Light Dependent Electron Transport, Using Dcpip

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Figure 1. Change in absorbance over time when under different treatments, measured against a control.

Discussion:

In all treatments the initial reading in the spectrophotometer was lower than expected, this may be due to the mixtures being poorly mixed and/or a delay of time between the addition of DCPIP and the readings being taken. In Tube 2 readings were only taken at 0 and 60 minutes the aforementioned point of the poorly mixed tubes could explain how the absorbance increased but as expected it did not decrease due to the mixture not being exposed to a light source. Tube 3 showed as significant decrease in absorbance as hypothesised, Tube 3 is most like that of a normal instance of a photosynthetic light dependant response. Tube 4 showed little to no change as the boiling denatured the chloroplasts so these results were expected. Again in the case of Tube 5, the mixture being poorly mixed is one possible reason for the increase in absorbance. In normal circumstances we would expect to see little to no change, this is because the DCMU inhibits electron transfer between the photosystems. Red light has a great potential for use as a light source to drive photosynthesis (Muneer et al. 2014) so it was expected to see a marked decrease in absorbance but as Fig.1 showed Tube 6 had only small decreases in absorbance, again the quality of mixing might have impacted results in the tube, as the expected decreases in absorbance were slightly greater than those seen in this experiment. Given that the pigments in chloroplasts are worst at absorbing green light (Muneer et al. 2014) it was expected to see little to no change in absorbance over the 60 minutes in Tube 7 and this is more or less what was seen in this experiment but given the errors in mixing and delay between readings (due to limited access to the spectrophotometer) the results overall weren’t exactly as hypothesised. Despite the small errors it was still possible to understand and witness how different levels of light and treatments affect the electron transfer.

References:

Muneer, S, Kim, E, Park, J, Lee, J (2014) Influence of Green, Red and Blue Light Emitting Diodes on Multiprotein Complex Proteins and Photosynthetic Activity under Different Light Intensities in Lettuce Leaves (Lactuca sativa L.). International Journal of Molecular Sciences 15, 4657-4670.