Page 15 - exemple

Chemical transformation of food by the
Activity 2
enzymes

The mechanical digestion consists of fragmenting and mixing the food with the digestive juices.
It prepares the food components to chemical reactions that are accelerated in the presence of
digestive enzymes.
A piece of bread impregnated with saliva, gets a sweet taste after a short period of time. Yet
neither starch nor saliva has this taste. Where does this taste come from? How can we prove that

starch turned into a reducing sugar? What is the role of enzymes? In which conditions do they
act?

1 Action and properties of enzymes

A- Definition of an enzyme

An enzyme is a molecule capable of accelerating the chemical reactions: Enzymes are considered
as biocatalysts that facilitate the hydrolysis of complex molecules into smaller ones and remain
intact at the end of the reaction.

B- Action of saliva on the digestion of starch

We perform “In vitro” (outside the organism), the
experiment below.
A B
Place in a water bath at 37°C, two test tubes A and B
containing cooked starch (dissolved starch in hot Cooked
Cooked starch +
water). Add fresh saliva to tube “B”. Perform the starch fresh
Fehling test and the iodine water test before and after saliva
adding saliva. 37ºC
The results are represented in the following table:
Doc 1- Digestion of starch in the presence
of saliva.

Before the addition of saliva 15 min after the addition of saliva
Fehling test Iodine test Fehling test Iodine test

Tube A Blue Violet blue Blue Violet blue
Brick red
Tube B Blue Violet blue Brown orange
precipitate

Doc 2- Results of the experiment.

Digestion 19