Q.Distinguish between the absorption spectrum and the action spectrum of a photosynthetic pigment, and explain how the close overlap between the two, for chlorophyll a, supports its role as the primary photosynthetic pigment.

[!TLDR]

Absorption spectrum = isolated pigment's light uptake by wavelength; action spectrum = living tissue's actual photosynthetic rate by wavelength; their overlap for chlorophyll a is key evidence for its primary role. …

The absorption spectrum is obtained in a test tube: a pigment is extracted from leaf tissue and its light absorption at each wavelength is measured directly with a spectrophotometer, independent of any living process. The action spectrum, by contrast, is obtained from a living, functioning organism, and plots the actual photosynthetic output (O2 evolution or CO2 uptake) measured at each wavelength of light supplied -- exactly the kind of measurement Engelmann achieved using bacterial clustering. If a pigment's absorption spectrum and the organism's action spectrum closely coincide, this is strong evidence that the pigment in question is genuinely responsible for driving the process, rather than merely happening to be present. For chlorophyll a specifically, this close overlap -- peaks in blue and red, trough in green, for both curves -- is precisely why chlorophyll a is id …

Separate "what the pigment alone absorbs" (absorption spectrum) from "what the living leaf actually does" (action spectrum) …

Treating the two spectra as interchangeable or identical by definition -- their overlap is an empirical finding that supp …