Determination of the Absorption Spectrum of a Pigment Extract

You should know that ordinary white light is a mixture of a variety of different wavelengths from the visible spectrum . If you shine white light on a pigment or pigment mixture, such as the chlorophyll extract, certain wavelengths are more readily absorbed and other wavelengths are transmitted through. Among other things this would give the researcher an idea of which wavelengths of light (if any) are most important to photosynthesis. The following exercise is designed so that you can examine the absorption of various wavelengths of light by a chlorophyll extract. The degree of absorbance of light by a substance as the wavelength is varied is referred to as the Absorption Spectrum.

One can readily obtain an absorption spectrum of any compound in solution by simply measuring the absorbance with an instrument called a spectrophotometer. This instrument generally allows us to select wavelengths from 380 nm to 980 nm.

A dilute pigment extract in 80% acetone will be provided. Your demonstrator will provide you with the instructions for operating the spectrophotometer.

Repeat these steps for 20 nm intervals up to 700 nm. Record your data in your notebook.

Graph the absorption readings against wavelengths (400-700 nm)

You have now established an absorption spectrum for the chlorophyll extract.

-What is the value of an absorption spectrum?
-How would you determine the absorption spectrum for each of the components of the pigment extract?
-Does an absorption spectrum tell you unequivocally the wavelengths of light that are active in the photosynthetic process?
-What is an action spectrum of photosynthesis ?

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