#39 Photosynthesis investigations - Principles and Starch test

Experiments can be used to find out what factors (CO₂, light, cholorophyll) are needed for photosynthesis. But first of all you need to destarch the plants. To be certain that they are thoroughly destarched, test a leaf for starch before you begin your investigation. 

Principles of investigations 

1. Investigations need controls

• Control plant (or leave) has all substances it needs.
• Test plant lacks one substance (light/chlorophyll/CO₂)

2. Plants must be destarched

• It is very important that the leaves you are testing should not have any starch in them at the beginning of the experiment.
• So, first of all, you must destarch the plants. Leave them in the dark for 48 hours. The plants use up all stores of starch in its leaves.

3. Starch test with Iodine solution 

• After a few hours, carry out the starch test on both plants: Iodine solution is used; a blue-black colour on the leave is positive.

• Boil the leaf in water for 30 second. This kill the cells in the leaf à break down the membrane à iodine solution gets through cell membrane to reach starch inside the chloroplasts and react with them.

Boil the leaf in water.

• Boil the leaf in alcohol (ethanol) in a water bath: The green colour of the leaf and the brown iodine solution can look black together, so you need to remove chlorophyll by dissolving it out with alcohol. Leave it until all the chlorophyll has come out of the leaf.

Boil the leaf in alcohol. 

• Rinse the leaf in water: Boiling the leaf in ethanol makes it brittle, the water softens it.
• Spread the leaf out on a white tile à easy to see the result.
• Add iodine solution to the leaf à blue- black colour is positive, starch is present. 

A leaf before (on the left) and after (on the right) starch test. 

 Additional resource: sjiiscience.blogspot.com