Photosynthesis

PRACTICAL 2

Introduction

Photosynthesis is a process which occurs in leaves of plants. It can synthesize organic compounds from inorganic raw materials in the presence of light. Photosynthesis uses carbon dioxide and water to produce organic compounds and is capable to convert chemical energy from solar energy to make sugar (Silverstein, Silverstein and Nunn, 1998). There are two stages of photosynthesis. The first stage is light reaction and the second one is known as dark reaction.

Light reaction occurs in the thylakoid membranes of chloroplast. During light reaction, it uses solar energy to split water, provide electrons and protons and byproduct of oxygen. Then, ATP is formed through chemiosmosis and NADPH is produced by passing down the electrons to electron transport chain (Reece et al., n.d.). These two products are later pass down to dark reaction. Dark reaction occurs in stroma of the chloroplast. It converts carbon dioxide to carbohydrate with the aid of ATP and NADPH produce during the light reaction.

This experiment is to investigate the photosynthetic electron transport with different treatments and compare their absorbance. When the rate of photosynthesis is high, the light reaction will occur at a faster rate. Thus, the DCPIP will decolourise at a faster rate.

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 spectrometer 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 thirty minute intervals, readings of absorbance were taken for all treatments, except for the dark tube which was kept in the light-proof tube for 90 minutes, after which its absorbance was measured.

Results

Table 1. Experimental design for the electron transport experiment.

Table 2. Absorbance readings taken at 30 minute intervals for each treatment.