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C1.3.8—CO₂ enrichment experiments

Carbon dioxide enrichment experiments test whether elevated atmospheric carbon dioxide changes photosynthesis or growth under realistic environmental conditions, connecting the mechanism to observable photosynthesis evidence.

Syllabus
First assessment 2025
Objective
C1.3.8
Level
SL

Exam analysis

Chance of appearing1%of analysed past papers
Latest appearanceMay 2010
Most common paperPaper3
Typical marks2

Common command terms

  • Discuss

Recent exam appearances

May 2010Paper3 ["SL"] · TZ1C1(c)[ 2 ]C1.3.8—CO₂ enrichment experiments

Coverage 2010–2010 · Updated 15 Jul 2026

CO₂ Enrichment Tests a Conditional Prediction

Extra CO₂ can raise photosynthesis or growth only while light, temperature, water, mineral nutrients and biochemical capacity remain sufficient. An enrichment result therefore supports a conditional prediction, not unlimited future growth.

Towers around a forest plot release carbon dioxide into open air for a FACE enrichment experiment.
Design Strength Main limitation
greenhouse or chamber close control of CO₂ and other conditions enclosure changes light, temperature, airflow and scale
FACE field plot exposes intact crops or ecosystems under realistic open-air conditions rainfall, light and other variables cannot be controlled and must be monitored

Compare enriched and ambient-CO₂ replicates, measure photosynthesis and biomass over time, and report variation. Differences among species, years or water treatments show interaction with other limiting factors.

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.

CO₂ enrichment experiments

[ 2 ]

Using the data in the graph, discuss whether rising carbon dioxide levels in the atmosphere will increase growth rates in maize.

Concept essentials

  • FACE experiments expose plants to elevated carbon dioxide under more realistic field conditions.
  • Higher carbon dioxide does not always cause a higher growth rate.
  • Temperature and other limiting factors can alter enrichment responses.
  • Carbon dioxide enrichment data must be interpreted from the experimental graph or design.