Starch can be divided into straight-chain starch (sugar starch) and branched-chain starch (gum starch). The former is a helical structure without branches; the latter is made of 24-30 glucose residues linked by α-1,4-glycosidic bonds at the beginning and end, and α-1,6-glycosidic bonds at the branched chains.
Straight-chain starch appears blue when exposed to iodine, while branched-chain starch appears purple-red when exposed to iodine. This is not a chemical reaction between starch and iodine (reaction), but an interaction, but the central cavity of the starch helix can accommodate iodine molecules, and through van der Waals forces, the two form a blue-black complex. Experiments have shown that iodine molecules alone cannot turn starch blue, but in fact it is the iodine molecule ion (I3) that turns starch blue.
Starch is a stored nutrient in the plant body, stored in seeds and tubers, and is found in high levels in all types of plants.