Photosynthesis, Relationship Between Light Dependent and Light Independent Reactions

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PHOTOSYNTHESIS

INVESTIGATE LIGHT-DEPENDENT ELECTRON TRANSPORT USING DCPIP

Introduction

Plants make their own food from the sunlight through photosynthesis. Photosynthesis breaks down carbon dioxide and water into glucose and oxygen with the sunlight energy.

CO2 + 6H2O —(light) ——> C6H12O6 + 6O2

This glucose is a necessary fuel for the respiration of plants. In the plant cell, an organelle called the chloroplast is responsible for the process of photosynthesis. There are two stages of photosynthesis, called light dependent reaction and light independent reaction. Light dependent reaction is also known as Photosystem II, which occurs in the inner surface of the grana thylakoid membrane. It breaks down water to release oxygen, hydrogen and electrons. These electrons enter the electron transport chain to pass the H+ ions to generate ATP for more photosynthesis. Therefore, this experiment uses DCPIP(2,6-Dichlorophenolindophenol) as electron receptor, which turns colourless as it is reduced. Therefore, we can measure the rate of DCPIP turning colourless as the rate of electron transport. Hypothesis is that the higher the rate of DCPIP turning colourless, the higher the rate of photosynthesis.

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 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, all tubes (1-7) were placed in larger plastic tubes; tube 2 in a light-proof (black) tube, and tubes 6 and 7 in tubes covered in red and green cellophane respectively.   All tubes were then placed horizontally on ice, under lights.  At fifteen minutes intervals, readings of absorbance were taken for all treatments, except for the dark tube which was kept in the light-proof tube for 60 minutes, after which its absorbance was measured.

Results

Absorbance readings taken at 15 minutes intervals for each treatment.

Figure 1. Plot of absorbance vs. time for DCPIP to turn colourless

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Brief description:

Tube 2: The absorbance level dropped, but lesser than other tubes.

Tube 3: Absorbance level decreased the most amongst other tubes.