Which pigment is present universally in all green plants?

What can we conclude from the statement that the action and absorption spectrum of photosynthesis overlap? At which wavelengths do they show peaks?

Which molecule acts as the reaction center in photosynthesis?

Suppose there were plants that had a high concentration of Chlorophyll-$b$,but lacked Chlorophyll-$a$. Would they carry out photosynthesis? Then why do plants have Chlorophyll-$b$ and other accessory pigments?

Pigment system-$I$ receives radiant energy and releases an electron from which reaction center?

The extensive study of pigments was carried out by .........