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

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Figure 1: Absorbance vs Time to determine the relationship between colour change and photosynthesis for each treatment

Discussion

The purpose of the experiment was to examine the rate of photosynthesis by amount of light absorbed by each treatment using Hills reaction (Hill 1937). The data generally supports the hypothesis but with qualifiers.

The light treatment displays a constantly decreasing trend which matches my prediction. Beneath the exposure of light, the light dependent reaction was operated by silverbeet chlorophyll, allowing the high energy electrons to be produced which reduces DCPIP. The addition of electrons allows the DCPIP to become colourless and transmit more light (Hickey 2000). The two reaction centres, photosystem I and photosystem II work together to form high energy molecules produced in the photosynthetic light reactions which increases the absorbance of light. The high rate production of electrons caused the absorbance to increase rapidly and then levelled off in the last fifteen minutes. It was observed that settling of substance at the bottom of the test tube caused the electrons to become less efficient. This meant that most of DCPIP had already turned colourless, showing a slower decrease in absorbance. Further research, involving the constant stirring of the solution, could be preformed to test this possibility.

The dark treatment indicates a slight colour change which doesn’t meet my hypotheses as presumed the treatment would not show any colour change. The slight absorbance could be caused by a few electrons, formed from light dependent reaction, that are left behind in the silverbeet leaves before putting the test tube into a larger tube covered in foil. This could be investigated by placing the spectrophotometer in a dark area so all the undesired light can’t be absorbed by the treatments.

There was an unexpected result in the boiled treatment with a slight change in colour that disagrees with the hypotheses. Boiling chloroplasts prevents DCPIP from being reduced due to the enzymes for the photosynthesis being destroyed causing chloroplasts to become denatured. However, a slight absorbance of light was caused by small amounts of proteins in the chloroplasts were still functioning where the proteins was not fully denatured. Further experiments could conclude exact temperature all enzymes are denatured therefore halting photosynthesis.

The DCMU treatment showed an unexpected trend as the absorbance recorded a higher value than the starting absorbance through the first fifteen minutes which does not support the hypotheses. DCMU is a competitive inhibitor that blocks the accepter of photosystem II, disallowing the electron flow to photosystem I. The DCMU has no effect on photosystem I. Only 0.10ml of 0.01ml DMCU is added into DCMU treatment. With small amounts of DMCU competitors being added, electrons have a higher chance from passing to photosystem I and eventually reduced DCPIP resulting a slim decrease. An increase in the first fifteen minutes may be caused by the DMCU interrupting the electron transport chain and blocking the chloroplast’s ability to absorb light. Further investigations are needed to see what exact amount of DCMU competitors are required to fully stop the electrons transportation

Red treatment displays a faster rate of photosynthesis than the Green treatment which supports my hypotheses. Chlorophyll absorbs light better in the red wavelengths and have lower photon energy than green treatment. Chlorophyll reflects and transmits green light due to the pigments being green. Chlorophyll absorbs all colours of light except for green and yellow (Chiras 1993). There was a slower decrease for the red treatment than expected due to the thickness of cellophane wrapped around the larger tube lowering the intensity of light penetrating the inner test tube. Additional experiments could be conducted using different coloured light bulbs which all share the same light intensity.

The data results verified the hypothesis but with qualifiers, where light treatment, red treatment and green treatment produced a constantly increased rate of photosynthesis. Dark treatment, boiled treatment and DCMU treatment displayed a slight rate of photosynthesis.

References:

Chiras, DD (1993) Biology: The Web of Life. (West Publishing co) pp. 896 (St Paul, Manhattan)

Hickey, MK (2006) Photosynetheis and Spectrophotometry. (Cornell Institute for Biology Teachers) Available at http://cibt.bio.cornell.edu/labs/phys/PHO_0004.PDF

Hill, R (1937) Discoveries in Photosynthesis (Govindjee, J.T Beatty, H. Gest, J.F Alle) pp. 110 (Springer, AA Dordrecht, Netherlands)