3.3.3Inquiry Process
Absorption Spectra — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 118 of 160
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.
Part 1 of 5Light and Pigments
Different pigments absorb different parts of the visible spectrum.
Visible light
- Visible light Visible light covers wavelengths from about 400 nm (violet) to about 700 nm (red), and each wavelength has a different energy.
Absorption
- Absorption A pigment absorbs some wavelengths, and the absorbed light energy can then be used to excite electrons.
Reflection and transmission
- Reflection and transmission Wavelengths not absorbed are reflected or transmitted, and these give the pigment its colour.
Absorption spectrum
- Absorption spectrum A graph of the amount of light absorbed by a pigment, plotted against wavelength.
Part 2 of 5Pigment Absorption
Chlorophylls
- Chlorophylls They absorb strongly in the blue-violet and in the red parts of the spectrum, with a trough in the green region.
Why leaves are green
- Why leaves are green Green light is reflected or transmitted by chlorophyll, so those wavelengths reach our eyes.
Carotenoids
- Carotenoids They absorb mainly blue-violet light, and cover wavelengths that chlorophyll absorbs poorly.
Chlorophyll a and b
- Chlorophyll a and b The two forms have slightly different peaks, so together they absorb a wider range than either could alone.
Several pigments
- Several pigments The presence of more than one pigment increases the proportion of the available light that can be used.
| Pigment | Absorbs strongly | Reflects or transmits |
|---|---|---|
| Chlorophyll a and b | Blue-violet and red | Green |
| Carotenoids | Blue-violet | Yellow, orange, red |
Part 3 of 5Using Spectra
Using Spectra
Reading a graph
- Reading a graph Peaks show wavelengths that are strongly absorbed, and low points show wavelengths that are reflected or transmitted.
Linking to photosynthesis
- Linking to photosynthesis Light that is absorbed can drive the light-dependent reactions, so absorption is the first step in using light.
Comparing pigments
- Comparing pigments Overlaying the spectra of several pigments shows how they complement each other.
Limitation
- Limitation An absorption spectrum shows absorption by an isolated pigment, not how much of the absorbed light is actually used.
Part 4 of 5Measuring an Absorption Spectrum
1
Extract the pigments from leaves with an organic solvent such as propanone to make a clear coloured solution.
2
Place the solution in a cuvette in a spectrophotometer or colorimeter and select one wavelength of light at a time.
3
Record the absorbance at each wavelength, using the pure solvent as a blank to set the zero reading.
4
Plot absorbance on the y-axis against wavelength in nanometres on the x-axis to give the absorption spectrum.
5
Identify the peaks and compare them with the colours of light that the pigment solution reflects.
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 successive peaks of a light wave, measured in nanometres.
- Chlorophyll Green pigments that absorb red and blue-violet light.
- Carotenoid An accessory pigment that mostly absorbs blue-violet light.
Common Mistakes
- Saying that chlorophyll absorbs green light, when it reflects it.
- Confusing absorption spectra with action spectra, which show the rate of photosynthesis.
- Writing that leaves appear green because they use all colours of light equally.
- Forgetting to give units such as nm for wavelength.
Last updated:
\n