Page 19 - exemple

Activity 3 The chemical digestion

Carbohydrates, proteins and lipids undergo in the digestive tract a hydrolysis that simplifies the
complex molecules into simpler ones.
Where and how are they digested? What are the nutrients resulting from their digestion?

1 The digestion of starch

Place in a water bath at 37°C two test tubes (A) and (B) containing a solution of cooked starch.
Add fresh saliva to tube (B).
Add few drops of iodine water to samples taken from (A) and (B); perform the Fehling test on
the rest.

The results are represented in the table below:

Results:

Time Tubes Iodine Fehling
(min) water solution A B

A + –
t = 0
B + –

A + –
t = 15
B – +

♦ With iodine water (brown orange):

(–) : no reaction.
(+) : violet blue coloration.

♦ With the Fehling solution (blue
coloration):
The absence of a brick The brick red
(–) : no reaction. red precipitate precipitate indicates
(+) : brick red precipitate. indicates the absence the presence of a
of reducing sugars. reducing sugar.
From starch to maltose: Doc 1- The digestion of starch.
a molecular simplification

Saliva that is secreted by the salivary glands contains an enzyme: the salivary amylase. This
enzyme favors the hydrolysis of starch into a reducing sugar: maltose (formed by two molecules
of glucose linked together).

Digestion 23