F4 Biology teaching notes Ch 6: NE 24

6.3  Digestion (Book 1A, p. 6-11)

A  Why is digestion necessary? (Book 1A, p. 6-11)

Ø   The wall of the alimentary canal is (1) differentially permeable. Large, complex food

molecules must be broken down into small, (2) soluble molecules so that they can pass through the wall and enter the blood.

Ø   The process of breaking down food into small molecules is called (3) digestion.

B  Physical digestion and chemical digestion (Book 1A, p. 6-11)

	(4)Physical / Mechanical digestion (機械消化)	(10)Chemical digestion (化學消化)
Principle	breaking down of food into smaller pieces by physical actions	involves (11) chemical reactions
Purpose / action	increases the (5) surface area of food in contact with the digestive juices.	Break down large, complex food molecules into small, simpler forms
brought about by:	n  Mechanical actions of alimentary canal: l   Chewing in the mouth cavity, i.e. (6) mastication l   (7) chuning (劇烈攪動) in the stomach l   (8) peristalsis along the alimentary canal n  (9) Emulsification (乳化) of lipids by bile salts (膽鹽)	Chemical reactions are catalysed by (12) digestive enzymes (消化酶)

Ø   Three main types of digestive enzymes in our digestive system:

Type	Function	Example
(13) Carbohydrases (碳水化合物酶)	Break down complex carbohydrates into simpler (14) sugars	Amylase (澱粉酶), sucrase (蔗糖酶), maltase (麥芽糖酶)
(15) Proteases (蛋白酶)	Break down proteins into polypeptides, peptides and (16) amino acids	Pepsin (胃蛋白酶), pancreatic proteases (胰蛋白酶), peptidase (肽酶)
(17) Lipase (脂肪酶)	Break down lipids into fatty acids and (18) glycerol	Pancreatic lipase (胰脂肪酶)

C  Digestion in the mouth cavity (Book 1A, p. 6-12)

Ø   In the mouth cavity, food is chewed into smaller pieces by the teeth and mixed with (19) saliva (唾液) produced by salivary glands. Saliva contains:

Component	Function
(20)    Salivary amylase (唾液澱粉酶)	Catalyses the breakdown of starch into (21) maltose
(22)  Mucus (黏液)	l  Helps bind food particles together l Moistens and (23) lubricates the food
Water (>95%)	l  Moistens and lubricates food l  (24) Dissolves soluble substances
Inorganic ions: Na+, Cl- & HCO3- etc	Maintain optimum pH (6.5 – 7.5) for amylase

Ø   After chewing, the tongue rolls the food into a (25) food bolus (食團), which is then

swallowed down the oesophagus (食道) through the (26) pharynx (咽)