Photosynthesis

INTRODUCTION

Photosynthesis was a very process for plants to continue living in this planet. Plants sustained themselves without consuming anything from others which means they were autotrophs. Furthermore, organic compounds formed from the process of photosynthesis by plants were harvested by heterotrophs. In photosynthetic system, light energy was converted into chemical energy. It was also being used in the assimilation of carbon dioxide and the oxidation of water to molecular oxygen. The word equation for the reaction was shown below:

(Carbon Dioxide + Water), under the presence of light and chloroplast → Glucose + Oxygen

Light-dependent reaction and light-independent reaction were the two parts of photosynthesis.

During light-dependent reaction, light energy was being absorbed and used to split water molecule into hydrogen ions and electrons and also released oxygen. The formation of NADPH from NADP was from the release of electrons. The light-dependent reaction equation was shown below:

2H20      →       4H+    +     4e-    +   O2

        (light)

The main aim of this experiment is to investigate the light-dependent electron transport using DCPIP (2,6 dichlorophenol-indophenol). The colour of DCPIP will reduce or turn colourless when it accepts electrons produced during the light reactions. So, the rate of electron transport is measured by the rate of DCPIP loses colour. The hypothesis of this experiment is the faster the rate of photosynthesis, the faster the DCPIP turns colourless and the more faster is the decrease in absorbance (as measured with a spectrophotometer).

METHODS

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 absorbance was calibrated using Tube 1, which contained chloroplasts and sucrose only, as the blank, to ensure that any changes in colour 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

The results were observed from Figure 1. plot of absorbance vs time in silverbeet chloroplast under different conditions and solutions. There was slightly decrease in absorbance where the reaction mix was kept in the dark (Tube 2) (Fig. 1). However, results decreased and produced a negative gradient in absorbance where the reaction mix was placed under the light (Tube 3) (Fig. 1). As in Tube 4 (Fig. 1), the results were not consistent but were between 0.67 to 0.69. The absorbance was slightly decrease and slightly increase in (Tube 5) (Fig.1). In Test Tube 6 (Fig. 1), the results decreased, thus produced a negative gradient on the plot. Lastly, there was decrease in absorbance where the reaction mix was kept in a large green plastic tube (Tube 7) (Fig. 1) and produced a negative gradient on the plot.