Bio 121 Photosynthesis PreLab - The photosynthetic pigments are responsible for absorbing and trapping light energy in the early steps of photosynthesis. The major pigments of photosynthesis are the chlorophylls. The two chlorophylls found in green plants are chlorophyll a (chl a) and chlorophyll b (chl b). Certain other chlorophylls (chlorophyll c and bacteriochlorophylls) are found in non-green algae, protistans, and photosynthetic bacteria. Other pigments include carotenoids and xanthophylls, sometimes referred to as the accessory pigments.

Chlorophylls - The chlorophylls have similar molecular structures. Each has a porphyrin ring and a long phytol side chain. Although the porphyrin ring resembles the prosthetic group of hemoglobin and cytochrome, it has a central magnesium atom instead of iron. The alternating double and single bonds of the porphyrin ring make chlorophyll an efficient light-absorbing molecule and determine the general shape of the absorption spectrum.

Paper Chromatography - Chromatography is a technique used to separate the components of a mixture. There are various types of chromatography (column, paper, thin-layer, gas), but in all cases the separation is achieved by distribution of components between a fixed or stationary phase and a moving or mobile phase. In paper chromatography, the components of a mixture are separable into discrete zones on a sheet of filter paper.

• A chromatography jar is prepared using an appropriate solvent
• The mixture to be separated is spotted or streaked near one end of a chromotography paper strip. Enough sample is applied so that there will be an adequate amount for subsequent analysis.
• The paper is placed in the jar so that the streak is above the level of the solvent.
• The solvent then moves up the paper by capillary action, past the sample, toward the end of the paper.
• During this process, the pigments separate and form discrete bands on the chromatogram.

Separation of the components can measured and given a "Rf value". The Rf value is determined by the equation:

In the numerator, the distance is measured from the origin either to the center or to the leading edge of each spot or band. The denominator is the distance from the origin to the solvent front. Rf values can be used to identify pigments in each of the various solutes if the experimental conditions are very carefully controlled.

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