Biology

Gilla:		Dela:

Video

Materials

• 1 spinach leaf (or other leaf with strong color and little water)
• 1 drinking glass
• 1 coin
• Isopropyl alcohol (rubbing alcohol) or acetone
• 1 white coffee filter
• 1 pair of scissors
• 1 ruler
• Plastic wrap

Short explanation

The alcohol or acetone moves up the filter paper and takes the plant pigments with it. Because the pigments move at different speeds through the filter paper, they are separated.

Long explanation

A pigment is a molecule that absorbs light. The color of the pigment depends on the wavelengths of light that are absorbed. In the leaves of plants, for example, there's a pigment called chlorophyll. Chlorophyll absorbs red and blue light, while reflecting all the other colors. Since the remaining colors are dominated by green, chlorophyll looks green.

Chlorophyll is the most common pigment in plants. The light it absorbs is used in photosynthesis to build the energy-rich molecules the plant then uses as energy and building material.

However, chlorophyll is not alone in absorbing light in plants. It would be a waste of light not to absorb other colors than red and blue, and therefore there are pigments that do just that. Carotenoids (xanthophyll and beta-carotene and others) are one group of such "helper pigments", which absorb green and blue light and thus look yellow. It is carotenoids that give carrots their color. In your demonstration, you will probably see chlorophyll and carotenoids. Maybe you will also see some other pigments.

In this demonstration, you have used a method called chromatography. This method is used to separate the different ingredients in a mixture. Chromatography roughly means "writing with colors", which fits well in this demonstration. In fact, it was precisely this demonstration that gave the method its name. However, it does not have to be pigments that are separated in chromatography, it can be anything. Chromatography works by moving a mobile phase (for example alcohol) through a stationary phase (for example a filter paper) and carrying the mixture to be separated. Because all substances have different properties, such as the size of the molecules and their electric charge, they move at different speeds through the stationary phase. And thus they are separated.

Why not use water instead of isopropyl alcohol or acetone? Well, the plant pigments are not very soluble in water and they would not follow the water up through the filter paper. However, the pigments dissolve well in alcohol and acetone.

Why wait for 20 minutes? Well, after about that long the pigments are well separated but not too scattered. If you wait longer, the pigments will gather again, as the alcohol only moves to a certain height. But decide for yourself when you want to stop the demonstration, it's a matter of taste.

Why do the leaves turn yellow and red in autumn?

Since the leaves will soon be shed, the tree absorbs nutrients and chlorophyll from the leaves and saves it in the branches and trunk. This is to save these precious molecules for spring. The chlorophyll can be broken down into its constituents, which are then reassembled.

There are actually some deciduous trees that shed green leaves. These are trees that are very efficient at utilizing, for example, the nitrogen in the soil, so they can easily obtain material and energy again for these molecules.

It is the carotenoids that make the leaves yellow. The pigments are present in the leaf all summer, but are not visible due to the chlorophyll.

The red colors come mainly from anthocyanins, which are formed in the cavities of leaf cells in the autumn. In the vacuoles, as the cavities are called, the trees form sugar during the day. During cold nights, the sugar is converted to anthocyanins. The amount of anthocyanin formed varies between different years and it probably depends mainly on the temperature.

No one knows for sure the evolutionary advantage of forming these beautiful and strong colors. It can be due to several interacting causes that are important for different processes in the trees. Examples include the reuse of nutrient resources such as nitrogen and phosphorus. This process differs between different species and geographical areas. Another suggestion that exists is that the color is the result of an interplay of development between insects and plants.

Experiment

You can turn this demonstration into an experiment. This will make it a better science project. To do that, try answering one of the following questions. The answer to the question will be your hypothesis. Then test the hypothesis by doing the experiment.

• What will de the result be from a range of different leaves?
• How can I get as strong colors as possible?

Variations

Try using the leaves of a flower - the petals - instead. These often have strong colors, and thus plenty of pigments. The pigments in a flower can have a different function than capturing light for photosynthesis, namely to attract insects for pollination.

If you want a real color explosion, you can try mashing petals of different colors in a mortar. First pour a little isopropyl alcohol or acetone in the mortar and then start mashing. Put a coffee filter over a glass and pour the puree into the coffee filter. Then place a coffee filter strip standing in the resulting liquid in the glass.

Gilla:		Dela: