Photosynthetic pigmentsEdexcel A-Level Biology B: Revision notes
Section 1
Absorption and action spectra
An absorption spectrum is a graph of how much light a pigment absorbs at each wavelength. An action spectrum is a graph of the rate of photosynthesis (for example oxygen release or carbon dioxide uptake) at each wavelength.
The action spectrum of a leaf is roughly the combined absorption spectra of its pigments: photosynthesis is fastest in the red and blue-violet regions and slowest in the green region. Differences between the two types of spectrum show that other pigments contribute.
An absorption spectrum is about one pigment's absorbance, an action spectrum is about the rate of photosynthesis. Do not mix them up.
Section 2
Why plants have several pigments
Chlorophyll a absorbs mainly red (about 660 to 680 nm) and blue-violet (about 430 nm) light and reflects green. It forms the reaction centre of each photosystem.
Accessory pigments such as chlorophyll b and the carotenoids (carotene and xanthophylls) absorb other wavelengths, including blue-green, and pass the energy to chlorophyll a. Having several pigments means a plant absorbs a wider range of wavelengths and so more light energy, which matters most in shaded habitats where the light is filtered. Carotenoids also protect chlorophyll from damage in intense light.
Link the answer: more wavelengths absorbed, then more electrons excited, then more ATP and reduced NADP.
Section 3
Core Practical 10: wavelength and rate
Pondweed (such as Elodea) or algal beads can be used. Light of different wavelengths is provided by coloured filters or LEDs, and the rate is measured as oxygen bubbles per minute or volume of oxygen collected.
Control the light intensity (keep the same distance and check with a light meter, since filters transmit different amounts), temperature (use a heat shield or water bath) and carbon dioxide (dissolved sodium hydrogencarbonate). Allow time to acclimatise at each wavelength, repeat and calculate a mean. The pondweed is a living organism, so handle it gently and return it to its pond afterwards.
Section 4
Core Practical 11: separating pigments
Grind leaf with sand and propanone to extract the pigments. Put a concentrated spot on a pencil origin line on chromatography paper or a thin-layer plate, and stand it in a covered tank with solvent below the origin. Mark the solvent front when it nears the top.
Rf = distance moved by pigment ÷ distance moved by solvent front. Pigments that are more soluble in the solvent, and less attracted to the stationary phase, travel further. In order from the top, you usually see carotene, xanthophylls, chlorophyll a (blue-green) and chlorophyll b (yellow-green).
Example: solvent front 8.0 cm, chlorophyll a 4.4 cm, so Rf = 4.4 ÷ 8.0 = 0.55. Propanone is flammable, so work in a fume cupboard away from flames.
Draw the origin in pencil, not ink, and keep the solvent level below it.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Photosynthetic pigments
- A student investigated the effect of wavelength on the rate of photosynthesis. A piece of pondweed in water containing dissolved sodium hydrogencarbonate was placed 15 cm from a lamp. Coloured filters were placed between the lamp and the pondweed in turn, and the number of oxygen bubbles released in five minutes was counted.The student found that very few bubbles were released when a green filter was used. Explain this result.2 marks
- A researcher measured how strongly a purified extract of chlorophyll a absorbed light of different wavelengths, and the rate of photosynthesis of whole leaves at the same wavelengths. The extract absorbed most strongly at about 430 nm and 660 nm. The leaves also photosynthesised at a substantial rate at 480 nm, where chlorophyll a absorbs very little.Suggest why the action spectrum of the leaves does not match the absorption spectrum of chlorophyll a alone.2 marks
- A student extracted pigments from spinach leaves by grinding them in propanone, then separated them by thin-layer chromatography. After the run the solvent front had moved 8.0 cm from the origin. Four pigment spots were visible, at these distances from the origin: carotene 7.6 cm, xanthophyll 5.6 cm, chlorophyll a 4.4 cm and chlorophyll b 3.6 cm.Calculate the Rf value of chlorophyll a and of xanthophyll.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).