3.3.4Inquiry Process
Absorption and Action Spectra- Skills — IB Biology HL Revision Notes & Topical Questions
5 min read 4 parts Lesson 119 of 160
Part 1 of 4Two Different Graphs
An absorption spectrum shows what is absorbed, whereas an action spectrum shows what is effective.
Absorption spectrum
- Shows how much light of each wavelength is absorbed by an extracted pigment or a pigment mixture.
Action spectrum
- Shows the rate of photosynthesis, such as oxygen release, at each wavelength of light.
Why compare them
- If absorption of a wavelength leads to photosynthesis, the two graphs will follow similar patterns.
Differences
- The curves are similar but not identical,
- because absorption by a pigment only drives photosynthesis if the energy reaches a reaction centre.
Part 2 of 4Obtaining an Action Spectrum
1
Illuminate a photosynthesising organism, such as an aquatic plant or algae, with light of one wavelength at a time using filters.
2
Keep other variables such as light intensity, temperature and carbon dioxide concentration constant.
3
Measure the rate of photosynthesis at each wavelength, for example by counting oxygen bubbles or using an oxygen sensor.
4
Plot rate of photosynthesis on the y-axis against wavelength on the x-axis.
5
Compare the shape of the curve with the absorption spectra of the pigments to show which pigments are most important.
Part 3 of 4Interpreting Graphs
Interpreting Graphs
Peaks in blue and red
- Both graphs have peaks in the blue-violet and red regions, showing that light absorbed by chlorophyll drives photosynthesis.
Low in green
- Rates are lowest in green light because it is mostly reflected, but they are not zero.
Engelmann's experiment
- Motile aerobic bacteria gathered along a filament of algae where oxygen release was greatest,
- which showed that red and blue light were the most effective.
Other evidence
- Modern work uses oxygen electrodes and dataloggers, which give more precise measurements than counting bacteria.
| Wavelength region | Absorption by chlorophyll | Rate of photosynthesis |
|---|---|---|
| Violet-blue (about 400 to 500 nm) | High | High |
| Green (about 500 to 600 nm) | Low | Low |
| Red (about 600 to 700 nm) | High | High |
Part 4 of 4Exam Checklist & Tips
3.3.4 – Key Points for the Exam
- Action spectrum A graph of the rate of photosynthesis against wavelength of light.
- Filter A coloured material that lets through only certain wavelengths of light.
- Oxygen sensor A device that measures dissolved or gaseous oxygen concentration.
- Accessory pigment A pigment that passes absorbed energy to chlorophyll a.
Common Mistakes
- Plotting the action spectrum with absorbance on the y-axis, when it should show rate of photosynthesis.
- Concluding that green light has no effect on photosynthesis, when the rate is low but not zero.
- Changing light intensity as well as wavelength, which makes the comparison unfair.
- Describing the two spectra as identical, when they are similar but have differences.
Last updated:
\n