IB Biology HLPhotosynthesisPaper 1 & 2~11 min read
Absorption Spectra
Leaves are green because green is the colour they throw away. Every other colour gets absorbed and used. An absorption spectrum is simply a graph of which wavelengths a pigment keeps.
📚 What you need to know
An absorption spectrum is a graph of the percentage of light absorbed by a pigment against wavelength.
Visible light runs from about 400 nm (violet) to about 700 nm (red).
Chlorophyll a is the main pigment, absorbing strongly in the blue-violet (around 430 nm) and the red (around 660 nm).
Chlorophyll b is an accessory pigment with peaks shifted slightly, at about 453 nm and 642 nm.
Carotenoids (carotene and xanthophyll) absorb in the blue and blue-green region, around 450–480 nm.
All the pigments absorb very little green light (around 500–570 nm), which is reflected or transmitted — so leaves look green.
Accessory pigments widen the range of wavelengths that can be used and pass the energy to chlorophyll a.
Absorption is measured with a spectrophotometer or colorimeter using a pigment extract.
Reading the graph
The coloured strip along the bottom is the visible spectrum lined up with the axis. Hold the valley against it and the reason leaves look green is obvious.
The three things to say about this graph
Two peaks. Chlorophyll absorbs most strongly in the blue-violet and again in the red.
One valley. Around 500–570 nm absorption is very low. Green light is mostly reflected and transmitted, which is what reaches your eye.
The peaks do not coincide. Chlorophyll b and the carotenoids peak at slightly different wavelengths from chlorophyll a, so together they cover more of the spectrum than any one pigment could.
Try answering “why are plants green?” without using the word absorb, and you will see why so many students get it wrong. The full answer is: the pigments absorb blue and red strongly but absorb green poorly, so green light is reflected into your eye. Green is the wavelength the plant has least use for.
Why more than one pigment?
Chlorophyll a is the only pigment that can pass electrons directly into the light-dependent reactions. Everything else is an accessory pigment.
Accessory pigments absorb wavelengths that chlorophyll a handles poorly, and pass the absorbed energy on to chlorophyll a. The plant therefore captures a wider range of the spectrum than chlorophyll a alone could manage, and photosynthesis runs faster in mixed light.
Pigment
Colour
Absorbs most strongly
Role
Chlorophyll a
Blue-green
About 430 nm and 662 nm
Primary pigment — found at the reaction centre
Chlorophyll b
Yellow-green
About 453 nm and 642 nm
Accessory pigment — broadens the range absorbed
Carotene
Orange
About 450–480 nm
Accessory pigment; also protects against damage by excess light
Xanthophyll
Yellow
Blue region
Accessory pigment
🧠
Primary versus accessory
Think of a relay team. The accessory pigments run the early legs, collecting energy from wavelengths they are good at. Only chlorophyll a crosses the finish line and hands the energy into the reaction.
Measuring an absorption spectrum
🧩 Using a spectrophotometer
Extract the pigments from leaf tissue, and if you want a single pigment, separate them first by chromatography.
Dissolve the pigment in a solvent and put it in a cuvette.
Calibrate the instrument with a cuvette of pure solvent, so the reading is due to the pigment alone.
Shine light of a known wavelength through the sample and record the absorbance.
Repeat across the visible range, in regular steps of wavelength.
Plot absorbance (or percentage absorbed) on the y-axis against wavelength on the x-axis.
Why the peaks shift slightly. The values quoted for peak wavelengths vary a little between sources because they depend on the solvent the pigment is dissolved in, and pigments behave slightly differently inside a living thylakoid membrane. Learn the approximate positions — blue-violet and red — rather than memorising exact numbers.
Worked examples
WE 1
Explain the colour of leaves
Use the absorption spectrum of chlorophyll to explain why leaves appear green. (3 marks)
Point 1: what is absorbed
Chlorophyll absorbs strongly in the blue-violet and red regions of the spectrum.
Point 2: what is not
Absorption is very low around 500 to 570 nm, the green region.
Point 3: the consequence
Green light is therefore reflected and transmitted rather than absorbed, and it is this reflected light that reaches the eye.
Green is the colour chlorophyll absorbs least, so it is the colour we seesay “reflected”, not “not used” — the mark is for what happens to the light
WE 2
Justify accessory pigments
Explain the advantage to a plant of containing accessory pigments as well as chlorophyll a. (3 marks)
Point 1: different peaks
Accessory pigments such as chlorophyll b and the carotenoids absorb at slightly different wavelengths from chlorophyll a.
Point 2: wider capture
Together they absorb a broader range of the spectrum than chlorophyll a could alone, so less light is wasted.
Point 3: the outcome
The energy is passed to chlorophyll a at the reaction centre, so more light energy is available and the rate of photosynthesis is higher.
A wider slice of the spectrum, all funnelled into chlorophyll athis matters most in shaded habitats, where the available light is limited and unusual in colour
Quote peaks as regions — blue-violet and red — and add approximate numbers if you are confident.
Say green light is reflected and transmitted, not simply “not absorbed”.
Distinguish the primary pigment (chlorophyll a) from accessory pigments.
Mention that accessory pigments pass energy to chlorophyll a.
Name the instrument: a spectrophotometer (or colorimeter).
⚠ Common mistakes
Saying chlorophyll absorbs green light. Green is the wavelength it absorbs least.
Saying no green light is absorbed at all. A small amount is; the graph does not touch zero.
Confusing absorption with the rate of photosynthesis. That is an action spectrum — the next page.
Calling chlorophyll b the primary pigment. Chlorophyll a is.
Saying carotenoids are useless because leaves are green. They absorb blue light and protect against excess light.
Putting wavelength on the y-axis. It is always the independent variable, so it goes on the x.
Up next: Absorption & Action Spectra (Skills). You now know which wavelengths are absorbed. The next page asks the harder question: does absorbing a wavelength actually mean the plant can use it?
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