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3.3.4Additional lessons

Absorption and Action Spectra Skills — IB Biology SL Revision Notes & Topical Questions

5 min read 6 parts Lesson 59 of 142

📚 IB Biology SL Hub / Topical Revision
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Part 1 of 5Action Spectrum

3.3.4 Absorption and Action Spectra Skills
Definition
  • An action spectrum shows the rate of photosynthesis at each wavelength of light.
Measurement
  • The rate can be measured as oxygen release or carbon dioxide uptake under light of different wavelengths.
Shape
  • Rates are highest in the blue and red regions and lowest in the green region.
Comparison
  • The action spectrum closely follows the absorption spectrum, showing that absorbed light drives photosynthesis.
Differences
  • Small differences arise because accessory pigments pass energy to chlorophyll with varying efficiency.

Part 2 of 5Using the Graphs

Using the Graphs
Reading peaks
  • Read off the wavelength at which absorption or rate is highest and state it with the unit nm.
Comparing
  • State the similarities and differences between the two curves, using data from the axes.
Conclusion
  • If the curves match, the pigments that absorb that light are responsible for the photosynthesis.
Evaluation
  • Check that light intensity was equal at each wavelength, as a fair test requires.

Part 3 of 5Interpreting Evidence

Interpreting Evidence
Evidence
  • Both graphs show that blue and red wavelengths are most effective and green least effective.
Conclusion
  • Leaves look green because they reflect green light, which is least useful for photosynthesis.
Limitations
  • Some green light is still used, so the rate at green wavelengths is low but not zero.

Part 4 of 5Worked Example

Question

  • An action spectrum shows a relative rate of 80 at 450 nm and 20 at 550 nm.
  • Calculate how many times greater the rate is at 450 nm.

  1. Ratio = 80 ÷ 20 = 4.
  2. The rate at 450 nm is four times that at 550 nm.

Answer Four times greater (example data).

Part 5 of 5Exam Checklist & Tips

3.3.4 – Key Points for the Exam

  • Action spectrum A graph of the rate of photosynthesis against wavelength of light.
  • Absorption spectrum A graph of the light absorbed by a pigment against wavelength.
  • Wavelength The distance between successive peaks of a light wave.
  • Aerobic bacteria Bacteria that need oxygen to respire and move toward it.

Common Mistakes

  • Confusing the y axis of the two graphs; one shows absorbance and the other shows the rate of photosynthesis.
  • Saying that the curves are identical when there are small differences.
  • Forgetting that the light intensity should be equal at all wavelengths.
  • Using the peak of a graph without stating the wavelength and its unit.

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