Nutrition and Digestion
Introduction
Nutrition is the process by which organisms obtain and utilise food for growth, energy, and maintenance. ICSE Class 9 covers types of nutrition, the human digestive system, balanced diet, and deficiency diseases.
Types of Nutrition
Autotrophic Nutrition (Self-feeding)
Organisms synthesise their own food from inorganic substances.
Photosynthesis: The process by which green plants convert light energy into chemical energy.
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (in presence of sunlight and chlorophyll)
Requirements: Carbon dioxide, water, sunlight, chlorophyll Site: Chloroplasts in leaves Events: Absorption of light, conversion of light energy to chemical energy, splitting of water, reduction of CO₂ to carbohydrates
Heterotrophic Nutrition (Feeding on others)
| Type | Description | Examples |
|---|---|---|
| Saprophytic | Feeds on dead and decaying matter | Fungi, mushrooms, bread mould |
| Parasitic | Lives on or inside a host organism | Tapeworm, leech, cuscuta |
| Holozoic | Takes in solid food and digests it internally | Humans, animals, amoeba |
| Symbiotic | Two organisms mutually benefit | Lichens (algae + fungi) |
Human Digestive System
Organs and Their Functions
Mouth (Buccal Cavity)
- Teeth: Mechanical breakdown of food (mastication)
- Tongue: Mixes food, taste perception, pushes food into pharynx
- Salivary glands: Produce saliva containing salivary amylase
- Salivary amylase breaks down starch into maltose
Oesophagus (Food Pipe)
- Carries food from pharynx to stomach
- Peristalsis: Rhythmic muscular contractions that push food down
Stomach
- J-shaped muscular organ
- Gastric glands secrete:
- Pepsinogen → Pepsin (digests proteins)
- HCl: Creates acidic medium, kills bacteria
- Mucus: Protects stomach lining
- Food becomes chyme (semi-fluid)
Small Intestine
- Duodenum: First part; receives bile from liver and pancreatic juice
- Jejunum and Ileum: Absorption of nutrients
- Villi: Finger-like projections that increase surface area for absorption
- Enzymes: Maltase, sucrase, lactase, peptidases
Accessory Organs:
- Liver: Produces bile (emulsifies fats); largest gland
- Pancreas: Produces pancreatic juice (trypsin, lipase, amylase)
Large Intestine (Colon)
- Absorbs water and minerals
- Forms and stores faeces
- Rectum: Stores faeces before elimination
Digestive Enzymes Summary
| Organ | Enzyme/Secretion | Substrate | Product |
|---|---|---|---|
| Mouth | Salivary amylase | Starch | Maltose |
| Stomach | Pepsin | Proteins | Peptides |
| Stomach | HCl | Activates pepsinogen | - |
| Pancreas | Trypsin | Proteins | Peptides |
| Pancreas | Lipase | Fats | Fatty acids + Glycerol |
| Pancreas | Amylase | Starch | Maltose |
| Small intestine | Maltase | Maltose | Glucose |
| Small intestine | Peptidase | Peptides | Amino acids |
| Liver | Bile (stored in gall bladder) | Fats | Emulsified fats |
Balanced Diet
A diet that contains all nutrients in the right proportions.
| Nutrient | Sources | Functions | Deficiency Disease |
|---|---|---|---|
| Carbohydrates | Rice, wheat, potato | Energy | Weakness, weight loss |
| Proteins | Eggs, milk, pulses | Growth, repair | Kwashiorkor, Marasmus |
| Fats | Butter, oil, nuts | Energy reserve | Weight loss |
| Vitamins | Fruits, vegetables | Various | Specific deficiency diseases |
| Minerals | Various foods | Various | Specific deficiency diseases |
| Fibre | Vegetables, whole grains | Digestion | Constipation |
| Water | Water, fluids | All metabolic processes | Dehydration |
Vitamin Deficiency Diseases
| Vitamin | Deficiency Disease | Symptoms |
|---|---|---|
| Vitamin A | Night blindness | Poor vision in dim light |
| Vitamin B₁ | Beriberi | Nerve disorders, weakness |
| Vitamin C | Scurvy | Bleeding gums, slow healing |
| Vitamin D | Rickets | Soft bones, bow legs |
| Vitamin B₁₂ | Anaemia | Weakness, pale skin |
Common Mistakes With Fixes
| Mistake | Correction |
|---|---|
| Digestion starts in the stomach | Digestion begins in the MOUTH (salivary amylase) |
| Bile digests fats | Bile EMULSIFIES fats; lipase digests them |
| All vitamins are produced by the body | Most vitamins must be obtained from food |
| Proteins are digested in the stomach only | Protein digestion continues in the small intestine (trypsin) |
ICSE Exam Focus
| Topic | Marks (approx.) | Frequency |
|---|---|---|
| Human digestive system (labelled diagram) | 5-6 marks | Very common |
| Digestive enzymes and their functions | 4-5 marks | Very common |
| Balanced diet and deficiency diseases | 3-4 marks | Common |
| Types of nutrition | 3 marks | Frequently asked |
Self-Test
Q1: Draw a labelled diagram of the human digestive system.
Q2: List the enzymes secreted by the pancreas and their functions.
Q3: What is the function of villi in the small intestine?
Q4: Name the deficiency diseases caused by: (i) Vitamin A (ii) Vitamin C (iii) Vitamin D
Q5: Differentiate between autotrophic and heterotrophic nutrition.
