BiologyICSE Class 9 · Nutrition and the Digestive System

Teeth and the Working of the Digestive System (ICSE Class 9 Biology)

Updated 17 October 2026 · Fact-checked

Humans have four types of teeth (incisors, canines, premolars and molars), with the adult dental formula 2123/2123. Food is digested in the mouth, stomach and small intestine by the enzymes of the salivary glands, stomach, pancreas and intestinal walls, with bile from the liver emulsifying fats (bile has no enzymes); absorption takes place through villi in the small intestine and water is absorbed in the large intestine.

Understand Teeth and the Working of the Digestive System

1. Teeth. Humans are *diphyodont* (two sets: 20 milk teeth and 32 permanent teeth) and *heterodont* (four kinds).

*Incisors*: chisel-shaped; cut food.
*Canines*: pointed; tear food.
*Premolars* and *molars*: broad with ridges; grind food.

Adult dental formula: I 2/2, C 1/1, PM 2/2, M 3/3 per half jaw, so 2 x (2 + 1 + 2 + 3) x 2 = 32. A tooth has a crown, neck and root; enamel (hardest tissue), dentine and a pulp cavity with nerves and blood vessels.

2. Digestion in the mouth. Saliva from the salivary glands contains *salivary amylase*, which changes starch into maltose. The tongue mixes food into a bolus.

3. Stomach. Gastric juice contains hydrochloric acid (kills germs, gives acid medium), *pepsin* (proteins into peptones) and mucus (protects the stomach wall). Food becomes chyme.

4. Small intestine. *Bile* from the liver, stored in the gall bladder, has no enzymes but emulsifies fats. *Pancreatic juice* has amylase, trypsin and lipase. *Intestinal juice* completes digestion: carbohydrates to glucose, proteins to amino acids, fats to fatty acids and glycerol.

5. Absorption and assimilation. The small intestine is long and its inner wall has finger-like *villi* with blood capillaries and lacteals, giving a huge surface for absorption. Absorbed food is carried by blood to cells and used (assimilation).

6. Large intestine. Absorbs water and minerals; undigested matter is stored in the rectum and egested through the anus.

Teeth and the Working of the Digestive System: key facts

Dental formula: Adult: I 2/2 C 1/1 PM 2/2 M 3/3
Total 32
Milk teeth 20
Teeth work: Incisor cut
Canine tear
Premolar and molar grind
Enzymes: $S a l i v a : am y l a se ∣ S t o ma c h : p e p s in ∣ P an cr e a s : am y l a se, t r y p s in, l i p a se$
Products: Glucose
Amino acids
Fatty acids and glycerol

Incisor	cut
Canine	tear
Premolar and molar	grind

Teeth and the Working of the Digestive System in real life

The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Chewing bread

Suppose a student chews a piece of bread for a long time and notices that it starts to taste sweet.

Explain the change.

Try it first, then show the solutionSolution

Saliva has amylase.
Amylase changes starch into maltose, a sugar.
The sweet taste appears.

Answer: Starch is digested to maltose in the mouth.

Memory hook: Chew long, taste sweet.

Tricks to remember Teeth and the Working of the Digestive System

Short memory aids. Say each one aloud once, then use it on a question the same day.

2123
Incisor 2, Canine 1, Premolar 2, Molar 3 in each half jaw.
Use it when: Dental formula.
Bile has no enzymes
Bile only emulsifies fat.
Use it when: Common mistake.

Teeth and the Working of the Digestive System: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Dental formula · 3 marks · Short answerBoard level

Write the dental formula of an adult human and show the total number of teeth. [3]

Try it first, then show the solutionSolution

I 2/2, C 1/1, PM 2/2, M 3/3.
Teeth in half jaws: upper 2 + 1 + 2 + 3 = 8; lower = 8.
For both sides: 16 x 2 = 32.

Answer: 2123/2123; total 32 teeth.

Example 2: Digestive juices · 3 marks · Short answerHarder board level

Name the gland, the juice and one enzyme involved at (a) the mouth, (b) the stomach, (c) the pancreas. [3]

Try it first, then show the solutionSolution

(a) Salivary glands, saliva, amylase.
(b) Gastric glands, gastric juice, pepsin.
(c) Pancreas, pancreatic juice, trypsin or lipase.

Answer: Saliva with amylase; gastric juice with pepsin; pancreatic juice with trypsin or lipase.

Example 3: Role of bile · 3 marks · Short answerHarder board level

Bile contains no enzymes yet is essential. Explain its two roles. [3]

Try it first, then show the solutionSolution

It emulsifies fats into tiny droplets, increasing surface area for lipase.
It makes the food alkaline so that pancreatic enzymes work.
It comes from the liver and is stored in the gall bladder.

Answer: Emulsification of fat and an alkaline medium for enzymes.

Example 4: Villi · 4 marks · Long answerHarder board level

Explain how the structure of villi is suited for absorption. [4]

Try it first, then show the solutionSolution

Finger-like projections increase the surface area.
Thin walls (one cell thick) allow quick diffusion.
Blood capillaries absorb glucose and amino acids.
Lacteals absorb fatty acids and glycerol.

Answer: Large surface area, thin wall, capillaries and lacteals.

Example 5: Case: the missing acid · 6 marks · Case-basedHOTS / competency

Case. A patient has very low acid secretion in the stomach. (i) Name the acid. [1] (ii) State two likely effects. [2] (iii) Which enzyme would work poorly? [1] (iv) Why does the stomach itself not get digested? [2]

Try it first, then show the solutionSolution

(i) Hydrochloric acid.
(ii) Poor protein digestion and more germs surviving in food.
(iii) Pepsin.
(iv) Mucus covers and protects its wall.

Answer: (i) HCl (ii) Weak digestion and more infections (iii) Pepsin (iv) Mucus protects the wall.

Where students lose marks in Teeth and the Working of the Digestive System

The slips examiners see most often, and what to write instead.

Mistake: Saying bile contains enzymes
Bile only emulsifies.
Do this: Write that bile has no enzymes.
Usually costs: 1 mark.
Mistake: Writing the dental formula as 32
The formula is incomplete.
Do this: Write 2123/2123.
Usually costs: 1 mark.
Mistake: Placing the end products in the wrong organ
Absorption sites are mixed.
Do this: Absorption of nutrients is mainly in the small intestine.
Usually costs: 1 mark.

Exam tips

Draw the alimentary canal with labels.
Name the gland, juice, enzyme, substrate and product in a table.
Always state the site of absorption.
Show the working for the dental total.

Practice questions on Teeth and the Working of the Digestive System

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
Which teeth are used for tearing food?
(a) incisors
(b) canines
(c) molars
(d) premolars
Hint
Pointed.
Answer
(b) canines.
Q2 · 1 mark · Very short answerBoard level
Name the hardest part of a tooth. [1]
Hint
Outer layer.
Answer
Enamel.
Q3 · 2 marks · Very short answerBoard level
What is the function of the large intestine? [2]
Hint
Water.
Answer
It absorbs water and minerals and stores undigested waste.
Q4 · 1 mark · Assertion-reasonHarder board level
Assertion (A): Fat digestion slows down if the bile duct is blocked.
Reason (R): Bile breaks fat into droplets and makes the medium alkaline.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Hint
Surface area.
Answer
(a).
Q5 · 2 marks · Very short answerBoard level
Why does a child have only 20 teeth? What is the term for having two sets? [2]
Hint
Milk teeth.
Answer
The jaw is small, so 20 milk teeth appear first; two sets is called diphyodont.
Q6 · 4 marks · Long answerHarder board level
Trace the digestion of a protein from the stomach to the final product, naming enzymes. [4]
Hint
Pepsin, trypsin.
Answer
Pepsin in the stomach forms peptones; trypsin in the small intestine continues; intestinal enzymes form amino acids.
Q7 · 2 marks · Very short answerBoard level
State two reasons why the small intestine is long and coiled. [2]
Hint
Time and area.
Answer
It gives more time for digestion and a large surface for absorption.
Q8 · 3 marks · Short answerHarder board level
Why should we chew food thoroughly? Give three reasons. [3]
Hint
Enzyme contact.
Answer
It breaks food into small pieces, mixes it with saliva for starch digestion, and makes swallowing and later digestion easier.
Q9 · 4 marks · Case-basedHOTS / competency
Case. A child complains of toothache that spreads to the jaw when eating sweets. (i) Name the part reached by decay. [1] (ii) Name the tissue with nerves. [1] (iii) Suggest two preventive measures. [2]
Hint
Pulp.
Answer
(i) Dentine and then pulp. (ii) Pulp cavity. (iii) Brush after meals and reduce sweet sticky food.
Q10 · 4 marks · Case-basedHOTS / competency
Case. A patient has an obstruction in the bile duct. (i) Predict the effect on fat digestion. [2] (ii) Explain why stools may become pale. [2]
Hint
Bile pigment.
Answer
(i) Fat digestion is poor as bile does not reach the intestine. (ii) Bile pigments give stool its colour; with no bile they are pale.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Enzyme in saliva?Show answer
Amylase.
2.Number of permanent teeth?Show answer
32.
3.Where is bile stored?Show answer
Gall bladder.
4.Organ of absorption?Show answer
Small intestine.
5.Site of water absorption?Show answer
Large intestine.

Teeth and the Working of the Digestive System: frequently asked questions

Why is HCl needed?

It kills germs and gives the right acidity for pepsin.

What are lacteals?

Lymph vessels in villi that absorb fats.

What is egestion?

Removal of undigested waste.