Absorption & Action Spectra (Edexcel International A Level Biology): Revision Note

Absorption & Action Spectra

• Chloroplasts contain several different photosynthetic pigments within photosystems embedded in the thylakoid membranes

• Different pigments absorb light of different wavelengths

  • Chlorophylls absorb wavelengths in the blue-violet and red regions of the light spectrum, reflecting green light and appearing green in colour

  • Carotenoids absorb wavelengths of light mainly in the blue-violet region of the spectrum, reflecting yellow and orange light

    • Carotenoids often remain in leaves after the breakdown of chlorophyll in the autumn, giving some leaves their yellow, orange, and red autumn colours

Examples of Photosynthetic pigments Table

• The amount of light at different wavelengths absorbed by a particular pigment gives that pigment's absorption spectrum (plural spectra)

  • Because each type of pigment absorbs light at different wavelengths the absorption spectrum of each pigment is different

Different photosynthetic pigments absorb light of different wavelengths, giving different absorption spectra

• A plant's rate of photosynthesis varies depending on the wavelengths of light available

• The changing rate of photosynthesis at different wavelengths is known as an action spectrum

• Action spectra are very closely correlated to the absorption spectra of the different pigments

  • Having several different pigments with different absorption spectra allows plants to photosynthesise under a wider variety of light wavelengths; this extends the action spectra of plants and maximises rates of photosynthesis

Plant action spectra are closely related to the absorption spectra of the different photosynthetic pigments