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Introduction

Name: Kern Pemberton Date: 7th November, 2008 Lab#: 5 Aim: To investigate the hill reaction Theory: Photosynthesis is a process carried out in plants, in the photosynthetic organ which is the leaf. CO2 + H2O --> (CH2O)n + O2 A plant takes in CO2 and H2O and absorbs sunlight in order for photosynthesis to take place. These reactants then travel to the chloroplasts in the palisade cells of the leaf, where the actual process takes place. In photosynthesis there are two reactions; the light dependant reaction and the light independent reaction. The light dependant reaction takes place in thylakoids of the grana of the chloroplasts whereas the light independat takes place in the stroma. The thylakoids of the chloroplasts contain pigments which can be divided into accessory and primary pigments. These pigments form light harvesting clusters which can then be divided into different photosystems, those being photosystem I and photosystem ll. In photosystem I, the accessory pigments trap energy from the sunlight and funnel it to the primary pigment or chlorophyll a. This then becomes excited and releases an electron which is accepted by an electron acceptor. The electron then moves through a series of electron carriers arranged in order of their redox potentials. Meanwhile, the photosystem is said to be unstable as a result of chlorophyll a losing an electron. ...read more.

Middle

7. 0.5 ml of the chloroplast suspension was placed in a test tube followed by 5 ml of DCPIP. This was then placed in the dark and the aluminium foil was not removed. 8. The time taken for a colour change from blue to green was then observed and recorded. Diagram: Diagram 1.0 shows setup of apparatus to check time taken for colour changes in tubes. Results: Tube Time taken for colour change from blue to colourless/min 10 cm away from light source 1.53 100 cm away from light source 15.55 Isolation in medium only No change Tube in dark No change Table shows different times for colour changes to be observed. Discussion: Photosystem ll as mentioned in the theory, NADPH and 02. In this process light strikes both photosystem l and ll, the energy is funneled down by the accessory pigments to the primary pigment and this causes the primary pigment from PS l and PS ll to release an electron. The electron from PS l is accepted by an electron acceptor and then passes through a series of electron carriers arranged in their redox potentials. The electron then is accepted by another electron acceptor and then neutralizes the H+ in the splitting of water. H2O - 2H+ + 2e- + O2 This H is then picked up by NADP and forms NAPH. ...read more.

Conclusion

This is because the isolation medium contained no chlorophyll for photosynthesis to take place. However, when the tube with the chloroplast suspension which includes the isolation medium was placed in a tube, but in the dark, there was no colour change either. This shows that light is needed for photosynthesis to take place. Precautions: * The amount of chloroplast suspension and DCPIP placed in the tube was standardized. * The distance of the light source from the tube was ensured with a centimeter ruler. * The salt/ice water was used to preserve the samples so that no biological activity would have taken place before the experiment had taken place. * The aluminium foil was used so that no light would have entered the sample before the experiment started. Limitations: * The DCPIP was blue in colour and the chloroplast suspension was blue in colour. Therefore, the colour change was not from blue to colourless and this could have affected one's observation of the colour change. Sources of error: * The experiment was done during the day and sunlight entering the room could have affected the results in some way of it had any exposure to the test tubes. Improvements: * It could have been ensured that the room was completely dark or sunlight was not reaching the tubes in any way. * The amount of light could have been reduced by putting layers of tissue paper between the light and the test tube to produce a range of light intensities. ...read more.

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