Absorption Spectra (DP IB Biology: HL): Revision Note

Absorption Spectra

• Light is made up of a mixture of all the visible wavelengths to include red, orange, yellow, green, blue, indigo and violet

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

• Within the chlorophyll, light energy results in the excitation of electrons which triggers transfer of electrons leading to a series of reactions which make up the process of photosynthesis

  • During photosynthesis, light energy is transformed to chemical energy when glucose is formed 

• 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

• The chemical structure of these molecules determines the wavelengths of light that can be absorbed

• The green part of the spectrum is largely reflected from the leaf and this is why leaves usually appear green

Absorption spectra of chlorophyll A, chlorophyll B and carotenoid pigments