Page 30 - exemple

Exercises

combustion, some of the food samples were 7 A mechanical or chemical
carbonized (turned black) such as sugar, meat digestion ?
and potato. Other food samples didn’t
The mechanical digestion allows the
carbonize such as water and mineral salts. decomposition of food into small pieces which

Classify these food components in two facilitates the chemical action of enzymes.
different categories: organic or mineral and To verify this hypothesis, bread pieces of
justify the answer. different sizes were placed in two test tubes in
the presence of the enzyme: salivary amylase
6 The convenient temperature for The conditions and the results of the
the activity of enzymes experiment are figured in the document

A- Two test tubes contain the same quantity of below.
cooked starch at 37°C.
We add: - fresh saliva to tube A Water
- boiled saliva to tube B Amylase
Half an hour later, we perform on both test
tubes the iodine water test and the Fehling Large
Small
test. The first test is positive in tube B only. pieces pH=7 pieces of
bread
The second test is positive in tube A and of bread
negative in tube B. 1 2
B- We distribute, in four test tubes C, D, E
and F, cooked starch and fresh saliva then Water bath : temperature = 37˚C
we place the test tubes in different
temperatures : 20 minutes
- Tube C in a water bath at 0°C
- Tube D in a water bath at 20°C
- Tube E in a water bath at 37°C
- Tube F in a water bath at 60°C
Half an hour later, the results of the iodine Disappearance Decrease in
water test and the Fehling test show that : of pieces the size of
pieces of
of bread
- Digestion didn’t take place in tube C bread
- Slow digestion occurred in tube D
- Digestion is impossible in tube F 1 2

a- Draw from these experiments the
temperature where: a- Represent, in the same table, the
- Digestion is the fastest conditions of the experiment and the
- The enzyme conserved its properties but results obtained.
stays without activity b- Pick up, from the text, the tested
- The enzyme is destroyed. hypothesis.
b- Interpret the results obtained in tubes A c- Interpret the obtained results and draw out
and B. a conclusion.
Lebanese Brevet 2007

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