Test for Starch in Leaves
Photosynthesis is the process by which green plants and some other organisms use sunlight to synthesize food from carbon dioxide and water. One product of photosynthesis in green plants is starch. The presence of starch can be con rmed by the addition of iodine solution.
To show that starch is a product of photosynthesis.
Young green leaves, ethanol*, iodine solution*, heat source, cooking pots, water, test tube*, white tile, dropper*, and cotton wool*.
Ethanol is very ammable! Make sure that student cover their test tubes with cotton wool to avoid excess release of ethanol vapour. If a test tube catches on re, instruct student to cover the tube with a non- ammable object to extinguish the ame.
1. Collect green leaves from the environment. Try to find leaves that do not have a very waxy outer coat.
2. Heat water to boiling using the heat source.
1. Choose one leaf and submerge a piece of it in the boiling water for about 3 minutes. (Note: the piece of leaf used should be no larger than a bottle cap.)
2. Remove the leaf from the water and insert it into a test tube containing methylated spirit and plug the test tube with a piece of cotton wool. The test tube should be less than half full of ethanol.
3. Submerge the test tube in the boiling water and leave it to boil until the leaf loses all of its colour.
4. Once the leaf has lost its colour, remove it from the ethanol solution and dip it brie y into the boiling water to remove the ethanol and soften it.
5. Spread the decolourized leaf on a white tile and add iodine solution until the whole leaf is covered. Record your observations.
The leaf is dipped in hot water to kill the cells. The leaf is then submerged in boiling ethanol to extract the colour from the leaf. The ethanol will change to a green colour while the leaf should lose all of its colour to become white. When iodine solution is added, it should turn dark blue/black colour which indicates the presence of starch in the leaf.
Figure 6.3: When iodine solution is added, the leaf will turn to a black-blue colour if it is positive for starch
Collect all the used materials, cleaning and storing items that will be used later. No special waste disposal is required.
It is important that the leaf does not contain a thick waxy coating. Before doing this experiment with students, test some leaves from the local envi-ronment to ensure that they respond well to the experiment. Leaves such as amaranthus, beans and commilina respond fast. Make sure that the leaf has been in sunlight for at least 6 hours prior to the experiment or there may not be enough starch present to detect. This practical should not be done in the morning.
Ethanol boils at a lower temperature than water, thus it can be boiled in a water bath. Ethanol is very ammable and it is possible that the top of test tube catches re. If this happens a non- ammable material such as glass or metal can be used to cover the ame and deprive it of oxygen.