A level-readyAll subjects · Biology · WBI14 (Unit 4)
Chloroplasts, pigments and spectra
Chloroplasts are built for photosynthesis. Several pigments absorb different wavelengths, and chromatography lets us separate and identify them.
Chloroplast structure
- Double membrane (envelope) surrounds the organelle.
- Thylakoids: flattened membrane sacs, stacked into grana and joined by lamellae. Their membranes hold the photosystems, electron transport chain and ATP synthase, and stacking gives a large surface area for the light-dependent reactions. The small thylakoid space lets a steep H+ gradient build up.
- Stroma: the fluid around the thylakoids, containing the enzymes of the Calvin cycle (including RuBisCO), starch grains, lipid droplets, circular DNA and 70S ribosomes (so some chloroplast proteins are made inside the chloroplast).
Absorption and action spectra
- An absorption spectrum shows how much light a pigment absorbs at each wavelength. Chlorophylls a and b absorb mainly blue-violet and red light and reflect green. Carotenoids absorb blue-violet light.
- An action spectrum shows the rate of photosynthesis at each wavelength.
The two spectra have a very similar shape, which is evidence that the light absorbed by these pigments is the light used in photosynthesis. Having several pigments widens the range of wavelengths a plant can use.
Chromatography of pigments
1. Grind a leaf with a solvent (such as propanone) to extract the pigments.
2. Spot the extract repeatedly on a pencil origin line on chromatography paper or a thin-layer (TLC) plate, drying between spots.
3. Stand it in a solvent below the origin line, cover, and let the solvent rise. Mark the solvent front.
4. Pigments separate because they differ in solubility in the solvent and in attraction to the paper or plate.
Rf = distance moved by the pigment / distance moved by the solvent front.
Rf is always less than 1. Comparing Rf values with known values identifies each pigment, for example chlorophyll a, chlorophyll b, carotene and xanthophyll.
Worked example
A pigment spot is 3.6 cm from the origin and the solvent front is 8.0 cm from the origin. Calculate Rf.
- Rf = distance moved by pigment / distance moved by solvent
- = 3.6 / 8.0
Answer: Rf = 0.45
Check yourself
- A pigment moves 2.1 cm and the solvent front moves 7.0 cm. Calculate the Rf value.
Show answer
0.30. 2.1 / 7.0 = 0.30.
- A pigment has an Rf of 0.95. The solvent front moved 6.0 cm. How far did the pigment move, in cm?
Show answer
5.7 cm. 0.95 x 6.0 = 5.7 cm.
- Why are thylakoids stacked into grana?
- To store starch
- To hold the Calvin cycle enzymes
- To give a large surface area for photosystems and electron carriers
- To make ribosomes
Show answer
To give a large surface area for photosystems and electron carriers. More membrane means more photosystems and more light absorbed.
- Which of these is found in the stroma?
- Photosystem II
- RuBisCO
- The electron transport chain
- Chlorophyll a reaction centres
Show answer
RuBisCO. The Calvin cycle enzymes are in the stroma; the others are in thylakoid membranes.
- What does an action spectrum show?
- How much light one pigment absorbs at each wavelength
- The distance each pigment moves in chromatography
- The colours a leaf reflects
- The rate of photosynthesis at different wavelengths of light
Show answer
The rate of photosynthesis at different wavelengths of light. Action = what the plant does; absorption = what the pigment absorbs.
- Why do most leaves look green?
- Chlorophylls absorb mostly green light
- Chlorophylls absorb little green light, so it is reflected
- Carotenoids absorb red light
- Green light is used fastest in photosynthesis
Show answer
Chlorophylls absorb little green light, so it is reflected. Chlorophylls absorb red and blue-violet strongly and reflect green.
- Why is the origin line on a chromatogram drawn in pencil, not ink?
- Pencil makes the pigments move faster
- Pencil absorbs the solvent
- Ink stops the solvent rising
- Ink would dissolve in the solvent and separate too
Show answer
Ink would dissolve in the solvent and separate too. Graphite is insoluble, so it does not run with the solvent.
Sources
Last updated 7 November 2026. Spotted a mistake? Tell us.