Carbon dioxide enrichment experiments

Molecules (Interaction and interdependence) · Photosynthesis · note 5 of 9

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Carbon dioxide enrichment experimentsSpec C1.3.8

In short

Carbon dioxide enrichment experiments grow plants at higher CO₂ concentrations than today's to predict future rates of photosynthesis and plant growth. Enclosed greenhouse experiments allow careful control of variables. Free-air carbon dioxide enrichment (FACE) experiments release CO₂ over plots in the field, so plants grow in natural conditions, but other variables are harder to control.

Atmospheric carbon dioxide concentration is rising. Carbon dioxide enrichment experiments grow plants at concentrations predicted for the future to find out how rates of photosynthesis and plant growth may change.

Two types of CO₂ enrichment experiment
	Enclosed greenhouse experiments	Free-air CO₂ enrichment (FACE)
Set-up	Plants grown in closed glasshouses or chambers with CO₂ added to the air	Rings of pipes release CO₂ around plots in a field or natural ecosystem; sensors adjust the release to hold a target concentration
Control of variables	Easier: temperature, light, water and minerals can be controlled	Harder: weather, soil and pests vary, but affect treated and control plots similarly
How realistic	Less natural: enclosure changes temperature, humidity and wind	More realistic: plants grow in field conditions with other species
Control	Identical greenhouse at current CO₂	Plots with the same ring structure but no added CO₂

Higher carbon dioxide generally increases the rate of photosynthesis and growth, at least for a time. However, several FACE studies of crops found smaller increases in growth and yield than enclosed experiments had predicted (the size of the gap is still debated). In the field, other factors such as water, temperature or soil nitrogen can become limiting, and the effect of extra CO₂ can shrink over the years.

Practical skill:

Be able to identify a controlled variable, such as plant species, light intensity in a greenhouse, or plot size in a FACE experiment. Field experiments are less controlled than laboratory ones, but some questions can only be answered in natural ecosystems.

Free-air CO₂ enrichment plot: a circular field of crop plants surrounded by a ring of vertical pipes releasing CO₂, more on the upwind side, with a central mast carrying a sensor for CO₂ concentration and wind direction; beside it a smaller control plot with the same ring but no added CO₂. — A FACE experiment: pipes release CO₂ around a field plot to hold a target concentration; control plots have the same rings but no added CO₂.

Written and checked against the IB Biology HL specification · Updated October 2026

Frequently asked questions

Why do plants absorb red and blue light but not green?

Chlorophyll absorbs mainly red and blue light because only photons with energy matching the gaps between its electron energy levels can excite its electrons. Green light does not match well, so most of it is reflected or transmitted, which is why leaves look green. Accessory pigments such as carotenoids absorb some other wavelengths.

What is the difference between an absorption spectrum and an action spectrum?

An absorption spectrum shows the percentage of light a pigment absorbs at each wavelength, while an action spectrum shows the rate of photosynthesis at each wavelength. Both peak in blue and red light. The action spectrum stays above zero in green light because accessory pigments absorb some of it, and the similar shapes show absorbed light drives photosynthesis.

How do you calculate Rf values in chromatography?

Divide the distance moved by the pigment by the distance moved by the solvent front, both measured from the origin line. Rf values lie between 0 and 1 and have no units. Pigments are identified by their colour and by comparing their Rf values with reference values for the same solvent and medium.

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