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Learning Outcome 3’s (LO3) are the practical aspects of a number of SQA courses. Pupils must pass at least one of these in order to be presented for the final examination. Pupils can only pass if their written report conforms to the SQA criteria. I am currently teaching my Higher Biology class the Chemistry of Photosynthesis.

 Photosynthesis is one of the most fundamentally important biological processes. All green plants can produce their own “food” simply by using the energy in sunlight to convert carbon dioxide and water into a carbohydrate. Although sugar (glucose) is the initial product of photosynthesis, it is usually stored in the form of starch or converted into other more complex carbohydrates (i.e. cellulose in the plant cell walls). The chemical reactions involved in photosynthesis are quite complex so I won’t bore you with excess details.

During this topic we look at the pigments found in various plant leaves. The LO3 for this topic is titled:

‘Extracting The Photosynthetic Pigments By Means Of Paper Chromatography’

Chromatography is a fairly simple technique used to separate and help identify unknown chemical compounds. This technique is based on the idea that different chemicals have a different “attractiveness” for the paper and the solvent used. In the lab the pupils ground up their chosen plant leaves and spotted/blotted the bottom of a piece of chromatography paper. The tip of the paper was then placed in a solvent which travelled up the paper by capillary action. The various pigments in the plant leaves separated out according to their affinity for the paper.

Chlorophyll b was the first pigment to separate out. Above that is Chlorophyll a, then Xanthophyll, and finally at the very top is Carotene.

Different plants contain different pigments. The pigments present will depend on a variety of factors including the environment and sometimes the season. Having carried out this practical many times over the years, I though I would share my top five plants (the ones that give the best results!)

5th- Geranium
4th- Spinach
3rd- Busy Lizzie
2nd- Common Seaweed (Fantastic for Chloropyll a)
1st- Spider Plant, Aspidistra.

If you have any suggestions of other alternatives, I would love to hear from you!