Leaf Chromatography

Age range: 9-12 ages

Duration: 30 minutes

Biomimicry Connection:

Nature uses only the energy it needs and relies on freely available energy. Most of the freely available energy comes from the sun and is converted by green plants into sugars. Plants contain chlorophyll, which is needed for the conversion. With this experiment we can show pupils chlorophyll and other chromatophores of leaves.

Activity Details:

Tools and Materials

• Mortar and pestle
• Ethyl alcohol (70 or 96%)
• Test tube (small size)
• Skewer/loop stick
• Filter paper (7-10cm diameter)

Description

Photosynthesis is a process that takes place in the chloroplasts of leaves, taking place with the help of coloured compounds (chlorophylls, xanthophyll, carotene, etc.). In this task, we separate these dyes.

1. Collect soft leaves (e.g. nettle, dandelion, grass) and grind them up in a mortar with a little alcohol. You can add some sand, but it works also without it.
2. Carefully pour the resulting dark green liquid into a small test tube (filter out any leaves or dirt).
3. Place a clean sheet of filter paper on a flat surface. Dip the end of the loop stick into the solution in the test tube, and touch it to the centre of the filter paper. Repeat the process 10 times. It is important to put the drops on the same spot. Let it dry out.
4. When the green spot has dried, dip the other end of the loop stick in pure alcohol and touch it to the centre of the circle. The colours start separating away from the centre in a concentric circle. Add alcohol as many times as necessary until the colours spreads into a ring with a diameter of a few cm. Let it dry, then observe what colour rings appeared. Which one is outside, which one is inside? Which one ran faster? Why?

Explanation

This method of separating plant pigments is called chromatography (chromo = colour; graphia = drawing). Using chromatography we can separate compounds based on their different properties. The principle is based on how well the molecules in the solution are able to bond to the surface of the filter paper: the easier the bonding the shorter the path; the harder the farther they travel before bonding with the filter paper.

Rubbing with alcohol destroys the cells, so the dyes are also released, which dissolve in the alcohol. By touching the loop stick to the filter paper at one point, a large amount of dye is applied to a small surface. The thin fibres of the filter paper act as capillaries; the pure alcohol is absorbed between them and carries the various dyes with it. The one that binds with more difficultly (xanthophyll, carotene) 'runs' farther; the one that binds more easily (chlorophyll) 'puts down' sooner. This is why we see different colours in different places.

Colours:

• Chlorophyll-a – blue-green
• Chlorophyll-b - yellowish green
• Xanthophyll – yellow
• Carotene - orange
• Anthocyanins - from red to purple

Extensions

This activity can be tried at different times of the year e.g. summer and autumn. Choose the leaves of several tree species, and within them also the different phases of colouration. An interesting comparison can be made if we look at the thickness of the rings containing each colour in different species or colourings.