Digestive System
ICSE Class 9 · Biology · Chapter 12
What this chapter covers
- Digestion and the stages of nutrition
- Types of teeth and their functions
- Structure of a tooth
- Dental formula of an adult
- Organs of the alimentary canal and their functions
- Digestive glands and their functions
- Enzymes and their functions in digestion
- Absorption of digested food
- Assimilation of digested food
Key definitions
Digestion is the process by which large, complex and insoluble food molecules are broken down into small, simple and soluble molecules that can be absorbed into the blood and lymph.
The alimentary canal is the continuous muscular tube, about 8 to 9 metres long in an adult, through which food passes from the mouth to the anus.
Digestive glands are glands that produce secretions containing enzymes or other substances that help in digestion.
Incisors are the chisel-shaped front teeth with a sharp, straight cutting edge, used for cutting and biting food.
Canines are the pointed, cone-shaped teeth placed one on each side of the incisors, used for tearing and piercing food.
Premolars are the broad teeth behind the canines, each with two raised points called cusps on its crown, used for crushing and grinding food.
Molars are the largest teeth at the back of the jaw, with broad, flat crowns bearing four or five cusps, used for grinding and chewing food into a fine paste.
Enamel is the hard, white, shining layer that covers the crown of a tooth, and it is the hardest substance in the human body.
Dentine is the hard, yellowish, bone-like substance that forms the main bulk of a tooth, lying beneath the enamel in the crown and beneath the cement in the root.
The pulp cavity is the hollow space at the centre of a tooth, filled with soft connective tissue called pulp, which contains blood vessels and nerves.
Cement is a thin, bone-like layer that covers the dentine of the root and helps fix the tooth in its socket.
The periodontal membrane is a layer of tough fibres that joins the cement of the root to the bone of the socket and holds the tooth firmly in place.
Key points
- Digestion breaks large, insoluble food molecules into small, soluble ones; it is mechanical (teeth, tongue, churning) and chemical (enzymes).
- The five stages of nutrition are ingestion, digestion, absorption, assimilation and egestion.
- Human teeth are thecodont, diphyodont and heterodont: 20 milk teeth, then 32 permanent teeth.
- Incisors cut, canines tear, premolars crush and grind, molars grind; the wisdom tooth is the third molar.
- A tooth has a crown, neck and root; its layers are enamel (hardest, non-living), dentine (living, bulk of the tooth), pulp cavity (blood vessels and nerves) and cement on the root, held by the periodontal membrane and surrounded by the gum.
- The adult dental formula is I(2)/(2) C(1)/(1) PM(2)/(2) M(3)/(3) = 32; the milk formula is I(2)/(2) C(1)/(1) PM(0)/(0) M(2)/(2) = 20.
- The alimentary canal runs mouth, pharynx, oesophagus, stomach, small intestine (duodenum, jejunum, ileum), large intestine (caecum with appendix, colon, rectum) and anus; peristalsis moves food along it.
- Digestive glands: salivary glands (parotid, submandibular, sublingual), gastric glands, liver (bile, stored in the gall bladder), pancreas (pancreatic juice) and intestinal glands (intestinal juice).
- Hydrochloric acid kills germs, makes the stomach acidic and activates pepsinogen; bile has no enzymes but neutralises acid and emulsifies fats.
- Starch: salivary and pancreatic amylase to maltose, maltase to glucose. Proteins: pepsin to peptones, trypsin to peptides, peptidases to amino acids. Fats: lipase to fatty acids and glycerol, after emulsification by bile.
- Absorption takes place mainly in the ileum through villi; sugars and amino acids enter blood capillaries and go to the liver by the hepatic portal vein, while fats enter the lacteals.
- The large intestine absorbs water and salts and forms faeces; it secretes no enzymes.
- Assimilation is the use of absorbed food by cells: glucose for energy or stored as glycogen, amino acids for growth and repair with the extra deaminated to urea, fats for membranes and stored as an energy reserve.
Questions answered
Define digestion and name its two parts.
Distinguish between egestion and excretion.
Human teeth are described as thecodont, diphyodont and heterodont. What does each term mean?
Name the four types of teeth and give the function of each.
Name the hardest substance in the body. Why can it not repair itself?
Why does a tooth become painful when decay reaches the dentine but not while it is confined to the enamel?
Write the dental formula of an adult human and show how it gives 32 teeth.
Which type of tooth is absent in the milk set, and how many milk teeth does a child have?
Past board exam questions
ICSE 2019 sample paper · 5 marks
Fill in the blanks with suitable words: (i) (1) ____________ xrightarrowRenin (2) ____________ (ii) (3) ____________ xrightarrowMaltase (4) ____________ (iii) Emulsified fat xrightarrowLipase (5) ____________ + (6) ____________ (iv) (7) ____________ xrightarrowErepsin (8) ____________ (v) (9) ____________ xrightarrowPepsin (10) ____________