Results of the iodine method: which statement is correct?

• A. Plant starch, amylose, amylopectin blue; glycogen and amyloid red-brown
• B. Glycogen blue; starch red
• C. All carbohydrates blue
• D. Only glycogen red-brown

Answer: (A)

The iodine test distinguishes carbohydrate structures by how they form complexes with iodine. Starch has a helical structure that readily traps iodine, giving a deep blue color. Both components of plant starch—amylose and amylopectin—contribute to this blue result because they present those helical regions iodine fits into. In contrast, glycogen, with its highly branched, shorter chains, forms a different, reddish-brown iodine complex. The same red-brown reaction is noted for amyloid in this context. So the pattern described—plant starch, amylose, and amylopectin appearing blue, while glycogen and amyloid appear red-brown—is the correct description of the iodine method’s results.