Two graphs, same x-axis, completely different y-axis. One measures what the pigments soak up; the other measures what the plant actually achieves. Lay them on top of each other and you get one of the neatest pieces of evidence in biology.
📚 What you need to know
An absorption spectrum plots absorbance against wavelength — a property of the pigment.
An action spectrum plots the rate of photosynthesis against wavelength — a property of the plant.
Rate is measured as oxygen produced or carbon dioxide used at each wavelength.
Both graphs have two peaks, in the blue-violet and the red, and a trough in the green-yellow.
That correlation is evidence that these pigments are the ones driving photosynthesis.
When drawing an action spectrum: 400–700 nm on the x-axis, a percentage scale on the y-axis, and a smooth curve.
To test wavelength experimentally, use colour filters and keep the lamp at a fixed distance.
The action spectrum
Instead of asking what a pigment absorbs, ask what the whole plant does. Give it one colour of light at a time and measure how fast it photosynthesises. Plot that against wavelength and you have an action spectrum.
The y-axis is normally given as a percentage of the maximum rate, which is why it needs no units — it is a comparison, not an absolute measurement.
Do not let the curve touch zero in the green region. Photosynthesis slows there; it does not stop.
🧩 Drawing an action spectrum
x-axis: wavelength / nm, running from 400 to 700, labelled every 100 nm.
y-axis: rate of photosynthesis / % of maximum rate, from 0 to 100. No units needed, because it is a percentage.
Two peaks, one near each end — blue-violet on the left, red on the right.
A trough in the middle, in the green region, but well above zero.
One smooth curve, drawn freehand rather than joined dot-to-dot.
Putting the two graphs together
Overlay the action spectrum on the absorption spectra of all the pigments and the shapes track each other closely.
The action spectrum is broader than any single pigment curve, because the plant is using all its pigments at once — the accessory pigments fill in the gaps.
What the correlation tells you
Both graphs peak in the blue-violet and the red. Where absorbance is high, the rate is high.
Both dip in the green-yellow. Where little light is absorbed, little photosynthesis happens.
That match supports the conclusion that the chlorophylls and carotenoids are the pigments responsible for capturing the light used in photosynthesis.
The fit is not perfect. The action spectrum sits a little higher in the green, partly because accessory pigments absorb there and partly because green light penetrates deeper into a leaf before being absorbed.
Be careful with the word “proves”. A correlation is strong evidence, and the mark scheme wants supports or is consistent with. Saying it proves the point is the sort of overclaim examiners mark down.
Compare
Absorption spectrum
Action spectrum
What is on the y-axis
Absorbance of light
Rate of photosynthesis
What is being studied
An isolated pigment
A whole photosynthesising organism
How it is measured
Light absorbed by a pigment solution
Oxygen produced or carbon dioxide used
Shape
Two peaks, trough in the green
Two peaks, trough in the green
Typical units
Absorbance, or a percentage
Percentage of the maximum rate
Measuring it in the lab
The set-up is the same one used for light intensity, with one change: this time the colour of the light is the independent variable, not its brightness.
🧩 Testing different wavelengths
Set up pondweed in water with sodium hydrogencarbonate, under an inverted funnel and tube, exactly as for a light intensity investigation.
Fix the lamp distance and never move it. Distance changes intensity, and you are testing wavelength only.
Put a colour filter in front of the lamp so the plant receives one region of the spectrum.
Count bubbles or collect oxygen for a fixed time, then repeat with each filter across the whole spectrum.
Run one trial with no filter as a white-light comparison.
Repeat each colour at least three times and take a mean.
A fair-test wrinkle worth mentioning: a coloured filter also blocks some light, so it dims the lamp as well as colouring it. A good answer notes this and suggests using filters of equal transmission, or measuring the intensity that reaches the plant.
Worked examples
WORKED EXAMPLE
From an action spectrum, the rate at 670 nm is 99% of the maximum and the rate at 550 nm is 30%. Calculate how many times faster photosynthesis is at 670 nm, and explain the difference. [3]
Step 1: divide the two rates99 ÷ 30 = 3.3Step 2: explain using absorbance
At 670 nm the chlorophylls absorb strongly, so much more light energy is captured.
Step 3: explain the low value
At 550 nm most green light is reflected, so little energy is absorbed.
3.3 times faster, because red light is strongly absorbed and green is largely reflectedread both values off the axis before you calculate — do not estimate from the shape
WORKED EXAMPLE
Explain what the close match between the action spectrum and the absorption spectra suggests. [2]
Step 1: describe the matchpeaks and troughs occur at the same wavelengths in both graphsStep 2: draw the conclusion carefully
The wavelengths the pigments absorb are the ones that drive the fastest photosynthesis.
It supports the idea that these pigments are the ones capturing light for photosynthesis“supports”, not “proves” — correlation is evidence, not certainty
WORKED EXAMPLE
A student investigating wavelength moves the lamp closer when using the green filter, because the tank looks dim. Explain why their results will be invalid. [2]
Step 1: name the variable that changed
Moving the lamp changes the light intensity, which is meant to be a control variable.
Step 2: state the consequence
Two variables now differ, so any change in rate cannot be attributed to wavelength alone.
Intensity and wavelength both changed, so the results are not valid“invalid” is about the wrong variable changing; “unreliable” is about scatter between repeats
💡 Exam tip
Check the y-axis first whenever you are given one of these graphs. Absorbance means absorption spectrum; rate means action spectrum.
When drawing, label 400 to 700 nm in 100 nm steps and use a 0 to 100 % scale.
Keep the curve smooth and keep the green trough above zero.
Use supports or is consistent with when writing about the correlation.
For the practical, state that the lamp stays at a fixed distance. It is the control variable examiners look for.
Name the measurement: oxygen produced or carbon dioxide used per minute.
⚠ Common mix-up
Swapping the two graphs. They look similar; only the y-axis tells them apart.
Drawing the green trough down to zero. The rate falls to roughly a third, not to nothing.
Saying the correlation proves the pigments cause photosynthesis. It supports it.
Putting units on a percentage y-axis. A percentage of the maximum needs none.
Changing lamp distance during a wavelength investigation. That is a second variable.
Forgetting the white-light trial. It gives you something to compare all the filters against.
Joining plotted points with straight lines. An action spectrum is drawn as a smooth curve.
Up next: Limiting Factors of Photosynthesis (Skills) — light intensity, carbon dioxide and temperature, and how to design an experiment where only one of them is allowed to change.
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