Digestion and Absorption — Part 2: Digestion, absorption and disorders: visual lesson, CBSE Class 11 Biology

Lesson

Digestion, absorption and disorders

Part 2 — how food is broken down along the canal, how the products are absorbed, and what can go wrong.

In this lesson you will

Explain digestion in the mouth, stomach and small intestine with enzymes and products
Explain the mechanisms of absorption, including fats
State where substances are absorbed, and describe assimilation and egestion
Describe common digestive and nutritional disorders

Concept

Digestion in the mouth

Teeth and tongue masticate food and mix it with saliva; mucus helps form a bolus, which is swallowed (deglutition) and carried down the oesophagus by peristalsis.

Table of the components of saliva and their roles
Component of saliva	Role
Electrolytes (sodium, potassium, chloride, bicarbonate ions)	Medium for enzymes
Salivary amylase	Hydrolyses about 30 percent of starch to maltose at optimum pH 6.8
Lysozyme	Antibacterial agent that prevents infections
Mucus	Lubricates and sticks food particles into a bolus

Worked example

Worked example 1: starch and acidity

(i) A meal has 50 g of starch, and about 30 percent is hydrolysed in the mouth. How much is left for the intestine? (ii) Saliva works at pH 6.8 and gastric juice at pH 1.8. How many times more hydrogen ions does gastric juice have?

(i) $50 \times \dfrac{30}{100} = 15$ g hydrolysed; $50 - 15 = 35$ g left.
(ii) $6.8 - 1.8 = 5$ pH units, so $10^{5}$ , that is 1,00,000 times.
Amylase stops working once the bolus is mixed with acid in the stomach.

Worked table: 15 grams hydrolysed, 35 grams left; stomach a hundred thousand times more acidic
Quantity	Value
Starch hydrolysed in the mouth	15 g
Starch left for the intestine	35 g
Acidity, stomach compared with saliva	1,00,000 times

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