Do you know that leaves of plants produce glucose through photosynthesis? The excess glucose is then converted into starch, which is the reserved food material in plants. Let us do a small experiment to test the presence of starch in leaves.
We need a leaf, boiling water, alcohol, iodine solution, test tube, spirit lamp. Take a leaf from a plant that has been exposed to sunlight for several hours. Place the leaf in boiling water.
This will kill the leaf, disrupt the cell membrane and soften the cell walls, allowing the iodine to enter the cells and react. Now put the leaf in a test tube and cover it with alcohol. Keep the test tube in the boiling water. The chlorophyll slowly dissolves into the alcohol. Note that alcohol turns green.
Removal of chlorophyll will help us observe the color change of iodine clearly. Place the bleached leaf in a petri dish. pour iodine solution over it Note the color of the leaf. It changes to dark green.
This is because iodine combines with starch present in the leaf and forms a violet colored complex. This shows that starch is present in leaves. Now cover a leaf with a black paper for couple of days and do the same test. See if it still has starch.