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Joseph Priestley through his Bell Jar Experiment demonstrated that when a burning candle and a live mice was placed inside a closed chamber made up of glass, the candle extinguished and the mice died after sometime. But when he placed a potted Mint plant in the same setup along with the mice and burning candle he noticed that the candle kept burning and the mice didn’t die. From this set of experiment, he was able to conclude that the burning candle which is showing combustion and the mice which is performing respiration released bad air in the closed chamber due to which the mice died and the angle extinguished but when a plant was placed in the same setup it consumed the foul air and converted it into good air which was later discovered as oxygen. Due to his work Priestley is credited with the Discovery of Oxygen.



Jan Ingenhousz was able to demonstrate that plants can release the good air or oxygen only in presence of light. He used an aquatic plant, Hydrilla for this experiment and observed the formation of bubbles of oxygen during daytime from green parts of the plant but during night time no such bubbles were produced. This experiment made it clear that sunlight is essential for photosynthesis.



Julius von Sachs proved that the product of photosynthesis is Glucose and it is stored in the form of Starch in the green parts of the plants now known as chloroplasts.



T.W. Engelmann, using a prism, a green filamentous algae Cladophora and some aerobic bacteria gave the Action Spectrum of Photosynthesis. A beam of visible light was allowed to pass through a prism to split it into its spectrum. When this visible spectrum was allowed to fall on the filamentous algae Cladophora, he observed that aerobic bacteria moved toward the Cladophora by showing aerotactic movement but accumulated only in the regions of red and blue wavelength and thus he concluded that photosynthesis occurs mainly in red and blue wavelength of the visible spectrum.



Fredrick Blackman concluded that photosynthesis in not a single step reaction but can be divided into two phases. One is the called Photochemical reaction or the Light reaction and the other is Dark reaction. Both the reactions occurs during the day time but the Light reaction is directly dependent on light while the Dark reaction is indirectly dependent on light. The site of Light reaction is the grana and stroma lamellae of Chloroplast but the dark reactions occurs in the stroma of the chloroplast. The product of Light reactions are ATP, NADPH and molecular oxygen O2 and these products are used in dark reaction to synthesize Glucose.



Otto Warburg performed the Intermittent Light Experiment on Chlorella. He observed when the algae is provided with continuous light the amount of Carbon dioxide CO2 reduced is less as compared to when the same algae is provided Intermittent light i.e. alternate light and dark periods. This resulted in more yield in the latter case. He proposed 2 explanations for the improvement in yield of the intermittent light. Either the reduction of CO2 continues in the dark or it proceeds twice as fast during the brief light flash as during the same length of time in continuous light.



Emerson and Arnold proved the existence of two distinct Photochemical process in the Light reaction of photosynthesis. One in which the flow of electrons follows a Non-Cyclic pathway and these electrons are ultimately accepted by an electron acceptor NADP which gets reduced into NADPH. In the second type, the flow of electrons occurs in such a way that the electron returns from where it is released thus forming a cyclic pathway.



Robin Hill isolated chloroplast from plant cells and placed them in a CO2 deficient setup along with electron acceptors. When these chloroplasts were illuminated with light the photolysis of water occurs and resulted in production of molecular Oxygen even in absence of CO2. He also noted that the electron acceptors got reduced.



Two scientists Ruben and Kamen, gave the experimental proof that O2 released during photosynthesis comes from photolysis of water and not from CO2. They used heavy isotope of Oxygen O18 to trace the products. When CO2 containing heavy isotope of Oxygen was used along with water having normal isotope of oxygen it was observed that the heavy isotope was present in Glucose and H2O but not in the Oxygen gas released. But when CO2 having normal isotope of oxygen and H2O having heavy isotope of Oxygen was used the heavy Oxygen was present in gaseous form but not in Glucose and H2O.



Melvin Calvin was able to trace all the intermediates involved in the dark reaction of photosynthesis and arranged it in the form of a pathway called C3 cycle or Calvin cycle. He used Radioactive isotope of carbon C14 ad radiotracer technique for his experiment. The organisms used were green algae Chlorella and Scenedesmus. For this discovery, he was awarded Nobel prize in the year 1961.



Hill and Bendall proposed the Z-scheme. Z-scheme is the arrangement of all the electron carriers involved in the Non-Cyclic Photophosphorylation on the basis of their redox potential and the explains the flow of electrons through it. The release of electrons from PS II due to photoexcitation is from high redox potential to low redox potential i.e. UPHILL. The flow of electron through the series of electron carriers to reach PS I is DOWNHILL. Photoexcitation of electron from PS I is again UPHILL and its flow through electron carriers to finally reach the terminal electron acceptor NADP is again DOWNHILL.




Peter Mitchell proposed the Chemiosmotic Hypothesis which explains the ATP formation in both mitochondria and chloroplast during Oxidative Phosphorylation and Photophosphorylation respectively. In both the cases, ATP formation requires presence of a Proton gradient which is created by with the help of proton pumps and membrane. When this proton gradient breaks through ATP synthase enzyme, it results in formation of ATP. He was awarded Nobel prize in 1978 for this hypothesis.




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