Photosynthesis

Introduction

Photosynthesis is a process used every day by plants, algae, and some bacteria to harness light energy and turn it into chemical energy. Chloroplasts play a large role in the process, chloroplasts are organelles much like mitochondria that contain a double membrane and third inner membrane called the thylakoid membrane. It is on the thylakoid membrane where the process takes place. Chloroplasts contain their own DNA which is a major contributor to the stability of this organelle. Without this essential organelle the photosynthetic process would not be possible.

Chlorophyll is the pigment located inside the chloroplasts which gives the plants their green color. It is the substance that absorbs energy from light which excites the electrons donated from water into photosystem II, transferring the excited electrons to the next chlorophyll. This process is repeated until it reaches a reaction center which transports the electrons from the chlorophyll in photosystem II to the chlorophyll in photosystem I where the electrons are reenergized by light in this photosystem.

An electron acceptor is another essential part of the functioning of photosynthesis, DCPIP was the electron acceptor used in this experiment. The DCPIP accepted the excited electrons passed through the photosystems and was reduced to DCPIPH.

While this process of reduction is taking place protons from water and DCPIP were being pumped into the lumen of the chloroplast. With the concentration of protons and electrons from the reduced DCIPH an electrochemical gradient is produced to create the production of ATP.

In week one of the experiment the effect of light and herbicide on the photosynthesis of chloroplasts was tested using the percent transmittance as a way of measurement. The first cuvette that was altered was the negative control which did not contain any chloroplasts, resulting in a percent transmittance that did not change. The second cuvette contained the chloroplasts and was not exposed to light. Since it was not exposed to light the percent transmittance was very low only increasing by about 1%. The third cuvette was the positive control which contained chloroplasts and was exposed to light. As expected this underwent a high percent transmittance because of the exposure to light. The fourth cuvette was exposed to light and contained chloroplasts and herbicide, a substance used to destroy unwanted plants. The percent transmittance was dramatically reduced compared to the positive control group.

With the consideration of week one’s results, the effect of insecticide on percent transmittance was tested in week two. Since herbicide inhibited the reduction of plastiquionone, electron transfer was reduced. Photosynthesis relies mainly on the transfer of electrons to form energy, which means that inhibiting the reduction of plastiquionone slowed the process of photosynthesis, which was evident by the greatly reduced percent transmittance. This same process was believed to be similar for the effect of insecticide on percent transmittance of photosynthesis. Insecticide is a substance commonly used on or around plants to stop insects from consuming and killing plants. It was hypothesized that insecticide would work in similar ways to inhibit the transfer of electrons in the photosynthetic electron transport chain. In an experiment done by R. Unteidt and M Blanke the effect of insecticide photosynthesis was tested on apple orchard trees. They found that insecticides had similar inhibiting effects as herbicides, inhibiting the transfer of electrons in photosystem II and I. slowing the phosphorylation which inhibits energy (Unteidt and Blanke, 2004). Along with this article and other research information the hypothesis, if insecticides are added to the chloroplast solution then the rate of percent transmittance would be reduced, was created.

Experimental Design

The experiment was set up with three cuvettes to measure the effect of insecticide on percent transmittance rate using a spectrometer. The first cuvette was the negative control which contained no chloroplasts and was exposed to light. The second cuvette was the positive control containing chloroplasts with exposure to light. The third cuvette was the experimental cuvette which contained chloroplasts and insecticide, this cuvette was also exposed to light. The insecticide used was deltamethrin, a part of the pyrethroid family, a common insecticide used on lawns and binds readily to soil particles making it likely to be absorbed by plants. All cuvettes contained DCPIP which was the electron carrier agent in photosynthesis.

The results of these three cuvettes were quantified by taking the percent transmitted of each after exposure to light after zero minutes, four minutes, eight minutes, twelve minutes, and sixteen minutes. The percent transmittance measures the amount of DCPIP being reduced in the photosynthesis reaction. The independent variable in the experiment was the insecticide and the dependent variable was the rate of photosynthesis measured by transmittance percentage.

There were some variables in the experiment which were controlled. The amount and type of light was controlled, the light was a white light and the amount of time it was exposed to the light. The condition of the chloroplasts was controlled as well by being kept in a cool bucket of ice until the solutions were ready for them in order to keep them from disassociating. As well as controlling the amount of time exposed to light it was made sure that the solutions were not exposed to light before taking the transmittance percentage after zero minutes of light exposure.

Results

The experiment was designed to test whether insecticide had similar effects on photosynthesis of isolated chloroplasts as herbicide had. This was tested by comparing the percent transmitted in the chloroplast solution with herbicide in the first week’s experiment to the percent transmitted in the solution with chloroplasts and insecticide in the second weeks experiment. The data was analyzed using a scatter plot with linear equations of the three cuvettes results of percent transmitted. The cuvette with no chloroplasts had a low starting transmittance rate of 54.4% and a slope of .01 (Table 1 and Figure 1). The cuvette with only chloroplasts has a starting transmittance rate of 53.4% and a slope of 1.85 (Table 1 and Figure 1). The cuvette with chloroplasts and insecticide had a starting transmittance rate of 84.9% and a slope of .066 (Table 1 and Figure 1). It is clear from the data that the positive control with a slope of 1.85 was the solution with the most DCPIP reduced and negative control essentially had no DCPIP reduced with its slope of nearly zero. The experimental solutions slope was closest to slope of the negative control. This showed that when the dependent variable, transmittance rate, was affected by the independent variable, the insecticide. The results of the experiment did come out as significant because the hypothesis that photosynthesis in the form of transmittance rate would be reduced in chloroplast solutions with insecticide was supported.

Discussion

The effect that insecticide has on the photosynthesis in chloroplasts was tested in this experiment. The hypothesis state that effect would result in reduced photosynthesis rate measured by the transmittance percentage rate of reduced DCPIP. The full hypothesis was if insecticides are added to the chloroplast solution then the rate of percent transmittance would be reduced.

The data supported our hypothesis that the transmittance percentage would be reduced in a solution of chloroplasts that contained insecticide. It was measured and quantified by taking the slop of the rate of transmittance which came out to be .066 very close the transmittance rate of our negative control. Our rationale also helped to see the significance of insecticides effect on photosynthesis because it had similar effects as herbicide. In the first weeks experiment the slope of the solution with chloroplasts and herbicide was less than one, close to the slop of the solution with pesticide. We researched why herbicide affected photosynthesis and found that the rationale for its effect had similar relations to insecticides effect on photosynthesis. After seeing the close relations of our data to the rationale and hypothesis it is clear that our hypothesis was supported that insecticide has a strong effect on the reduction of transmittance rate of DCPIP therefore reducing photosynthesis rate.

The experiments results were also supported from findings that others had using similar or the same insecticide as ours. An experiment done on potato plants using the insecticide deltamethrin showed that electron activity was much lower in the plants treated with the insecticide compared to the positive and negative controls (Fidalgo et al. 1993). Another study was conducted by testing the effect of a pyrethroid insecticides on photosynthetic pigments. The experiment was looking into different aspects of the effect that the insecticide had but still found useful information that pertained to our experiment, which included the conclusion that insecticides effected the enzyme activity in photosynthesis by binding to different areas of the enzymes and acceptor sites which would affect the rate of photosynthesis (Mohapatra et al. 2003).

Although the hypothesis was supported some flaws were encountered in the experiment. One of these flaws was that the starting rate of transmittance percentage in the insecticide solution started at a much higher percentage than the other two cuvettes. Another flaw was that the solution that contained herbicide was not retested in the second week so we had to compare our results of the insecticide to the first week’s results for herbicide. If we reconducted the experiment we would add another cuvette that contained the herbicide in order to have more accurate information to compare the results to. We also would have done multiple trials to make sure the high starting transmittance rate was not a flaw in the preparation of the solution.

With consideration to the outcomes and flaws to the experiment a new hypothesis would be if bactericides are added to the chloroplast solution then the rate of percent transmittance would be reduced. Bactericide is under the classification of a pesticide as is herbicide and insecticide. Considering it is another form of pesticide it can be rationalized that bactericide would have similar results to herbicide and insecticide. Bacteria would also most likely have even clearer results because it undergoes photosynthesis as means of creating energy to stay alive. An overall consensus could eventually be made by further testing that all pesticides will have a reduced percent transmittance.