Overview
Paper chromatography is a useful technique for separating and identifying pigments and other molecules from cell extracts that contain a complex mixture of molecules.
The solvent moves up the paper by capillary action, which occurs as a result of the
attraction of solvent molecules to the paper and the attraction of solvent molecules to
one another. As the solvent moves up the paper, it carries along any substances
dissolved in it, in this case pigments. The pigments are carried along at different rates
because they are not equally soluble in the solvent and because they are attracted, to
different degrees, to the cellulose in the paper through the formation of hydrogen bonds.
A large number of different chromatographic systems are used to separate various
molecular components of cells. They differ in the type of support (paper in this case)
and in the type of solvent system used. In most cases special stains are needed in
order to visualize the molecules that have been separated.
Chlorophyll a is the primary photosynthetic pigment in all plants. A molecule of
chlorophyll a is located at the reaction center of photosystems. Other chlorophyll a
molecules, chlorophyll b, and the carotenoids (i.e., carotenes and xanthophylls) capture light energy and transfer it to the chlorophyll a at the reaction center. Carotenoids also protect the photosynthetic system from the damaging effects of bright sunlight.