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3.3.3Additional lessons

Absorption Spectra — IB Biology SL Revision Notes & Topical Questions

5 min read 5 parts Lesson 58 of 142

📚 IB Biology SL Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Part 1 of 5Light and Pigments

Visible light
  • Visible light Visible light covers wavelengths from about 400 nm (violet) to 700 nm (red).
Absorption
  • Absorption A pigment absorbs some wavelengths and reflects or transmits others.
Chlorophyll
  • Chlorophyll Chlorophyll absorbs mainly blue-violet and red light, and reflects green light.
Green leaves
  • Green leaves The reflected green light enters the eye, so leaves appear green.
Accessory pigments
  • Accessory pigments Carotenoids absorb other wavelengths, mainly blue-green light, and pass the energy on.

Part 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.

Part 3 of 5Why It Matters

Why It Matters
Energy capture
  • Energy capture Absorbed light is passed to the reaction centre in photosystems in the thylakoid membrane.
Variety
  • Variety Different pigments allow the plant to use more of the light available in different environments.
Shade
  • Shade Plants in shade receive light enriched in green and far-red light, and some have more accessory pigments.
Autumn colours
  • Autumn colours When chlorophyll breaks down in autumn, carotenoids remain and the leaf looks yellow or orange.

Part 4 of 5Light and Photosystems

Light and Photosystems
Photosystem
  • Photosystem Pigments are grouped in photosystems, which pass energy to a reaction centre containing chlorophyll a.
Excited electrons
  • Excited electrons Absorbed light excites electrons in chlorophyll, which provide energy for ATP production.
Range
  • Range Absorption by several pigments means that more light energy can be used for photosynthesis.

Part 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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