Ncert Exercises & Solutions

52. In the figure given below, the black line (upper) indicates action spectrum for photosynthesis and the lighter line (lower) indicates the absorption spectrum of chlorophyll-a, answer the following.

(a) What does the action spectrum indicate? How can we plot an action spectrum? Explain with an example

(b) How can we derive an absorption spectrum for any substance?

(c) If chlorophyll-a is responsible for light reaction of photosynthesis, why do the action spectrum and absorption spectrum not overlap?

(a) The effectiveness of different wavelengths of light on photosynthes is is measured and the rate of photosynthesis is plotted. This is called the action spectrum of photosynthesis.

(b) Absorption of different wavelengths of light by a particular pigment is plotted and the graph is called the absorption spectra of that pigment.

(c) Chlorophyll-a is responsible for light reaction of photosynthesis, but the action spectrum and absorption spectrum do not overlap because, though chlorophyll-a is the main pigment responsible for absorption of light, other thylakoid pigments like chlorophyll-b, xanthophylls. carotenoid, which are accessory pigments also absorb and transfer the energy to chlorophyll-a
Indeed they not only enable a wicler range of wavelength of incoming light to be utilised
for photosynthesis but also protect chlorophyll-a from photooxidation.

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