IB Biology HLPhotosynthesisPaper 1 & 2~11 min read
CO2 Enrichment Experiments
Carbon dioxide makes up a tiny fraction of the air, and for a plant in bright sunlight it is usually the bottleneck. So what happens if you give plants more of it? Growers, and climate scientists, have spent decades finding out.
📚 What you need to know
Atmospheric CO2 is around 0.04% (about 420 parts per million), which is often limiting for photosynthesis in good light.
CO2 enrichment means deliberately raising the concentration and measuring the effect on photosynthesis or on growth.
Raising CO2 increases the rate up to a point, then the curve plateaus as another factor becomes limiting.
Enrichment works only if light, temperature and water are not the limiting factors.
Enclosed greenhouse or growth chamber studies control conditions tightly but are not realistic.
FACE (Free-Air CO2 Enrichment) experiments release CO2 over open plots of real vegetation, so the conditions are realistic but harder to control.
Results are compared with control plots at ambient CO2, and repeated across plots and seasons.
Long-term responses are often smaller than short-term ones, because plants acclimate and other nutrients run short.
Why enrichment works at all
Carbon dioxide is the substrate of the light-independent reactions: it is the molecule that gets fixed. On a bright, warm day with plenty of water, light and temperature are not the problem — the plant simply cannot get hold of enough CO2. That is a textbook limiting factor situation, and raising the supply raises the rate.
The shape explains both the opportunity and its limit. Ambient air sits low on a steep slope, but the slope runs out — enrichment has a ceiling.
Look at where 420 ppm falls on that curve. Plants evolved in air that leaves them permanently short of their own substrate. That is a genuinely strange fact, and it is why commercial growers pump CO2 into greenhouses at all.
Two ways to run the experiment
Enclosed chamber or greenhouse
FACE (Free-Air CO2 Enrichment)
Set-up
Plants sealed in a chamber with controlled gas
Rings of pipes release CO2 over open plots of field or forest
Control of variables
Excellent — light, temperature and humidity all controlled
Poor — weather, soil and herbivory all vary
Realism
Low — conditions are artificial and plants are often young or potted
High — mature plants in their natural community and soil
Scale and cost
Small and relatively cheap
Large and very expensive
Main weakness
Results may not apply to real ecosystems
Hard to attribute differences to CO2 alone
Neither design is simply better. The chamber gives you a clean answer to a narrow question; FACE gives you a messy answer to a realistic one. Good experimental design questions want you to say that.
Designing the study
🧩 What a valid enrichment experiment needs
Control plots at ambient CO2, treated identically in every other way.
In FACE studies, control rings often have the same pipework blowing ordinary air, so any effect of the equipment itself is controlled for.
Replication: several plots at each concentration, to allow a mean and to expose anomalies.
Random allocation of treatments to plots, so soil differences do not bias the result.
A clearly defined dependent variable: rate of CO2 uptake, or biomass, or yield — with units.
Long enough duration to see whether the initial response is sustained across seasons.
The result that surprised people. Short-term enrichment often produces a large jump in photosynthetic rate, but the long-term gain in biomass is usually smaller. Plants acclimate, and growth soon becomes limited by something else — typically nitrogen or water. A limiting factor was removed, and a different one simply took its place.
Worked examples
WE 1
Explain a greenhouse decision
A grower adds CO2 to a greenhouse in summer and sees a large increase in yield, but adding the same amount in winter has almost no effect. Explain. (4 marks)
Point 1: summer conditions
In summer, light intensity and temperature are high, so neither is limiting and CO2 is the limiting factor.
Point 2: the summer result
Raising CO2 therefore increases the rate of the light-independent reactions and so increases yield.
Point 3: winter conditions
In winter, light intensity (and probably temperature) is low, so one of those is limiting instead.
Point 4: the winter result
Increasing a factor that is not limiting has no effect on the rate, so the extra CO2 is wasted.
Enrichment only pays when CO₂ is the factor in short supplythe general principle in point 4 is worth stating explicitly — it is often a mark on its own
WE 2
Calculate a percentage increase
At 420 ppm CO2 a crop fixes carbon at 68 arbitrary units; at 1000 ppm it fixes 83 units. Calculate the percentage increase. (2 marks)
Step 1: the increase
83 − 68 = 15 unitsStep 2: as a percentage of the original
(15 ÷ 68) × 100 = 22.06…
an increase of about 22%divide by the original value, not the new one — that error turns 22% into 18%
WE 3
Evaluate an experimental design
A scientist grows seedlings in a sealed chamber at 1000 ppm CO2 and concludes that forests will grow 20% faster as atmospheric CO2 rises. Evaluate this conclusion. (4 marks)
Point 1: a strength
Conditions in the chamber are tightly controlled, so the effect can confidently be attributed to CO2 concentration.
Point 2: the organisms are not comparable
Seedlings are not mature forest trees, and their growth response may be quite different.
Point 3: the conditions are not realistic
Real forests face variable weather, competition, herbivory and limited soil nutrients, especially nitrogen, which may become the new limiting factor.
Point 4: the improvement
A long-term FACE study on mature vegetation with replicated ambient control plots would give more valid evidence.
A sound experiment, but extrapolated far beyond what it can supportgood evaluation names the specific limitation and the specific improvement
💡 Exam tips
Say CO2 is the substrate of the light-independent reactions, not just “needed for photosynthesis”.
Always check whether the other factors are limiting before predicting an effect.
Know both designs and one strength and one weakness of each.
For percentage change, divide by the original value.
Distinguish a rise in photosynthetic rate from a rise in biomass or yield — they are not the same measurement.
Mention replication and ambient control plots in any design answer.
⚠ Common mistakes
Assuming more CO2 always means more growth. Only if CO2 is the limiting factor.
Forgetting the plateau. Very high concentrations give little further benefit.
Treating a greenhouse result as proof about ecosystems. The conditions are not comparable.
Leaving out the control. Without ambient plots there is nothing to compare against.
Ignoring other nutrients. Nitrogen and water often become limiting instead.
Confusing ppm with percentage. 1000 ppm is 0.1%, not 1%.
Up next: Photosystems. Enough of the whole-plant view. From here on the topic zooms into the thylakoid membrane, starting with the structures that actually capture the light.
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