Effect of Light Wavelength on Photosynthetic activity.

Introduction

Light travelling to earth from the sun carry energy, which is converted to chemical energy by a process called photosynthesis. Pigments within the photosynthetic organism absorb light energy and initiate the electron transfer reaction via a series of redox reactions (Robbinson and Croft, 1983). The electrons pass along a chain of proteins in the electron transport chain (ETC) in order to capture energy in the form of NADPH and ATP (Junge and Auslander 1974). ATP and NADPH are used by photosynthetic organisms to carry out biological functions such as growth and repair.

Photosynthesis depends on the effectiveness of different wavelengths of light at generating electrons (Edward and Burge, 1996). If a pigment absorbs light energy, the energy may trigger a chemical reaction, such as photosynthesis (Bjorkman and Adams, 1995). Chlorophyll is a green pigment common to all photosynthetic cells. It absorbs all wavelengths in the visible light spectrum except green, which is reflected back, thus giving leaves its green colour (Johkan et al., 2012).

DCPIP is a dye that provides an alternative pathway for electrons (Larom et al., 2010). As DCPIP absorbs electrons, it becomes reduced and changes from blue to colourless (Law et al., 2003). Photosynthetic activity can therefore be spectrophotometrically quantified by determining the absorbance of DCPIP. DCPIP reduction is proportional to electron flow in the ETC and is thus proportional to photosynthetic activity (Law et al., 2003).

In this experiment, DCPIP combined with chloroplast isolated from spinach beet were exposed to lights of different wavelengths. The absorbance due to DCPIP reduction was then measured to determine the affects of different wavelengths of the visible spectrum on the rate of photosynthesis. A previous study found that chloroplast under blue light had the highest photosynthetic activity (Horst, 1982), thus it is hypothesised that the chloroplast isolate exposed to blue light will be absorbed the most and thus increase the rate of photosynthesis. Considering the colour of chlorophyll and previous studies showing that chlorophyll absorb all visible light except green (Johkan et al., 2012), we also hypothesise that green light will be absorbed the least and will therefore lead to the lowest rate of photosynthesis.
References

Bjorkman, O. & Adams, D. (1995) Regulation of Photosynthetic Light Energy Capture, Conversion, and Dissipation