Absorbance Spectra of Photosynthetic Pigments

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Contents

1 Prelab Exercise2 Stop and Think: Reflectance3 The Role of Light in Carbohydrate Synthesis4 Measuring Absorbance

Prelab ExerciseFill the Color field in the table below1.Use plot.ly to create a line graph with the 3 samples below (A, B, C)2.

plot % Reflectance on the Y-axis and Wavelength (nm) on the X-axis

% ReflectanceColor nm A B C

400 68 92 78425 40 71 77450 90 38 51475 97 49 57500 100 92 45525 100 100 66550 96 97 100575 98 96 100600 96 98 100625 97 80 100650 79 71 100675 56 96 100700 88 100 100

Stop and Think: ReflectanceA sign of plant health is viewed through the near infra-red. While we cannot see thisspectrum of light with our eyes, we can use other sensors to detect this light. Compare the

Absorbance Spectra of Photosynthetic Pigments

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images of the Black & White with the Infra-red image. What differences can you see in the 2images that will help you understand how this is a useful measure of plant health? How doyou think this is corresponds to the table above?

The English Garden

Reflectance Slider shows an overlay of the next images

Absorbance Spectra of Photosynthetic Pigments

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The English Garden (black & white)

Absorbance Spectra of Photosynthetic Pigments

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The English Garden (near infra-red)

Visible light wavelengths (between 400nm-700nm) are strongly absorbed by the pigments inleaves (Chlorophylls, Xanthophylls, Carotenoids). These pigments utilize the energy of thesewavelengths to take part in the light reactions. The cellular structure of leaves do notabsorb wavelengths longer than these wavelengths (>700nm in the infra-red range). Bycomparing the amount of visible light to the amount of near infra-red light that arereflected, one can gauge the relative health of leaves, forests or jungles . This is the roughdescription of the Normalized Differential Vegetation Index (NDVI) that scientists use inconjunction with satellite imagery to assess the health of vegetation.

The Role of Light in Carbohydrate SynthesisPick a leaf from a geranium exposed to light and one kept in the dark for 48 hours.1.

keep the stem on the leaf grown in lightremove the stem from the leaf grown in the dark

Hydrolyze the cell walls of the geranium leaves by boiling in a water bath for2.5 minutes or until it looks like over-cooked vegetables)“Bleach“ the leaves by removing the pigments. Place the leaves in hot alcohol for 73.minutes or until they turn white.

Save this green solution for Absorbance Spectrum exercise1.Remove the leaves and place it in a petri dish.4.Add iodine to the dish. If starch is present, the leaf will turn a deep bluish-black color.5.Photograph the leaf with your phone to document the effects of light on carbohydrate6.storage.

Measuring AbsorbanceConnect the Spectrovis to the LabQuest21.Turn on the Labquest2 units2.Choose the Labquest app3.Select the icon that looks like X|Y4.Press the green Play button on the bottom left5.Press OK to calibrate6.Let the machine calibrate for 90 seconds7.Choose “Finish calibration”8.Insert the Geranium pigment from the bleaching reaction9.

Do NOT use Acetone in these plastic cuvettes since it will frost over the plastic1.

Absorbance Spectra of Photosynthetic Pigments

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Press the Red Stop button10.Students should record the absorbance values at every 10 nm from 380nm-700nm11.The professor will prepare Spirulina extract diluted in ethanol in a cuvette and obtain12.the continuous absorbance spectrum.Plot Relative Absorbance against wavelength using a line graph and compare the13.absorption spectrum of the extracts.

Relative Absorbance sets the maximum value in each dataset as a denominator1.Every value is divided by this maximum value2.

Tags: quantitative reasoning