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
Section 1 of 5Photosynthetic Pigments
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.
Section 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.
| Feature | Absorption spectrum | Action spectrum |
|---|---|---|
| Plots | Absorbance against wavelength | Rate against wavelength |
| Measured on | Isolated pigment | Whole plant or chloroplasts |
| Instrument | Colorimeter or spectrophotometer | Gas production or CO₂ uptake |
Section 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.
Section 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.
Section 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.
Last updated:
\n