A LevelBiologyCambridge (CIE)Revision Notes13. PhotosynthesisPhotosynthesis as an Energy Transfer ProcessAbsorption Spectra & Action Spectra

Absorption Spectra & Action Spectra (Cambridge (CIE) A Level Biology): Revision Note

Exam code: 9700

Phil

Written by: Phil

Reviewed by: Alistair Marjot

Updated on

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

Graph showing light absorption by chlorophylls and carotenoids across the spectrum, with peaks in blue-violet and red regions, and notes on absorption differences.
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)

Graph showing photosynthesis rate versus light wavelength. Peaks at blue-violet (450 nm) and red (680 nm) indicate higher photosynthesis efficiency.
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)

Graph showing photosynthetic and absorption rates against wavelength for action spectrum, chlorophyll a, chlorophyll b, and β-carotene, with key.
An overlay of the photosynthetic absorption and action spectra shows there is a strong cumulative correlation

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    Author
    Reviewer
    Phil

    Author: Phil

    Expertise: Biology Content Creator

    Phil has a BSc in Biochemistry from the University of Birmingham, followed by an MBA from Manchester Business School. He has 15 years of teaching and tutoring experience, teaching Biology in schools before becoming director of a growing tuition agency. He has also examined Biology for one of the leading UK exam boards. Phil has a particular passion for empowering students to overcome their fear of numbers in a scientific context.

    Alistair Marjot

    Reviewer: Alistair Marjot

    Expertise: Environmental Systems and Societies & Biology Content Creator

    Alistair graduated from Oxford University with a degree in Biological Sciences. He has taught GCSE/IGCSE Biology, as well as Biology and Environmental Systems & Societies for the International Baccalaureate Diploma Programme. While teaching in Oxford, Alistair completed his MA Education as Head of Department for Environmental Systems & Societies. Alistair has continued to pursue his interests in ecology and environmental science, recently gaining an MSc in Wildlife Biology & Conservation with Edinburgh Napier University.