. On the other hand, the third reaction is much complex and does not require light, it is therefore known as a dark reaction. Thus the overall process of photosynthesis may be divided into two phases:

  1. Light reaction

  2. Dark Reaction

Light reaction

Light reaction of photosynthesis is controlled by light. Only due to this reaction, it is known as a light reaction. This reaction takes place in grana of chloroplast. In light reaction, light energy is used, through which ATP and reducing power NADPH2 is produced. This phase of photosynthesis was discovered by Robert Hill (1937). Therefore it is also known as Hill reaction. The light reaction is completed in the following four steps:

  • Absorption of light energy by chloroplast: Different types of chlorophyll molecules and other pigments absorb the energy of visible spectrum of solar radiation and transfer to the reaction centre.

  • Transfer of light energy from accessory pigments to chlorophyll-a: All the photosynthetic pigments other than Chl-a are called antenna or accessory pigments. These antenna chlorophylls absorb light energy and transfer them into the photoreaction centre or energy trapping centre. In PS-I energy trapping centre is P-700 whereas in PS-II it is P-680.

  • Activation of chlorophyll-a by photons of light: When these photoreaction centres (P-700 or P-680) absorb a photon of light, it is converted into an excited state (more energy state) from ground state and releases electrons which can cause photolysis of water molecules.

  • Photolysis of water: The photolysis of water molecules takes place during PS-II in the presence of Mn++ and Cl- ions. In photolysis of water, oxygen gas is released. Photolysis of water takes place in the presence of a strong oxidant which is unknown. It has been named as "Z".

  • Electron transport and production of assimilatory power: The electron expels from P-680 and P-700 after travelling through Electron Transport System (ETS) of photosynthesis, are either consumed in reducing NADP+ to NADPH + H+ or cycled back. The extra light energy is utilized in the formation of ATP molecules at different places during its transport. It is known as photosynthetic phosphorylation or simply photophosphorylation.

  • Arnon made an important piece of work in the field of conversion of light energy into chemical energy. According to Arnon, photophosphorylation may be of two types:

    1. Non-cyclic photophosphorylation

    2. Cyclic photophosphorylation

    Non-cyclic Photophosphorylation

    Hill and Bendall (1960) and Rabinowitch and Govind (1965) proposed a "Z" scheme to explain the mechanism of non-cyclic photophosphorylation. According to them, the two types of photochemical reactions (PS-I and PS-II) in the light phase of photosynthesis take place in a series. The product of one is consumed by others. For the first time, Robert Hill proposed that during photophosphorylation, chloroplast make use of cytochrome as it is used in respiration inside mitochondria. P-700 loses its electron when it gets energy from PS-I. The electron reaches to ferredoxin reducing substance (FRS). From FRS the electron reaches to Ferrodoxine. The electron from reduced ferredoxin then reduces NADP to NADPH with the help of H+ released from water.

    When a quantum of the wavelength of light of lower wavelength is received by PS-II its reaction centre P-680 loses an electron to a substance which is probably a Quinone. The electrons then travel downhill and fall back to +4 eV in a dark reaction through a series of PS-I. The carriers are cytochrome-b (Cyt-b), plastoquinone (PQ), cytochrome-f (Cyt-f) and plastocyanin (PC). The electron thus does not complete the cycle as it starts from PS-II and is drained off in the carbohydrates produced by carbon dioxide reduction. The energy released in the transfer of an electron from PQ to cytochrome-f is utilized to convert ADP and inorganic phosphate to ATP. The ATP synthesis resulting from this type of non-cyclic electron transport chain is known as non-cyclic photophosphorylation. Water molecular is utilized as a source of the electron in this system. In this process, two molecules of ATP are formed per two molecules of NADP+ reduced or one molecule of oxygen evolved or two molecules of water oxidized.

    2ADP+2iP+2NADP++2H2) = 2ATP+2NADPH+2H++O2

    In this way, we see that the electrons released from PS-II do not return to PS-II again. Therefore, this is known as non-cyclic autophosphorylation.

    Cyclic Photophosphorylation

    In addition to non-cyclic photophosphorylation, there is another pathway of ATP formation which involves only PS-I and wavelength of light greater than 680nm is used. This is cyclic photophosphorylation which takes place under certain conditions. E.g., when the amount of available NADP is low or PS-II is absent. It involves only PS-I and therefore, photolysis of water and the consequent evolution of oxygen does not take place. Non-cyclic electron transfer does not take place and NADPH is not formed. The electron lost by P-700 reaches to X, FRS, Fd and cytochrome-b6, cytochrome-f and plastocyanin and comes back into P-700. ATP molecules are synthesized from ADP and inorganic phosphate when electron is transferred from ferredoxin to cytochrome-b6 and cytochrome-b6to cytochrome-f. In this type of phosphorylation, the electrons ultimately reach to P-700 from which it was released. Therefore, the cycle is known as cyclic autophosphorylation.

    Difference between cyclic and non-cyclic photophosphorylation

    The differences between cyclic and non-cyclic photophosphorylation are given below:

    1. Cyclic photophosphorylation involves only PS-I but non-cyclic photophosphorylation involves PS-I and PS-II.

    2. An external source of electrons is not required because the same electrons get recycled in cyclic photophosphorylation but non-cyclic photophosphorylation requires an external electron donor.

    3. Oxygen is not released in cyclic photophosphorylation whereas oxygen gas is released in non-cyclic photophosphorylation.

    4. Cyclic photophosphorylation synthesizes only ATP but non-cyclic photophosphorylation synthesizes ATP and NADPH.

    5. Water is not consumed in cyclic photophosphorylation whereas water is consumed in non- cyclic photophosphorylation.

    6. Photolysis of water does not take place in cyclic photophosphorylation but photolysis of water takes place in non- cyclic photophosphorylation.

    Salient Features of light reaction

    The mechanism of light reaction proves that this is purely a photochemical reaction in which the following events occur:

    1. Photolysis of water and release of oxygen.

    2. Production of 18 molecules of ATP

    3. Production of 12 molecules of NADPH2

    The ATP and NADPH2 produced in light reaction are utilized in the process of carbon dioxide reduction during the dark reaction. In fact ATP and NADPH2 function as carrier of light energy to dark reaction. ATP and NADPH2 are regarded as assimilatory power. Likewise, NADPH2 functions as reducing power.


    Author: Umesh23 Jul 2020 Member Level: Diamond   Points : 2

    This is an elaborate article on Light reaction of photosynthesis explaining the process in details. Photosynthesis in plants is a very important activity which gives rise to oxygen which is one of the most important materials for the survival of life on Earth. As the sunlight is a continuously available entity the plants would go on manufacturing oxygen for us. It is imperative that plants being such an important part of our ecosystem, it is our duty to preserve them through forest maintenance and new plantations.

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