Absorption Spectra & Action Spectra (Cambridge (CIE) A Level Biology)

Absorption Spectra & Action 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 Graph

Absorption spectra of chlorophyll A, chlorophyll B and carotenoid pigments

• 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, as 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 Graph

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

Overlay of Absorption Spectra and Action Spectra Graph

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