An Investigation of Photosynthetic Electron Transport of Isolated Chloroplasts from Silverbeet Leaves

An Investigation of Photosynthetic Electron Transport of Isolated Chloroplasts from Silverbeet Leaves

Introduction

Photosynthesis is a vital process where plants use energy from sunlight to produce sugar for growth. Photosynthesis relies on two essential reactions with light, dependent and independent. Light dependent reactions occur in the first stage and requires the plant to capture light, using this as energy. Photons then split water molecules in half, releasing oxygen and hydrogen which drives ATP production during phosphorylation and yield NADPH. Light independent reactions then use carbon dioxide, the products of ATP and NADPH from light dependent stage to form glucose.

The aim of this report is to investigate the effects of different light treatments on photosynthetic electron transport by using isolated chloroplasts from silverbeet leaves. The indicator DCPIP (blue dye) as a substitute for NADP+; can determine the rate of photosynthesis. DCPIP turns colourless as it is reduced by accepting electrons, so a rapid decease in dye colour will give an estimation on the rate of photosynthesis.

Given this, I predict white light treatment to have the fastest rate of photosynthesis, followed by red light treatment, which will proceed at a more rapid rate of absorbance than the green light treatment. I also predict that dark treatment, boiled treatment and DCMU treatment, will produce no change in colour and therefore no photosynthesis reaction.  

Method

Seven spectrophotometer tubes were numbered and solutions A-D were added according to the volumes shown in Table 1.  Tube 1 was capped and inverted several times. The Spectrophotometer was calibrated using Tube 1, which contained chloroplasts and sucrose only, as the blank, to ensure that any changes in absorbance for the other treatments could be attributed to the reduction of the dye DCPIP.  At time zero (mins), absorbance was recorded for all treatments immediately after addition of DCPIP and mixing of contents.  Immediately following the time zero reading, tube 2 was wrapped in foil and tubes 6 and 7 were placed into larger tubes covered in red and green cellophane respectively.  Tubes 1-5 were also placed into larger tubes.  All tubes were then placed horizontally on ice, under lights.  At fifteen minute intervals, readings of absorbance were taken for all treatments, except for the dark tube which was kept wrapped in foil for 60 minutes, after which its absorbance was measured.

Table 1.  Experimental design for the electron transport experiment.

Results

To calculate the rate of photosynthesis absorbance values of light treatments were plotted against time (Figure 1). There was a massive decrease in absorbance for the light treatment (Tube 3)(Fig 1) as at start, showed a value of 0.806 and after 60 minutes, showed a value of 0.185. The red and green light treatments (Tubes 6 and 7)(Fig 1), showed slower decreases of absorbance: 0.355 (0.812 to 0.457) and 0.168 (0.765 to 0.597), respectively. There was a minor decrease in the Dark treatment (Tube 2)(Fig 1) from 0.805 to 0.779. The reaction mix in the DCMU treatment (Tube 5)(Fig 1) has demonstrated an increase in the first 15 minutes from 0.771 to 0.810, then a slight decrease in the next 45 minute down to 0.767. The boiled treatment (Tube 4)(Fig 1) has the least decrease of all treatments from 0.810 to 0.789 of absorbance.