Plants prepare food in the form of glucose, which is stored in the form of starch. Let us see what is starch.
We all know that the simplest organic compound is methane (CH4). A methane molecule joins another molecule to form a longer molecule, like C2H5, C3H7, etc.
Similarly, the addition of OH group makes alcohol like CH3OH, C2H5OH.
Glucose has a formula of C6H12O6. Here six OH molecule combines with C6H13 to form glucose. We call glucose as a unit of saccharide (sugar) as it is sweet in nature. Many sugar molecules combine and form a polysaccharide called starch. Thus starch is a polymer of glucose.
The plant produces glucose by photosynthesis. This glucose has osmotic pressure which may interfere with the water balance of the body. So as an alternative, plants convert it to starch and store them in leaves, roots, stems, and fruits for later use.
An iodine solution act on the joint of a sugar molecule and gives a blue test. Thus An iodine solution givest test for starch but not for sugar.