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Digestion begins

الأسبوع 4 · الفصل 3 «الهضم»: 3-1 أين يذهب الطعام، 3-2 وظيفة اللعاب · 3 دروس
2

Saliva, the digestive juice in your mouth

Science P6: Term 1, Chapter 3, pp. 24–29
GoalExplain what saliva does in the mouth and how chewing makes bread taste sweet.
New words
saliva · the watery liquid made in the mouthstarch · a nutrient found in bread, rice and potatoesdigestive juice · a liquid that helps break food down
Explanation

Saliva is a watery liquid made in the mouth. It wets food, so it is easier to swallow. But it also helps digestion. Saliva starts to break starch down into sugar. Bread, rice and potatoes contain a lot of starch. That is why a piece of plain bread tastes slightly sweet if you chew it for a long time. A liquid that helps to digest food is called a digestive juice. Saliva is the first digestive juice. Scientists in laboratories test this with a starch solution and a special test that turns blue-black with starch. The test uses chemicals and warm water, so only adults do it. You can do a safe version at home: chew a plain cracker slowly and notice how the taste changes.

QuestionAnswer
Where is saliva made?In the mouth
What does it break down?Starch, into sugar
What name do we give a liquid like it?A digestive juice
Worked examples
cracker1 minute
Karim chews a plain cracker for one minute with his mother’s help. At first it tastes plain. Near the end, it tastes slightly sweet. The saliva has begun to change the starch into sugar.
TubeStarch left (test)
with waterstill plenty
with salivamuch less
The laboratory test (adult only): starch solution is mixed with saliva in one tube and with water in another. Later, the tube with water still shows starch, but the tube with saliva shows less. This shows that saliva changes starch.
Why do we chew a sandwich well before swallowing? Small, wet pieces mix better with saliva and are easier to swallow and digest.
Class activity

Chew and notice. Only if there is no allergy, with a parent, chew a plain cracker or a piece of bread for one full minute before swallowing. Write what the taste is like at the start and at the end. Why do you think it changed?

Practice
1
Name a food that contains a lot of starch.
2
What does saliva do to starch?
3
Why might chewed bread taste sweet?
4
Is saliva a digestive juice?