Absorption Spectra & Action Spectra (Cambridge International A-Level Biology)

Spec point — 13.1.5 · interpret absorption spectra of chloroplast pigments and action spectra for photosynthesis

Absorption spectra & action spectra

Absorption spectra

• An absorption spectrum is a graph that shows the absorbance of different wavelengths of light by a particular pigment

  • Chlorophylls absorb wavelengths in the blue-violet and red regions of the light spectrum
  • Carotenoids absorb wavelengths of light mainly in the blue-violet region of the spectrum

Absorption spectra of chlorophyll A, chlorophyll B and carotenoid pigments

Action spectra

• An action spectrum is a graph that shows the rate of photosynthesis at different wavelengths of light

  • The rate of photosynthesis is highest at the blue-violet and red regions of the light spectrum
  • This is because these are the wavelengths of light that plants can absorb (i.e. the wavelengths of light that chlorophylls and carotenoids can absorb)

Photosynthetic action spectrum

• There is a strong correlation between the cumulative absorption spectra of all pigments and the action spectrum:

  • Both graphs have two main peaks (at the blue-violet region and the red region of the light spectrum)
  • Both graphs have a trough (in the green-yellow region of the light spectrum)

An overlay of the photosynthetic absorption and action spectra shows there is a strong cumulative correlation