13.1.5—Absorption and action spectra

An absorption spectrum shows how strongly a particular photosynthetic pigment absorbs different wavelengths of light. An action spectrum shows the rate of photosynthesis at different wavelengths, so comparing them links pigment absorption to photosynthetic activity.

• Read an absorption spectrum: identify the axes, then locate peaks where the named pigment absorbs strongly and troughs where absorption is low. The graph describes light absorbed by that pigment, not the rate of photosynthesis.
• Read an action spectrum: use the wavelength axis and photosynthesis-rate axis to locate wavelengths where photosynthesis is fastest or slowest. High action-spectrum values indicate effective photosynthetic activity under those wavelengths.
• Compare the graphs: the combined absorption of the chloroplast’s pigments should broadly match the action spectrum. Peaks in the blue-violet and red regions and a trough in the green-yellow region show the relationship supported by the pigment mixture.
• Interpret the link: wavelengths absorbed by chlorophylls and carotenoids can provide energy for photosynthesis, so strong cumulative absorption can correspond to a higher photosynthesis rate. Keep the graph boundary: an absorption spectrum is not itself a photosynthesis-rate measurement.

Do not read an absorption spectrum as if it directly measured photosynthesis, infer that a pigment’s visible colour is its main absorbed wavelength, or add chromatography and full light-reaction detail to this graph-reading objective.