Photosynthesis: Role of Sunlight in Photosynthesis and Absorption and Action Spectra (For CBSE, ICSE, IAS, NET, NRA 2022)

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  • Light consists of small particles or packages of energy called photons. A single photon is also called quantum.
  • Chlorophyll molecules absorb light energy and get into an excited state and lose an electron to the outer orbit. No substance can remain in an excited state of long, so the energized and excited chlorophyll molecule comes down to a low energy state known as ground state and releases the extra amount of energy. This energy can be lost as heat or as light (fluorescence) or can do some work. In photosynthesis, it works by splitting water molecule to produce H + and OH- ions.
  • Carotene is orange yellow pigment present along with chlorophylls in the thylakoid membrane. A carotene molecule breaks down into the vitamin A molecules. It is this pigment which gives carrot its color.

Absorption and Action Spectra

  • A plot (graph) showing absorption of light of different wavelength a pigment is called absorption spectrum. A solution of a pigmy placed in a spectrometer. It gives the wavelength absorbed by pigment. The light absorbed by the pigment is plot against wavelength gives absorption spectrum. Both chlorophyll a and b show maximum absorption in the violet and orange-red region of visible spectrum. Green and light is absorbed very little. The blue and red light are used by the green plants as energy source for photosynthesis.
  • The plot showing relative effectiveness of different wavelengths of ht in photosynthesis is called action spectrum. Action spectrum ratifies the pigments actually involved in the photosynthesis.
  • All wavelengths of light are not equally effective in photosynthesis i.e.. the rate of photosynthesis is more in some and less in others.
    Image Showing (A) Relative Absorption (B) Action Spectrum

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