Investigating photosynthetic pigments through TLC

This practical on photosynthetic pigments was developed for the Scottish Highers and is in the appropriate format. An illustrated worksheet, technical notes and students’ questions are available to download here: Student Sheet 10.

Photosynthetic pigments in plants absorb light energy and convert it to chemical energy. This energy is transferred to and trapped inside chemicals such as glucose and starch.

This resource is designed to support students in

• developing their knowledge and understanding of the photosynthetic pigments
• developing their knowledge and understanding of the absorption spectra of different pigments and relate this to the action spectrum of the plant.

Several different pigments are involved in the absorption of light. This experiment shows how these photosynthetic pigments can be extracted from plants and then separated.

Chromatography is a method used to separate chemically similar substances. Separating photosynthetic pigments using thin layer chromatography (TLC) has a number of advantages:

• Only small volumes of propanone are required for extraction so build up of toxic fumes is avoided.
• The pigments take about three minutes to separate, so there is little chance of time running out at the end of the period or the student forgetting about it.
• The resulting chromatogram gives very clear results, allowing calculations of Rf values.
• Other pigments apart from the usual four will probably be visible on the chromatogram. Their presence helps students appreciate why the absorption spectra of pigments can be different from the action spectrum of the plant.