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C1.3.6—Absorption vs. action spectra

Absorption spectra show pigment light absorption, while action spectra show how effectively each wavelength drives photosynthesis in living systems, connecting the mechanism to observable photosynthesis evidence.

Syllabus
First assessment 2025
Objective
C1.3.6
Level
SL

Exam analysis

Chance of appearing5%of analysed past papers
Latest appearanceNovember 2022
Most common paperPaper3
Typical marks1–2

Common command terms

  • Sketch
  • Draw
  • Explain
  • Compare
  • Contrast
  • Predict
  • Deduce
  • Identify

Recent exam appearances

November 2022Paper1 ["SL"] · TZ012[ 1 ]C1.3.6—Absorption vs. action spectra
May 2021Paper1 ["SL"] · TZ116[ 1 ]C1.3.6—Absorption vs. action spectra
May 2019Paper1 ["SL"] · TZ211[ 1 ]C1.3.6—Absorption vs. action spectra
November 2018Paper3 ["SL"] · TZ01(c)(ii)[ 1 ]C1.3.6—Absorption vs. action spectra
November 2018Paper3 ["SL"] · TZ01(c)(i)[ 1 ]C1.3.6—Absorption vs. action spectra

Coverage 2013–2022 · Updated 15 Jul 2026

Absorption and Action Spectra Answer Different Questions

Spectrum What is varied What the y-axis measures What it tests
absorption wavelength light absorbed by a pigment or extract which wavelengths pigments capture
action wavelength rate of photosynthesis, such as O₂ production or CO₂ uptake which wavelengths drive the whole process

Both normally show strong effects in blue and red regions and weaker effects in much of the green region. The curves are related, not identical: an action spectrum integrates all pigments, energy transfer and every later rate-limiting step.

To compare wavelengths fairly, keep incident photon supply, temperature, CO₂ availability and plant material constant; a coloured filter that also changes intensity confounds the conclusion.

SL Checkpoint: Read Photosynthesis as Matter, Energy and Evidence

light energy → excited pigment electrons → ATP and reduced NADP → CO₂ reduced using hydrogen from water → organic carbon compounds; photolysis of water releases O₂

  • A chromatogram separates pigments; Rf compares their movement under matched conditions.
  • An absorption spectrum measures captured wavelengths; an action spectrum measures whole-process rate.
  • A limiting-factor experiment changes one condition, controls the others and measures O₂ production or CO₂ uptake per unit time.

When a rate curve plateaus, identify which factor was relieved and which may now be limiting. When evaluating CO₂ enrichment, separate strong control from ecological realism and do not extrapolate beyond the tested conditions.

Absorption vs. action spectra

[ 3 ]

Draw a fully labelled graph of the action spectrum for photosynthesis.

Drawing canvas

Concept essentials

  • Absorption spectra measure light absorbed by pigments at each wavelength.
  • Action spectra measure photosynthetic activity at each wavelength.
  • Action spectra usually reflect the combined absorption of several pigments.
  • Blue and red wavelengths often produce high photosynthetic activity in green plants.