Question

In this laboratory experiment, you wanted to see the effect of Amylase with starch. So you performed the following:

In tube 1 - Combine equal volume of Amylase with Starch solution

In tube 2 - Combine Amylase with water

In tube 3 - Combine water with Starch solution

Let all three tubes sit in a 37C waterbath for 20 minutes. After the 20 minutes incubation was over, then you would test for the presence of starch. In the meantime, what would you predict? Based on what you know about enzymatic digestion, what do you expect here in these tubes? After the incubation period we added Iodine solutions to each tube to test for starch. Here are the results:

In tube 1 - white cloudy solution

In tube 2 - Yellowish solution

In tube 3 - Dark red solution

Explain this result, what happened in each tube? Why is adding iodine important to detect the presence of starch?

Answer 1

Starch amylase test

Amylase is the enzyme which catalysis the hydrolysis of starch. It acts on disaccharide into glucose. As the reaction progress, there will be more glucose and less starch.

In tube 1 - Combine equal volume of Amylase with Starch solution - Both enzyme and substrate present. Enzyme catalyze the reaction and products formation begins.

In tube 2 - Combine Amylase with water- No substrate in this tube. Only enzyme with water.

In tube 3 - Combine water with Starch solution- No enzyme and therefore no product formation.

Iodine is the indicator for starch test. It reacts with starch but not with glucose. Iodine gives linear triiodide anion with starch which gives deep-blue color pigment. The more the amylase, greater the effect and speed of enzyme. This will be reflected by the color formation after the addition of iodine. Higher the amylase concentration, the lighter the color of starch.

After incubation and iodine addition,

In tube 1 - white cloudy solution - This implies occurrence of reaction. Cloudy white color infer that tube is left with 5% amylase and product mixed

In tube 2 - Yellowish solution - Enzyme and water gives yellowish color upon addition of iodine

In tube 3 - Dark red solution - Upon addition of iodine, starch would give deep blue color solution.

Usually starch gives blue color with iodine. But when starch has more amylopectin than amylose, it would develop purple-red coloring. Amylopectin give purple-red coloring when added to iodine.