Hills Reaction to Demonstrate the Electron Transfer during the Light Reaction of Photosynthesis - Essay Example

? HILL’S REACTION TO DEMONSTRATE THE ELECTRON TRANSFER DURING THE LIGHT REACTION OF PHOTOSYNTHESIS By: Presented Aim The aim of the practical is to learn about cell fractionation and how isolated thykaloid membranes can be used for photosynthetic reactions (Lab 6, n.d.; Marietta University, n.d.). It also helps in learning how quantitative measurements of photosynthetic electron transport can be made through the Hill reaction. It is aimed at demonstrating the Hill reaction of photosynthesis and the reaction conditions that affect it. At the end of the practical, the learner should produce a report that can be used by other scientists to understand the light dependent reaction in photosynthesis and the electron transfer to high energy molecules like ATP and NADP+ and how they are substituted by DCPIP (Dichlorophenol Indophenol) in the Hill reaction to demonstrate the same. The learner should also explain the source of the electrons that are transported in the light reaction of the light dependent reaction of photosynthesis. Introduction Photosynthesis takes place in plants which have chlorophyll, in some algae and some bacteria. Photosynthesis needs chlorophyll-the green coloring pigment found in the chloroplasts. The process takes place in two phases: the light dependent reaction and the light independent reaction. The light-dependent reaction takes place in the thykaloid membranes while the light independent reaction takes place in the stroma of chloroplast. At this phase light is taken up by chlorophyll molecules and transferred to electrons of a water molecule. This energy causes the water molecules to split releasing an oxygen atom and two hydrogen atoms. The electrons carry energy which used in the synthesis of high energy molecules like ATP and NADPH2. 2H2O + 3 ADP + NADP +3Pi = O2 +3ATP + NADPH2                 Light energy is needed in the process. The high energy bond molecules produced in the light-dependent reaction i.e. ATP and NADPH2 are used in the carbon fixation process where carbon dioxide is converted into simple carbohydrates like glucose. Carbon fixation takes place in the stroma of the chloroplast while the light reactions take place in the thykaloid membranes. The Hill reaction is aimed at proving that the two reactions can take place independent of each other. This can be done if the thykaloid membranes (sites for light reaction) and the stroma (site for carbon fixation) are uncoupled or separated by cell fractionation. Using the isolated thykaloids, the light reactions will proceed without carbon fixation if light is present. It is an in-vintro type of reaction. DCPIP is used in Hill’s reaction to be the electron acceptor to accept the electrons after water molecules are split. It is blue in when oxidized state and colorless when in the reduced state. DCPIP (oxidized) + 2e = DCPIP (reduced) (Blue)                                    (Colorless) This way the electron transport rate in the Hill’s reaction can be accounted for spectrophotometrically on a 620 nm wavelength by the change in absorbance of DCPIP as it accepts electrons by the conversion of water molecules in the chain. The rate of electron transport is a function of light intensity. Method Hill’s reaction was used to show how photosynthesis occurs in two distinct reactions. It also shows the role of light in photosynthesis which is shown by the difference in the readings of the spectrophotometer between tubes 3 and 4 where tube 3 is placed in the presence of light and tube 4 in the dark. Also boiling the solution destroys the chloroplasts in it, thus there is no observable change in absorption. This shows there was little or no photosynthetic reaction taking place. Ice cold solutions were used to stop any reactions that could have affected or altered the results in the Hill’s reaction in the electron transport chain. The reaction conditions were light to produce the needed energy, carbon dioxide and water which is broken down to produce the electrons which are needed in the transport chain. Results In tube 2, the absorbance at 620 nm showed a negligible change. This shows that the DCPIP was not reduced. It shows that the chloroplasts could have been denatured thus hindering the electron transfer chain. In tube 3 the absorbance at a wavelength of 620 nm shows that the DCPIP was reduced and that explains the reduction in the values from 0.124 to 0.002. This is because the factors are optimum and hence photosynthesis takes place successfully. The electron transport chain takes place, which leads to the reduction of the DCPIP hence the change in absorbance. In tube 4, the absorbance at a wavelength of 620 nm shows that there was partial reduction of the DCPIP. It shows that even in darkness there was some process taking place which could be the carbon fixation as carbon dioxide is converted into simple carbohydrates like glucose. Discussion From the results above, the learner got results from three tubes: 2, 3 and 4. In tube 2 the solution is boiled and hence reduction of DCPIP which would cause the absorbance to reduce was not witnessed. This shows that the chloroplasts had been denatured hindering photosynthesis from taking place. The reaction in the thykaloids does not proceed. In tube 3, the solution is not boiled and all the factors needed for the light reaction are available. The DCPIP is reduced because the electrons are available after the water molecules are split. DCPIP accepts these electrons hence it is reduced from its blue color to colorless. This accounts for the change in absorbance in the spectrophotometer. In tube 4, there is partial reduction of DCPIP which shows there is minimal reaction thus accounting for the change in absorbance. A possible explanation could be that there is conversion of carbon dioxide into simple carbohydrates like glucose. Conclusion Hill’s reaction successfully explained that the process of photosynthesis occurs in two phases (reactions) and that it involves the breakdown of water molecules which act as electron donors and DCPIP replaces NADP+ as the electron acceptor. The color change which signifies change in absorbance explains this. This experiment proves that the process of photosynthesis involves a chain of electron transfer and hence the aim was attained. Bibliography Marietta University, n.d. Measuring Photosynthetic Electron Transport (Hill reaction). [online] Laboratory 6, n.d. 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