Question

Please help :3 .... Explain why a negative iodine test for starch would indicate a positive result for the enzymatic degradation of starch by amylase.

Answer 1

The dark blue / purple color you get from iodine + starch results from the iodine embedding itself within the polysaccharide and causing a complex to be formed. The hydrolysis of starch by amylase results in the breakdown of the polysaccharide, yielding much smaller units which do not permit the iodine / starch complex to form; thus, no purple color is formed. I'll try to add some more to this answer since it's not too good right now, but hope that gives you the basic idea.

Answer 2

Yeah, I appreciate the help so much, so thanks! :)

Answer 3

Perhaps this would help you understand things more visually: http://www.elmhurst.edu/~chm/vchembook/548starchiodine.html

Scroll down to the second picture, on the left. That's the starch-iodine complex that would give you a positive result (dark color). Now imagine once you add amylase, the outside "coil" would be broken apart into numerous little pieces - the iodine would float out free, and the test would come out as negative because the complex was not formed. Does that help?

Answer 4

Yes, thanks so much. This was exactly what i was trying to find so that i dont just have the answer, but understand it. =D