Pigments of the chloroplast: Chlorophyll is in fact a mixture of very closely related pigments of which chlorophyll is just one of them. You can verify the composition of this plant pigment by extracting the pigment from the leaves of plants with acetone and separating them into their component pigments using paper chromatography. Five distinct pigments can be identified from this separation:
(i) Chlorophyll a (blue-green)
(ii) Chlorophyll b (yellow-green)
(iii) Xanthophylls (yellow)
(iv) Carotene (orange)
(v) Phaeophytin (grey, which is a breakdown product of chleorophyll)
When separate solutions of each of the above listed pigments are made and their absorption spectrum determined, it can be clearly shown that chlorophyll a and chlorophyll b absorb light from both the red and blue/violet parts of the spectrum. It would also show that xanthophylls and carotene absorb light only from the blue/violet part. Chlorophyll a is the most abundant pigment and is of wide and universal occurrence in all photosynthesizing plants. The function of chlorophyll a is to absorb light energy and convert it into chemical energy. Other pigments also perform this function and then hands over the energy converted to chlorophyll a.
The chlorophyll structure is quite complex. It belongs to a group of organic radical called porphyrins and has magnesium as its metallic radical which is a characteristic feature of porphyrins because they form complexes with heavy metals. Other porphyrin pigments include haemoglobin and some respiratory pigments but the ability of chlorophyll to absorb light may not be unconnected with the presence of the metal magnesium unlike haemoglobin which contains iron.