Class: BE-210
Group: W7
Members: Brian Fabella, Jinwah Lau, Nicole Rhoden, Mahesh Swaminathan
Date: Spring 1997
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Abstract:
The absorption spectrum was determined for a mixture of chlorophyll a and b from spinach and romaine lettuce using a Milton-Roy Spectronic 20D Spectrophotometer. Also, an extraction method was found that was suitable for the experiment and that considered the availability of materials. Chloroplasts were extracted separately for spinach and lettuce using a modified version of Whatley and Arnon. Then 80% acetone was added creating the chlorophyll solution in which the absorbance was measured at wavelength intervals of five nm. Two trials of three samples each were measured for each vegetable. Absorbances vs. Wavelengths were plotted, and the chlorophyll content and chlorophyll a/b ratio were calculated. The molar absorption coefficients were also calculated to plot of Molar absorption coefficient vs. Wavelength. The absorption spectra of the spinach and lettuce were then compared to each other and to the literature spectra of chlorophyll a and b. Literature values, respectively for the two peaks of chlorophyll a and b are 420 nm and 435 nm for the first peak, and 663 nm and 645 nm for the second peak. Experimentally, the first absorption peak for spinach occurred at 431 nm and 430 nm for trial1 and trial 2 respectively. The second peak occurred at 665 nm for both trials 1 and 2. The molar absorption coefficient at the first peak was 1.261E+05 M-1*cm-1 and 1.884E+05 M-1*cm-1 for trial 1 and trial 2 respectively. At the second peak, the molar absorption coefficient was 4.400E+04 M-1*cm-1 for trial 1 and 7.230E+04 M-1*cm-1 for trial 2. For Romaine lettuce, the first peak occurred at 430 nm both trials. The second peak occurred at 665 nm for both trials as well. For trial1, the molar absorption coefficient at 430 nm was 9.764E+05 M-1*cm-1 and 4.422E+04 M-1*cm-1 for 665 nm. For trial 2, the molar absorption coefficient was 1.406E+05 M-1*cm-1 at 430 nm and 4.720E+04 M-1*cm-1 for a wavelength of 665 nm. After analyzing the experimental data, we ultimately came to two conclusions: 1. The absorption spectrum for a mixture of chlorophyll a and b is not just the simple addition of the individual chlorophyll a and chlorophyll b spectra. Rather, the resultant spectrum demonstrates intermediate characteristics, depending on the particular peak under consideration (the blue maximum or the red maximum). 2. The chlorophyll a to b ratio is essentially constant for higher order plants.