Interpret absorption and action spectra
Compare pigment absorption spectra with photosynthesis action spectra and draw conclusions from wavelength evidence.
Section 13.1 · Photosynthesis as an energy transfer process
A Level; Cambridge International Biology 9700, 2028–2030. Written practice is self-checked; practical investigations need supervised work and human subject-teacher review remains pending.
Before you begin
- Chloroplasts contain photosynthetic pigments, and enzymes catalyse metabolic reactions.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which spectrum plots pigment absorption against wavelength?
What you will learn
- Distinguish absorption spectra from action spectra
- Interpret wavelength patterns without equating the measured quantities
Build the explanation
An absorption spectrum plots how strongly a pigment or pigment mixture absorbs different wavelengths. An action spectrum plots the rate or effectiveness of photosynthesis at different wavelengths. Peaks in the action spectrum often occur where chloroplast pigments absorb strongly, because absorbed light can supply energy to photosynthesis. Green wavelengths are commonly less effective than red or blue regions, but use the actual scale and data supplied.
Apply it carefully
Absorption and action spectra measure different things: light absorption versus photosynthetic activity. A similar pattern supports a relationship but does not make their vertical axes interchangeable. Check whether the graph shows relative absorbance, oxygen production or another rate, and describe the observed wavelengths before explaining them.
Pause and explain
If both spectra peak in blue light, are the y-values the same quantity?
Worked example
What does an action spectrum show that an absorption spectrum does not directly show?
Show the worked solution
- Compare the plotted response rather than the pigment property.
- An action spectrum shows photosynthetic effectiveness or rate across wavelengths.
Answer An action spectrum shows how photosynthetic rate varies with wavelength; an absorption spectrum shows pigment absorption.
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A graph shows strong pigment absorption and a high photosynthetic rate in red light, with lower values in green. Interpret the relationship.
Give me a hint
Identify each spectrum and connect absorbed light to photosynthetic energy transfer.
Compare my reasoning
- The absorption spectrum shows pigments absorb red strongly and green less strongly.
- The action spectrum shows higher photosynthetic effectiveness in red; the similar pattern is consistent with absorbed light supplying energy.
Pigments absorb red light strongly and green light less strongly in the supplied absorption spectrum. The action spectrum shows higher photosynthetic rate in red than green. The similar pattern is consistent with absorbed light supplying energy, but the two axes measure different quantities.
Look for these in your work
- Absorption pattern stated
- Action pattern stated
- Relationship and limit stated
A researcher doubles absorbance in a pigment test but does not measure a plant. Can the photosynthetic rate be reported as doubled? Explain.
Give me a hint
Distinguish the instrument's pigment measurement from an action-spectrum rate.
Compare my reasoning
- The measurement is absorbance, not photosynthetic rate.
- A separate photosynthesis measurement is needed to determine the rate response.
No. The pigment test measured absorbance. It may provide evidence about light capture, but photosynthetic rate must be measured separately, for example using gas exchange under controlled conditions.
Look for these in your work
- Measured quantity identified
- Rate inference rejected
- Separate measurement suggested
Two action spectra have peaks at similar wavelengths but different vertical scales. State one valid comparison and one invalid comparison.
Give me a hint
Compare wavelength positions, not unsupported absolute heights.
Compare my reasoning
- The peak wavelength positions can be compared if axes use the same wavelength unit.
- Absolute rates cannot be compared unless the scales and experimental conditions are comparable.
The wavelength positions of peaks can be compared if both use the same wavelength units. Absolute rate heights cannot be compared without compatible scales, normalization and conditions.
Look for these in your work
- Valid peak comparison
- Scale limitation stated
- Conditions acknowledged
Common mistakes
- Calling absorbance a photosynthesis rate
- Comparing graph heights without checking scales
- Assuming correlation proves the spectra are identical
What can you explain now?
Which spectrum plots pigment absorption against wavelength?
Compare with the explanation
An absorption spectrum
An action spectrum plots photosynthetic rate or effectiveness.
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Make it stick
What is plotted on an absorption spectrum and what is plotted on an action spectrum?
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