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5.1.6Unit 5: Respiration, Internal Environment, Coordination and Gene Technology

Absorption and Action Spectra — IAL Edexcel Linear Revision Notes & Topical Questions

5 min read 5 parts Lesson 13 of 36 Unit quiz

📚 IAL Edexcel Linear Hub / Topical Revision
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Part 1 of 5Photosynthetic Pigments

5.1.6 Absorption and Action Spectra

Different pigments absorb different wavelengths of light.

Chlorophyll a
  • Absorbs mainly in the blue-violet and red regions of the spectrum and is the primary pigment.
Chlorophyll b
  • An accessory pigment that absorbs slightly different blue and red wavelengths.
Carotenoids
  • Accessory pigments that absorb mostly blue-green light and protect chlorophyll from damage.
Reflection
  • Green light is reflected and transmitted most, which is why leaves look green.
Peak absorption
  • Chlorophyll a absorbs most strongly near the ends of the visible spectrum, not in the green region.

Part 2 of 5Two Types of Graph

Two Types of Graph
Absorption spectrum
  • Shows how much light of each wavelength a pigment absorbs, usually measured on an extract.
Action spectrum
  • Shows the rate of photosynthesis at each wavelength of light in a whole plant or tissue.
Comparison
  • The shapes are similar because absorbed light drives the light-dependent reactions.
Differences
  • The action spectrum is broader because accessory pigments pass energy to chlorophyll a.
Measuring
  • A colorimeter with coloured filters can be used to measure absorption by a pigment extract.
FeatureAbsorption spectrumAction spectrum
PlotsAbsorbance against wavelengthRate against wavelength
Measured onIsolated pigmentWhole plant or chloroplasts
InstrumentColorimeter or spectrophotometerGas production or CO₂ uptake

Part 3 of 5Experimental Evidence

Engelmann's experiment used bacteria to detect oxygen.

1
A filament of green algae was lit with light split into separate colours.
2
Oxygen-seeking bacteria were placed along the filament.
3
The bacteria gathered mainly in the blue and red regions where more oxygen was released.
4
This showed that blue and red light drive photosynthesis most strongly.

Part 4 of 5Why Several Pigments?

Wider range
  • Having several pigments lets the plant use more of the visible spectrum.
Energy transfer
  • Energy absorbed by accessory pigments passes to the reaction centre chlorophyll a.
Reaction centres
  • P680 and P700 are the chlorophyll a forms that absorb at the peaks of their photosystems.
Light limitation
  • In shade, extra pigments help capture whatever wavelengths are available.

Part 5 of 5Exam Checklist & Tips

5.1.6 – Key Points for the Exam

  • Absorption spectrum A graph of how well a pigment absorbs each wavelength of light.
  • Action spectrum A graph of the rate of photosynthesis at each wavelength of light.
  • Accessory pigment A pigment that captures light and passes the energy to chlorophyll a.
  • Wavelength The distance between wave peaks, measured in nanometres for light.

Common Mistakes

  • Saying that plants do not absorb green light at all; they absorb it weakly.
  • Mixing up the axes of the two graphs.
  • Writing that the action spectrum measures pigment absorbance; it measures the rate of photosynthesis.
  • Saying chlorophyll alone absorbs all the light used; accessory pigments contribute.

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