Introduction Modern researches have proved that at least three types of reactions take place in photosynthesis. These reactions are as follows:
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:
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:
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:
Non-cyclic PhotophosphorylationHill 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:
Salient Features of light reaction The mechanism of light reaction proves that this is purely a photochemical reaction in which the following events occur:
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.
Hakimuddin Kuwakhedawala is based in India with over 15 years experience as a teacher. Teaching and writing are his passion. Most of the articles of Hakimuddin are related to education, exam, environmental problems, etc.
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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.