Materials and Equipment
tripod & gauze
white spotting tile
test tube holder
Before The Experiment
Destarch the plant by leaving it in darkness for 48 hours.
1. Cover one leaf entirely with tinfoil to exclude all light.
2. Expose plant to bright light for at least 4 hours.
3. Remove covered leaf and one other leaf. Make sure you know which is which.
4. Test both leaves for starch as outlined below.
5. Note which one is positive for starch.
6. You should only find the blue black colour in the leaf that was exposed to light.
7. This indicates that light is necessary for photosynthesis.
Starch Test Procedure
1. Bring water bath to boil.
2. Put leaf to be tested in the boiling water for about 30 seconds.
(This softens and kills the leaf)
3. Remove the leaf from the water and place in boiling tube of alcohol. Making sure that no bunsen is lighting, put the tube back into the beaker of hot water. (The alcohol will remove the chlorophyll).
4. When the leaf is white, (about ten minutes) remove the leaf from the alcohol and place it once again in the hot water for a few seconds. (The leaf will be brittle when you take it out of the alcohol and the hot water softens it).
5. Place the leaf on the spotting tile and cover it with iodine. Leave for a minute or two.
6. The presence of a blue/black colour indicates the presence of starch.
1 Make sure that there is no naked flame lighting anywhere near the alcohol. It is particularly important if using a bunsen for the water bath that the bunsen is turned off before putting the alcohol into the bath.
2 Do not pour the alcohol down the sink.
3 Take care when using sodium hydroxide (soda lime) as it is very corrosive.
*Suitable Plants: Variegated geranium is most suitable. It is a very good idea to keep a variegated geranium in the lab throughout the year; also busy-lizzie.
Iodine Solution: Grind 1g iodine and 1g potassium iodide in a mortar with distilled water. Pour into a graduated cylinder and dilute to 100cm3. Store in a dark glass bottle. When ready for use dilute by adding 5cm3 of the stock solution to 100cm3 of distilled water. This will not keep well. The success of the experiment depends on having fresh iodine.