Spectrophotometer Lab.

Figure 1:

Purpose:

To determine the % of absorption and transmission of light from spinach leafs at different wavelengths of energy.

Hypothesis:

The wavelength with the greatest transmission % will be 550 nm because the spinach leaf reflects green light (hence, why plants are green).  It is expected that the highest absorption % will be between 420-450 nm and 650-700 nm.  The reason for this is because Plants absorb blue light and red light the best, and 650-700 nm is the range in which photosystem 1 and 2 absorb light to being the light dependant reactions.

Materials:

• 6.1 mL Spinach solution
•  2 Cuvettes
• Spectrophotometer

Variables:

Controlled:

• Room temperature
• Concentration of spinach
• Amount of spinach
• Type of spectrophotometer
• Pressure

Dependent:

• Transmittance percentage
• Absorption Percentage

Independent:

• Wavelength of light

Procedure:

1. The instrument was turned on by rotating the amplifier control (c) in a clock-wise direction. 5 minutes were allowed for the instrument to warm up.
2. A blank cuvette was placed in the sample compartment and the guide mark was aligned on the cuvette at the front of the sample compartment. The lid was closed. It was often checked that the meter needle reads 100% Transmittance when the blank cuvette was in the sample compartment with the lid closed.
3. The amplifier control was adjusted with the sample compartment lid closed until the meter needle read 0 on the % Transmittance scale.
4. The blank cuvette was removed and the ground-up spinach was inserted. The cuvette was placed in the sample compartment. The guide mark on the cuvette was aligned with the guide mark at the front of the sample compartment. The lid was closed.  
5. The wavelength was set to zero nm using the wavelength control knob.
6. The transmittance percent and absorption were recorded from the Transmittance/Absorbance scale.
7. It was often checked that the meter needle readss 0 on the % Transmittance scale with the sample compartment empty and the lid closed.
8. Steps 1 – 8 were repeated using the wavelengths of 360 nm to 900 nm, at every 20 nm.

Observations:

Chart 1

Figure 2:

Figure 3:

Figure 4:

Sources of Error:

When grinding up the spinach leaves, impurities, such as dust particles, may have entered the substance, changing the transmittance percentage slightly. In addition, the cuvettes may have been scratched, or have had fingerprints on them, despite efforts to keep them clean. This would also alter the transmittance percentage. Also, there may have been human errors of misreading the spectrophotometer, making our results have a small degree of uncertainty. Finally, when turning the knobs to the desired wavelength, the numbers may have been slightly off.

Discussion/Conclusions:

In conclusion, chlorophyll inside of an intact chloroplast that has absorbed light gives different results when the chloroplast is not intact. (Campbell, Neil) When inside of a thylakoid membrane, chlorophyll acts in a photosystem. In a photosystem, the energy gained from the absorbed light is transferred to an electron transport chain. (Campbell, Neil) However, when the spinach was ground up, the chloroplasts were broken up. Therefore, the photosystems were not functioning, and the chlorophyll fluoresces since there were no electron acceptors to prevent the excited electrons from dropping back down to ground state. Initially, it was hypothesized that the wavelength with the greatest transmission % would be 550 nm.  However, by analyzing the graph, it becomes apparent that although there was a peak in the graph in the region of 550 nm, the region of 700 nm contained the highest transmittance percentage. This unexpected peak can be accounted for by the fact that when the thylakoids were mashed up, the photosytems, which are best at absorbing wavelengths of light at 680 nm and 700 nm, were disabled.  Therefore, the energy from the absorbed light that would normally be transferred by the photosystems to the electron transport chain was fluoresced, resulting in a higher transmittance percentage.  In addition it was expected that the highest absorption % would be between 420-450 nm and 650-700 nm.  650-700 nm is the range in which photosystem 1 and 2 absorb light to being the light dependant reactions. It was true that for the purple and blue light, absorbance was very high. Also, as expected, absorbance in the green region was very low, since green light is the least effective wavelength that can be used in photosynthesis. (Campbell, Neil) However, the unexpected low occurred in the 650-700 nm region, where absorbance was expected to be high. The rise in the absorbance graph in the range of 550 nm to 600 nm can be explained by the presence of xanthophyll (which absorbs yellow wavelengths) and carotene (which absorbs yellow wavelengths orange-yellow), which are also present in spinach.  This can be explained in the same way the unexpected high in the 650-700 nm range for transmittance percentage: the photosystems were disabled, and therefore that range of light was fluoresced. A comparison of the two graphs shows a relationship between transmittance percentage and absorbance. When the transmittance percentage increases, the absorbance tends to decrease. This relationship is represented by the equation Absorbance = log 100/ Transmittance Percent.

References:

1. Campbell Neil, Reece Jane, Mitchell Lawrence, 1996. BIOLOGY-Fifth edition
2. Buchanan, B.B., Gruissem, W., and Jones, R.L.

                             2002. Biochemistry and Molecular Biology of Plants.

                            John Wiley and Sons, Inc.

3. Hickey, Mary Kay, The Cornell Institute, 200.

                          cibt.bio.cornell.edu/labs/phys/PHO_0004.PDF

4. Pavia, D.L.; Lampman, G.M.; Kriz, G.S.; Engel, R.G. Isolation of Chlorophyll and Carotenoid Pigments from Spinach Introduction to Organic Laboratory Techniques: A Microscale Approach 3rd Edition Saunders College Publishing: New York, NY, l999.
5. Brown, Theodore, The Central Science, Seventh Addition. Upper Saddle River, New Jersey
6. Chemical  Instrumentation  Laboratories, Spectroscopy.