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Chlorophyll is the green photosynthetic pigment present in chloroplasts which provides the energy necessary for photosynthesis.

Chemical structure

Chlorophyll is a porphyrin pigment, as are several other important organic compounds, notably Heme. At the center of the ring is a magnesium ion. The side chains vary somewhat between the different forms of chlorophyll found in different organisms - chlorophyll a is always present, but chlorophylls b and c also occur in various groups.
Common structure of chlorophyll a and b (Image in the PD)

Absorbance spectra
Absorbance of light of chlorophyll a(green) and b(red) (Image in the PD)

Evidence for chorophyll
It can be shown that chlorophyll is vital for photososynthesis by destarching a variegated plant's leaves and exposing it to light for several hours. Variegated plant leaves have green areas which contain chlorophyll and white areas that have none. When tested with iodine solution a colour change showing starch present is only evident in regions of the plant that were green and therefore contained chlorophyll. This shows that without chlorophyll photosynthesis is impossible.