IB Biology HLPhotosynthesisPaper 1 & 2~11 min read
Absorption & Action Spectra (Skills)
Two graphs, same axes, completely different measurements. One records what the pigments catch; the other records what the plant actually achieves. Laying them on top of each other is one of the neatest pieces of evidence in biology.
📚 What you need to know
An absorption spectrum shows the percentage of light absorbed by a pigment at each wavelength.
An action spectrum shows the rate of photosynthesis at each wavelength.
The two curves have a similar shape: high in the blue-violet and red, low in the green.
That correlation is evidence that the pigments measured are the ones driving photosynthesis.
The action spectrum is broader and does not fall as low in the green, because accessory pigments absorb there and pass energy on.
A correlation is not proof of cause on its own — but combined with the pigment data it is strong evidence.
Action spectra are measured by illuminating a plant with light of one wavelength at a time and recording oxygen production or carbon dioxide uptake.
Light intensity must be kept constant at every wavelength, or the comparison is meaningless.
What each graph measures
Absorption spectrum
Action spectrum
y-axis
Light absorbed / %
Rate of photosynthesis
x-axis
Wavelength / nm
Wavelength / nm
What is measured
Absorbance of a pigment extract
Oxygen released or CO2 taken up by a whole plant or alga
Instrument
Spectrophotometer or colorimeter
Oxygen sensor, gas syringe or bubble counter
Tells you
Which wavelengths the pigment captures
Which wavelengths actually drive the process
Keep the distinction in one sentence each: absorption is about the pigment, action is about the plant. If the y-axis mentions a rate, you are looking at an action spectrum, whatever the shape.
Overlaying them
The gap in the middle is the interesting part. If chlorophyll a were the only pigment, the red curve would sink into the valley with the green one.
Describing the relationship
Questions almost always ask you to describe the relationship and then explain it. Split your answer the same way.
Describe the similarity. Both curves peak in the blue-violet (about 430–450 nm) and again in the red (about 650–670 nm), and both fall to a minimum in the green (about 500–570 nm). They are positively correlated.
Explain the similarity. The wavelengths absorbed most are the ones whose energy is available to drive the light-dependent reactions, so the rate of photosynthesis is highest at those wavelengths.
Describe the difference. The action spectrum is broader and its trough is shallower — the rate in green light is well above zero.
Explain the difference. Accessory pigments such as chlorophyll b and the carotenoids absorb wavelengths that chlorophyll a does not, and pass the energy to chlorophyll a. An absorption spectrum for a single pigment cannot show that.
Correlation is not causation. The two curves matching does not by itself prove that chlorophyll drives photosynthesis — something else might absorb at the same wavelengths. It becomes strong evidence when combined with other findings: that chlorophyll is present in the chloroplast, that mutants lacking it cannot photosynthesise, and that isolated chloroplasts behave the same way. Examiners like the phrase “supports, but does not on its own prove“.
Measuring an action spectrum
🧩 Method outline
Place a photosynthesising organism — pondweed or an algal suspension is ideal — in a sealed vessel with a hydrogencarbonate solution to supply CO2.
Illuminate it with light of a single wavelength, using coloured filters or a monochromator.
Measure the rate of photosynthesis as oxygen produced per unit time, or as CO2 uptake.
Repeat at regular intervals across the visible spectrum.
Keep light intensity, temperature and CO2 concentration constant, and use the same mass of tissue throughout.
Plot rate against wavelength, and repeat the whole series to obtain a mean.
🧠
The control that everyone forgets
Coloured filters cut out light, so a red filter and a green filter do not transmit the same intensity. If you do not correct for that, you are measuring brightness, not wavelength.
Worked examples
WE 1
Compare the two spectra
Describe and explain the relationship between the absorption spectrum of chlorophyll a and the action spectrum of photosynthesis. (4 marks)
Describe the match
Both are highest in the blue-violet and red regions and lowest in the green, so they are positively correlated.
Explain the match
Light absorbed by chlorophyll a provides the energy for the light-dependent reactions, so the rate is greatest where absorption is greatest.
Describe the difference
The action spectrum is broader and does not fall as low in the green region.
Explain the difference
Accessory pigments absorb those wavelengths and transfer the energy to chlorophyll a, so some photosynthesis still occurs in green light.
Same overall shape, but the action spectrum is broader and shallowertwo describe marks and two explain marks — give both halves for each point
WE 2
Evaluate the evidence
A student concludes that the correlation between the two spectra proves chlorophyll causes photosynthesis. Evaluate this conclusion. (3 marks)
Point 1: what the data supports
The close correlation is strong supporting evidence that chlorophyll is responsible for absorbing the light used.
Point 2: the limitation
A correlation does not establish causation — another pigment absorbing at similar wavelengths could produce the same pattern.
Point 3: what would strengthen it
Further evidence is needed, such as testing plants that lack chlorophyll, or using isolated pigments, before a causal claim is justified.
Supports the conclusion strongly, but does not prove it alone“evaluate” always wants a limitation as well as a strength
WE 3
Predict from the graph
A greenhouse grower is choosing between green and red supplementary lamps. Use the spectra to advise them. (3 marks)
Point 1: the recommendation
Choose the red lamps.
Point 2: the reason from the absorption spectrum
Chlorophyll a and b both absorb strongly around 650 to 670 nm but absorb green light poorly, so most green light would be reflected and wasted.
Point 3: the reason from the action spectrum
The action spectrum confirms the rate of photosynthesis is far higher in red light than in green, so the same energy input yields more growth.
Red light is absorbed and used; green light is largely reflecteduse both graphs when a question gives you both — each is worth a separate mark
💡 Exam tips
Check the y-axis first to tell the two graphs apart.
Answer “describe and explain” questions in two clearly separated halves.
Explain the shallower green trough using accessory pigments.
Use the phrase positively correlated when describing the match.
Say the evidence supports rather than proves a causal link.
For method questions, control light intensity as well as temperature and CO2.
⚠ Common mistakes
Mixing up the two spectra. Absorption is a percentage; action is a rate.
Saying no photosynthesis occurs in green light. The rate is reduced, not zero.
Claiming the correlation proves causation. It supports it.
Explaining the difference using chlorophyll a alone. The point is the other pigments.
Forgetting to control light intensity when changing wavelength.
Describing without explaining. Half the marks are usually for the reason.
Up next: Limiting Factors of Photosynthesis (Skills). Wavelength is only one thing you can change. The next page deals with light intensity, carbon dioxide and temperature — and with the idea that at any moment, only one of them is holding the plant back.
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