Nutrition and the Digestive System ICSE Class 9: Food, Deficiency Diseases, Teeth and Digestion

What Nutrition and the Digestive System is about

Weightage: CISCE publishes no chapter weightage for ICSE Biology. Nutrition and the Digestive System is topic 6, Human Anatomy and Physiology (i), of the Class IX part of the 2028 syllabus; the Class IX paper (80 marks, 2 hours) is set and marked by the school.Source: CISCE ICSE Biology (Science Paper 3) syllabus, examination year 2028 (Class IX)

Key concepts

Classes of food and their functions

Nutrient	Main function	Sources
Carbohydrates (sugars, starch)	main energy-giving food (about 4 kcal per gram)	cereals, potato, sugar, fruits
Fats	concentrated energy (about 9 kcal per gram); energy store; insulation; carry fat-soluble vitamins	butter, ghee, oils, nuts
Proteins	body-building: growth and repair; enzymes, hormones, antibodies	pulses, milk, eggs, fish, meat, soya
Mineral salts	protective/regulatory; build bones, blood	milk, green vegetables, salt
Vitamins	protective: regulate body processes; prevent deficiency diseases	fruits, vegetables, milk, eggs
Water	medium for reactions; transport; temperature control (sweat); removal of wastes	drinking water, fruits
Roughage (cellulose)	not digested by humans; adds bulk, helps peristalsis, prevents constipation	whole grains, fruit and vegetable skins, salads

Four minerals in the syllabus:

• Calcium: bones and teeth, blood clotting, muscle contraction; deficiency: weak bones, rickets-like defects; sources: milk, cheese, green leaves.
• Iodine: making thyroxine (thyroid hormone); deficiency: goitre (swollen thyroid), poor growth; sources: iodised salt, sea food.
• Iron: making haemoglobin of red blood cells; deficiency: anaemia (tiredness, pale skin); sources: green leafy vegetables, jaggery, liver, eggs.
• Sodium: nerve impulses, muscle contraction, water balance of body fluids; deficiency: muscle cramps, low blood pressure; source: common salt.

Worked example

Why is iodised salt recommended for everyone, especially in hilly regions?

Try it first, then show the solutionSolution

1. Iodine is needed to make thyroxine.
2. Soil and water in many hilly regions are poor in iodine, so goitre is common.
3. Iodised salt supplies iodine in a daily food.

Answer: To prevent iodine deficiency (goitre).

Vitamins: sources, functions and deficiency diseases

Vitamins are needed in very small amounts. A, D, E, K are fat-soluble; B-complex and C are water-soluble (not stored, so needed daily).

Vitamin	Sources	Function	Deficiency disease
A (retinol)	carrot, green leaves, milk, butter, liver, fish-liver oil	healthy eyes (night vision), skin	night blindness, xerophthalmia (dry eyes)
B1 (thiamine)	whole cereals, pulses, yeast	carbohydrate use, nerves	beriberi (weak muscles, nerve damage)
B3 (niacin)	meat, groundnuts, cereals	energy release, skin	pellagra (skin, diarrhoea, mental problems)
B12	milk, eggs, meat, fish	making red blood cells	pernicious anaemia
C (ascorbic acid)	citrus fruits, amla, tomatoes, guava, sprouts	healthy gums, skin, healing, immunity	scurvy (bleeding, swollen gums, slow healing)
D (calciferol)	sunlight on skin, milk, eggs, fish-liver oil	absorption of calcium: bones and teeth	rickets (soft, bent bones in children)
E (tocopherol)	vegetable oils, green leaves, nuts	protects cells; normal reproduction	reduced fertility (sterility in rats)
K	green leafy vegetables; made by gut bacteria	blood clotting	excessive bleeding (haemorrhage)

Worked example

Children kept indoors all the time and fed little milk develop bow legs. Name the deficiency and disease, and suggest two remedies.

Try it first, then show the solutionSolution

1. Vitamin D (made in skin in sunlight) helps absorb calcium for bones.
2. Its lack causes rickets.
3. Remedies: daily outdoor play in sunlight; milk, eggs or fish-liver oil.

Answer: Vitamin D deficiency: rickets.

Balanced diet and malnutrition: kwashiorkor and marasmus

Balanced diet: a diet that contains all the nutrients (carbohydrates, fats, proteins, vitamins, minerals, water and roughage) in the right amounts to meet the body's needs for energy, growth and health. Needs differ with age, sex, body size, activity, climate and conditions such as pregnancy and illness.

Malnutrition: a condition caused by a diet lacking one or more nutrients (undernutrition) or by too much of some (overnutrition, e.g. obesity).

	Kwashiorkor	Marasmus
Cause	mainly protein deficiency in a diet with some carbohydrate	deficiency of both protein and energy (calories): near-starvation
Age	children 1-5 years, often after weaning	usually infants under 1 year
Symptoms	swollen belly and legs (oedema), puffy face; thin limbs; dry, scaly skin with patches; thin, discoloured (reddish) hair; poor appetite, irritable; stunted growth	extreme thinness, ribs show; wrinkled, loose skin ('old man' face); no oedema; very weak; diarrhoea
Prevention	protein-rich foods: milk, eggs, pulses, soya, fish	breastfeeding; enough food with proteins and calories; nutrition schemes

Worked example

A 2-year-old fed mostly on rice water after weaning has a swollen belly but thin arms. Name the disease and give the cause.

Try it first, then show the solutionSolution

1. The diet has some carbohydrate but very little protein.
2. Swollen belly (oedema) with thin limbs is typical.

Answer: Kwashiorkor: protein deficiency.

Teeth: structure, types and dental formula

Structure of a tooth (vertical section):

• Crown: the part above the gum; neck: at the gum line; root: embedded in the jaw bone socket.
• Enamel: covers the crown; the hardest substance in the body (mostly calcium salts); non-living.
• Dentine: bone-like layer under the enamel, forming most of the tooth; living, with fine canals.
• Pulp cavity: central space with pulp: blood vessels and nerves that nourish the tooth and make it sensitive; it narrows into the root canal.
• Cement: bone-like layer covering the root, fixing the tooth in the socket with the periodontal membrane (ligament).
• Gum: soft tissue around the neck.

Types of teeth (humans are heterodont: different kinds):

Type	Shape	Function
Incisors (front)	chisel-like, sharp edge, one root	cutting and biting
Canines	pointed, one long root	tearing
Premolars	broad, two cusps, one or two roots	grinding and crushing
Molars (back)	broad, several cusps, two or three roots	grinding and crushing

Dental formula of an adult: the number of each type in one half of the upper and lower jaw:
$\frac{I2,C1,PM2,M3}{I2,C1,PM2,M3}=\frac{8}{8}\times 2=32$
The last molars (wisdom teeth) appear at about 17-25 years.

Worked example

Using the adult dental formula, find the total number of grinding teeth.

Try it first, then show the solutionSolution

1. Premolars + molars in half a jaw: 2 + 3 = 5.
2. Four half-jaws: 5 × 4 = 20.

Answer: 20 grinding teeth (8 premolars + 12 molars).

Organs and glands of digestion: mouth to stomach

The alimentary canal: mouth → pharynx → oesophagus → stomach → small intestine (duodenum, jejunum, ileum) → large intestine (caecum with appendix, colon, rectum) → anus. Digestive glands: salivary glands, gastric glands, liver, pancreas, intestinal glands.

Mouth: teeth break food into small pieces (mechanical digestion); the tongue mixes it with saliva.

• Salivary glands (three pairs) secrete saliva with salivary amylase (ptyalin): starch → maltose. Saliva also moistens food to form a bolus.

Oesophagus (food pipe): carries the bolus to the stomach by peristalsis (waves of muscle contraction). No digestion.

Stomach: a muscular bag; its walls churn food. Gastric glands secrete gastric juice:

• Hydrochloric acid: makes the medium acidic (needed for pepsin); kills germs.
• Pepsin: proteins → peptones (smaller chains).
• Rennin (mainly in infants): curdles milk: soluble caseinogen → insoluble casein, keeping milk longer in the stomach.
• Mucus: protects the stomach lining from acid and pepsin.

Food becomes a semi-liquid chyme.

Worked example

Why does a piece of bread taste slightly sweet if you chew it for a long time?

Try it first, then show the solutionSolution

1. Salivary amylase digests starch in bread.
2. Starch → maltose, a sugar, which tastes sweet.

Answer: Starch is converted to maltose by saliva.

Small intestine, absorption, assimilation and large intestine

Duodenum receives two juices:

• Bile from the liver (stored in the gall bladder): no enzymes; its bile salts emulsify fats (break them into tiny droplets for lipase to act on); it is alkaline and neutralises the acid chyme.
• Pancreatic juice from the pancreas (alkaline):
• Trypsin: proteins and peptones → peptides
• Pancreatic amylase: starch → maltose
• Lipase: fats → fatty acids + glycerol

Intestinal juice (succus entericus) from glands of the small intestine completes digestion:

• Maltase: maltose → glucose
• Sucrase (invertase): sucrose → glucose + fructose
• Lactase: lactose → glucose + galactose
• Peptidases (erepsin): peptides → amino acids
• Lipase: fats → fatty acids + glycerol

Absorption takes place mainly in the ileum, whose lining has millions of finger-like villi (with microvilli), giving a huge surface. Each villus has thin walls, blood capillaries (absorb glucose, amino acids, minerals, water-soluble vitamins) and a central lacteal (lymph vessel; absorbs fatty acids and glycerol). Blood from the intestine goes to the liver first (hepatic portal vein).

Assimilation: absorbed food is used by body cells: glucose is respired for energy (excess stored as glycogen in the liver and muscles, or as fat); amino acids build proteins for growth and repair (excess amino acids are broken down in the liver).

Large intestine: absorbs water and some salts from undigested food; the remaining faeces are stored in the rectum and passed out through the anus (egestion). The appendix has no digestive role in humans.

Worked example

Bile contains no enzyme. How does it still help digestion?

Try it first, then show the solutionSolution

1. Bile salts break large fat globules into tiny droplets (emulsification).
2. This gives lipase a much larger surface to act on.
3. Its alkali also neutralises the acid chyme, giving the right pH for intestinal enzymes.

Answer: By emulsifying fats and neutralising acid.

Nutrition and the Digestive System in real life

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

Topics in this chapter

• Teeth and the Working of the Digestive System
• Vitamins, Balanced Diet and Malnutrition

Nutrition and the Digestive System: key facts

Adult dental formula

$\frac{2,1,2,3}{2,1,2,3}\times 2=32$

I, C, PM, M in half of each jaw.

Starch digestion

$\text{starch}\stackrel{\text{amylase}}{\to }\text{maltose}\stackrel{\text{maltase}}{\to }\text{glucose}$

Mouth, duodenum, small intestine.

Protein digestion

$\text{protein}\stackrel{\text{pepsin}}{\to }\text{peptones}\stackrel{\text{trypsin}}{\to }\text{peptides}\stackrel{\text{erepsin}}{\to }\text{amino acids}$

Stomach, duodenum, small intestine.

Fat digestion

$\text{fats}\stackrel{\text{bile (emulsify)}}{\to }\text{droplets}\stackrel{\text{lipase}}{\to }\text{fatty acids}+\text{glycerol}$

Bile has no enzyme.

Energy values

$\text{carbohydrate }4,\text{protein }4,\text{fat }9\text{kcal/g}$

Approximate.

Nutrition and the Digestive System: diagrams to practise

Practise each one on paper: draw it from the checklist, then tick off every label.

How the school exam asks Nutrition and the Digestive System

The question patterns to expect, and the method that works for each.

Tricks to remember Nutrition and the Digestive System

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

• Fat-soluble vitamins: 'ADEK' (a deck)

  A, D, E, K dissolve in fat; B and C in water.

  Use it when: Vitamin solubility.

• Deficiency pairs

  A-night blind, B1-beriberi, C-scurvy, D-rickets, K-bleeding.

  Use it when: Vitamin diseases.

• Dental 2-1-2-3

  Two incisors, one canine, two premolars, three molars in each quarter: 8 × 4 = 32.

  Use it when: Adult dental formula.

• Kwashiorkor Keeps the belly swollen; Marasmus Makes the body shrink

  Oedema in kwashiorkor; wasting in marasmus.

  Use it when: Malnutrition diseases.

• Pepsin in the Pouch, Trypsin in the Tube

  Pepsin works in the stomach (acid); trypsin in the duodenum (alkaline).

  Use it when: Protein enzymes.

Nutrition and the Digestive System: exam-level solved questions

Ordered from exam level to HOTS. Cover the solution, write your own, then compare step by step.

Case-study practice (Section B style)

Read the situation, then answer every part. In a school exam set on the board pattern this is a 5-mark question split into 2 + 1 + 2 marks.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Where students lose marks in Nutrition and the Digestive System

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

• ✗Mistake: Bile treated as an enzyme

  Bile is part of fat digestion, so students assume it digests.

  Wrong working

  Bile digests fats into fatty acids and glycerol.

  Right working

  Bile emulsifies fats; lipase digests them.

  ✓Do this: Bile has no enzyme.

  Usually costs: 1 mark.

• ✗Mistake: Dental formula for the whole mouth

  The formula shows half of each jaw.

  Wrong working

  I 4/4, C 2/2, PM 4/4, M 6/6

  Right working

  I 2/2, C 1/1, PM 2/2, M 3/3, multiplied by 2 = 32.

  ✓Do this: Write half a jaw; multiply by 2.

  Usually costs: 1 mark.

• ✗Mistake: Kwashiorkor and marasmus swapped

  Both are malnutrition diseases of children.

  Wrong working

  Marasmus: swollen belly due to protein lack.

  Right working

  Kwashiorkor: oedema, protein lack; marasmus: wasting, protein and calorie lack.

  ✓Do this: K = Keeps swelling.

  Usually costs: 1 mark.

• ✗Mistake: Enzyme without source or product

  Answers name the enzyme only.

  Wrong working

  Trypsin acts on proteins.

  Right working

  Trypsin (pancreatic juice) converts proteins and peptones into peptides in the duodenum.

  ✓Do this: Enzyme + source + substrate → product.

  Usually costs: 1/2 mark each.

• ✗Mistake: Absorption placed in the stomach or large intestine

  Students think food is absorbed wherever it is.

  Wrong working

  Digested food is absorbed in the large intestine.

  Right working

  Digested food is absorbed mainly in the ileum (villi); the large intestine absorbs water.

  ✓Do this: Ileum for food, colon for water.

  Usually costs: 1 mark.

Practice questions on Nutrition and 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.

1. Q1 · 1 mark · MCQExam level

   Goitre is caused by deficiency of:
   (a) iron
   (b) iodine
   (c) calcium
   (d) sodium

   Hint

   Thyroid.

   Answer

   (b)

2. Q2 · 1 mark · MCQExam level

   Trypsin is secreted by the:
   (a) stomach
   (b) liver
   (c) pancreas
   (d) salivary glands

   Hint

   Pancreatic juice.

   Answer

   (c)

3. Q3 · 2 marks · Very short answerExam level

   Name the deficiency disease: (a) vitamin C, (b) vitamin B1, (c) iron, (d) vitamin A.

   Hint

   Scurvy...

   Answer

   (a) Scurvy (b) Beriberi (c) Anaemia (d) Night blindness

4. Q4 · 2 marks · Short answerExam level

   What is a balanced diet? Give two factors on which a person's dietary needs depend.

   Hint

   All nutrients in the right amounts.

   Answer

   A diet containing all nutrients (carbohydrates, fats, proteins, vitamins, minerals, water, roughage) in the right amounts for health. Needs depend on age, sex, activity, body size, climate, pregnancy or illness (any two).

5. Q5 · 3 marks · Short answerHarder exam level

   Give the function of (a) HCl in gastric juice, (b) mucus in the stomach, (c) villi in the ileum.

   Hint

   Acid, protect, absorb.

   Answer

   (a) Makes the medium acidic for pepsin and kills germs. (b) Protects the stomach lining from acid and pepsin. (c) Increase surface area for absorption of digested food.

6. Q6 · 2 marks · Short answerHarder exam level

   Why are water-soluble vitamins needed in the diet every day, while fat-soluble vitamins are not?

   Hint

   Storage.

   Answer

   Water-soluble vitamins (B, C) are not stored in the body; excess is lost in urine. Fat-soluble vitamins (A, D, E, K) are stored in fat and the liver.

7. Q7 · 3 marks · Short answerHarder exam level

   Explain the role of the liver and the pancreas in digestion.

   Hint

   Bile; three enzymes.

   Answer

   Liver: secretes bile, which emulsifies fats and neutralises acid chyme (it also stores glycogen). Pancreas: secretes pancreatic juice with trypsin (proteins → peptides), amylase (starch → maltose) and lipase (fats → fatty acids + glycerol).

8. Q8 · 3 marks · Short answerHarder exam level

   Define assimilation. What happens to excess glucose and excess amino acids absorbed after a meal?

   Hint

   Glycogen; liver.

   Answer

   Assimilation is the use of absorbed food by body cells for energy, growth and repair. Excess glucose is stored as glycogen in the liver and muscles (or as fat); excess amino acids are broken down in the liver (deamination) and the nitrogen removed as urea.

9. Q9 · 2 marks · Short answerHarder exam level

   Why do canines of a dog differ from those of a human, and what does this tell you about diet?

   Hint

   Tearing.

   Answer

   A dog's canines are long and sharp for tearing flesh, showing a meat-eating diet. Human canines are small, as humans eat a mixed diet and cook their food.

10. Q10 · 3 marks · Short answerHOTS / competency

    A person has had his gall bladder removed. Which kind of food should he eat less of and why?

    Hint

    Bile store.

    Answer

    Fatty foods. The gall bladder stores and concentrates bile; without it, less bile is available at once to emulsify large amounts of fat, so fat digestion is poor and may cause discomfort.

11. Q11 · 3 marks · Short answerHOTS / competency

    In an experiment, test tube A has starch solution + saliva at 37 °C, B has starch solution + boiled saliva at 37 °C, and C has starch solution + saliva at 0 °C. After 20 minutes iodine is added to each. Predict and explain the colours.

    Hint

    Enzymes need the right temperature.

    Answer

    A: no blue-black (starch digested by salivary amylase at body temperature). B: blue-black: boiling destroyed the enzyme. C: blue-black: the enzyme is inactive at 0 °C, so starch remains.

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. 1.Mineral needed to make thyroxine?Show answer

   Iodine.

2. 2.Deficiency disease of vitamin B1?Show answer

   Beriberi.

3. 3.Hardest substance in the body?Show answer

   Tooth enamel.

4. 4.Total number of teeth in an adult?Show answer

   32.

5. 5.Enzyme in saliva?Show answer

   Salivary amylase (ptyalin).

6. 6.Enzyme that curdles milk in infants?Show answer

   Rennin.

7. 7.Where are fatty acids and glycerol absorbed?Show answer

   Into the lacteals of the villi.

8. 8.Role of cellulose in our diet?Show answer

   Roughage: adds bulk, aids peristalsis, prevents constipation.

Quick revision checklist

The night before the exam: can you do each of these without looking?

• ☐Can you give the role and a deficiency effect of Ca, I, Fe and Na?
• ☐Can you name the deficiency disease of vitamins A, B1, C, D and K?
• ☐Can you distinguish kwashiorkor and marasmus in three points?
• ☐Can you label a tooth and write the adult dental formula?
• ☐Can you name each enzyme, its source, substrate and product?
• ☐Can you explain how villi absorb food?

Nutrition and the Digestive System: frequently asked questions