Question

In a test tube containing dilute starch, solution saliva is added kept at pH 2.8 and temp 38C. After half an hour, a drop of iodine is added and it is found that solution turns
A. Bluish and sweet in taste
B. Colourless and sweet in taste
C. Bluish and tasteless
D. Colourless and tasteless

Answer 1

Hint: Enzymes are proteins that act as catalysts for biological reactions. Enzymes are bio-catalysts that speed up reactions without being used up themselves. Enzymes lower the activation energy of a reaction and hence speeding up the reaction. All biochemical reactions are catalysed by enzymes. Enzymes are proteins in nature and thus can be denatured easily. They are most active under mild conditions. Generally enzymes have their optimum activity in neutral pH and at body temperature.

Complete answer: Saliva contains the amylase enzyme. This enzyme is responsible for hydrolysing starch. Salivary amylase hydrolyses starch into smaller polysaccharides such as dextrin, maltose, and glucose.
This enzyme is used to test for the presence or absence of starch in a solution using iodine. When iodine reacts starch, it forms a blue-black complex and it does not react with glucose. When iodine is added to a glucose solution, the only colour seen is the red or yellow colour of iodine.
The faster the blue colour is lost, the faster the amylase enzyme is working. Once amylase is inactivated, it can no longer hydrolyse starch, hence the blue colour of the starch-iodine complex will persist.
Hence in this scenario, when salivary solution is added to dilute starch, the salivary amylase present in saliva will hydrolyse starch and break into maltose and glucose. Therefore, when iodine is added to this solution, it will not form a blue-black complex as starch is absent. Therefore it will be a colourless solution. Since glucose is present, it will be sweet in taste.
The solution turns colourless and sweet in taste. So, the correct answer is option (B).

Note: When we add saliva to a starch solution, the salivary amylase in saliva gradually acts on the starch molecules and converts it into maltose. Until fully digested into maltose, starch gives out a blue colour with iodine. But at this point, no blue colour is formed and thus it is referred to as the end point or achromic point.