3.3.3Additional lessons
Absorption Spectra — IB Biology SL Revision Notes & Topical Questions
5 min read 5 parts Lesson 58 of 142
Section 1 of 5Light and Pigments
Visible light
- Visible light covers wavelengths from about 400 nm (violet) to 700 nm (red).
Absorption
- A pigment absorbs some wavelengths and reflects or transmits others.
Chlorophyll
- Chlorophyll absorbs mainly blue-violet and red light, and reflects green light.
Green leaves
- The reflected green light enters the eye, so leaves appear green.
Accessory pigments
- Carotenoids absorb other wavelengths, mainly blue-green light, and pass the energy on.
Section 2 of 5Absorption Spectrum
1
The absorption spectrum is a graph of the amount of light absorbed against wavelength.
2
Chlorophyll a has absorption peaks in the blue region and in the red region.
3
Chlorophyll b has peaks at slightly different wavelengths, so it widens the range of usable light.
4
Absorption is low in the green region, so little green light is used.
5
Together the pigments absorb a wider range of wavelengths than any single pigment.
Section 3 of 5Why It Matters
Why It Matters
Energy capture
- Absorbed light is passed to the reaction centre in photosystems in the thylakoid membrane.
Variety
- Different pigments allow the plant to use more of the light available in different environments.
Shade
- Plants in shade receive light enriched in green and far-red light, and some have more accessory pigments.
Autumn colours
- When chlorophyll breaks down in autumn, carotenoids remain and the leaf looks yellow or orange.
Section 4 of 5Light and Photosystems
Light and Photosystems
Photosystem
- Pigments are grouped in photosystems, which pass energy to a reaction centre containing chlorophyll a.
Excited electrons
- Absorbed light excites electrons in chlorophyll, which provide energy for ATP production.
Range
- Absorption by several pigments means that more light energy can be used for photosynthesis.
Section 5 of 5Exam Checklist & Tips
3.3.3 – Key Points for the Exam
- Absorption spectrum A graph showing how much light a pigment absorbs at each wavelength.
- Wavelength The distance between two peaks of a light wave, measured in nanometres.
- Accessory pigment A pigment that absorbs light and passes the energy to chlorophyll.
- Carotenoid A yellow, orange or red pigment found in chloroplasts.
Common Mistakes
- Saying that chlorophyll absorbs green light, when it reflects most of it.
- Confusing absorption of light with reflection when explaining the colour of a leaf.
- Believing that only chlorophyll a takes part in photosynthesis.
- Reading a graph peak as the amount of photosynthesis, which would need an action spectrum.
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