Nutrition in Animals - Class 7 Science (CBSE)
Based on the 2025-26 NCERT syllabus for Class 7 Science. This chapter covers how animals obtain and process food, focusing on the human digestive system, digestion in ruminants, and in simple organisms like amoeba.
1. Why this chapter matters
Understanding our own digestive system helps us make better food choices and recognise digestive health issues. In CBSE exams, this chapter contributes 8-10 marks with diagram-based questions on the digestive system.
2. The human digestive system
The human digestive system consists of the alimentary canal and associated digestive glands.
Organs of the digestive system
- Mouth (buccal cavity)
- Food pipe (oesophagus)
- Stomach
- Small intestine
- Large intestine
- Anus
Associated glands
- Salivary glands
- Liver
- Pancreas
3. Step-by-step digestion
Mouth
Digestion begins in the mouth:
- Teeth break food into smaller pieces (physical digestion).
- Saliva (from salivary glands) contains the enzyme salivary amylase that breaks down starch into sugar.
Oesophagus (food pipe)
The oesophagus is a muscular tube that carries food from the mouth to the stomach through wave-like muscular contractions called peristalsis.
Stomach
- The stomach is a J-shaped muscular organ.
- Gastric glands in the stomach wall secrete:
- Hydrochloric acid (HCl): Kills bacteria and creates an acidic medium.
- Pepsin: An enzyme that breaks down proteins.
- Mucus: Protects the stomach lining from acid.
- Partial digestion of proteins occurs here.
Small intestine
The small intestine is the longest part of the digestive tract (about 6-7 metres).
- The liver produces bile (stored in gall bladder) that breaks down fats.
- The pancreas produces pancreatic juices containing enzymes for digesting carbohydrates, proteins, and fats.
- The intestinal wall has finger-like projections called villi that absorb digested food.
Absorption in small intestine
Villi increase the surface area for absorption. Each villus has blood vessels that transport absorbed nutrients to all parts of the body.
Large intestine
The large intestine absorbs water and minerals from undigested food. The remaining solid waste is stored in the rectum and eliminated through the anus.
4. Digestive enzymes and their functions
| Enzyme | Produced by | Digests | Product |
|---|---|---|---|
| Salivary amylase | Salivary glands | Starch | Sugar |
| Pepsin | Stomach | Proteins | Amino acids |
| Bile (not an enzyme) | Liver | Fats | Fat droplets |
| Pancreatic amylase | Pancreas | Starch | Sugar |
| Trypsin | Pancreas | Proteins | Amino acids |
| Lipase | Pancreas | Fats | Fatty acids + Glycerol |
5. Digestion in amoeba
Amoeba is a single-celled organism found in pond water. It feeds on microscopic organisms.
Process of nutrition in amoeba
- Ingestion: Amoeba uses pseudopodia (false feet) to engulf food particles. It forms a food vacuole around the food.
- Digestion: Digestive enzymes are secreted into the food vacuole to break down food.
- Absorption: Nutrients are absorbed into the cytoplasm.
- Assimilation: Nutrients are used for growth and energy.
- Egestion: Undigested waste is expelled from the cell.
This process is called phagocytosis (cell eating).
6. Digestion in ruminants
Ruminants are animals that eat grass and have a specialised stomach with four compartments.
Compartments of a ruminant stomach
| Compartment | Function |
|---|---|
| Rumen | Grass is partially digested by bacteria. Cud is formed |
| Reticulum | Cud is formed into small balls and regurgitated for chewing |
| Omasum | Water is absorbed from the partially digested food |
| Abomasum | True stomach where digestive enzymes break down food |
The rumination process
- The animal eats grass quickly and stores it in the rumen.
- Bacteria in the rumen break down cellulose (which humans cannot digest).
- The partially digested food (cud) is regurgitated into the mouth.
- The animal chews the cud thoroughly to break it down further.
- The chewed food is swallowed again and passes through the remaining compartments.
Cellulose digestion
Ruminants can digest cellulose because of bacteria present in the rumen. Humans cannot digest cellulose.
7. Comparison of digestive systems
| Feature | Humans | Ruminants | Amoeba |
|---|---|---|---|
| Digestive system | One stomach, long intestine | Four-chambered stomach | No organs |
| Digestion type | Intracellular and extracellular | Extracellular | Intracellular |
| Cellulose digestion | No | Yes (bacteria in rumen) | No |
| Feeding method | Ingestion through mouth | Grazing | Pseudopodia |
8. Worked examples
Example 1: Where is bile produced and what is its function?
Bile is produced in the liver and stored in the gall bladder. It breaks down fats into smaller droplets (emulsification), making them easier for enzymes to digest.
Example 2: What is the function of villi in the small intestine?
Villi increase the surface area of the small intestine for maximum absorption of digested food. Each villus has blood vessels that transport nutrients.
Example 3: How does an amoeba capture its food?
Amoeba extends pseudopodia around its food particle, engulfing it to form a food vacuole where digestion occurs.
9. Common mistakes and how to fix them
| Mistake | Fix |
|---|---|
| Thinking digestion begins in the stomach | Digestion begins in the mouth with saliva |
| Saying bile digests fats | Bile emulsifies fats; lipase actually digests them |
| Confusing rumen and abomasum | Rumen is storage/fermentation; abomasum is the true stomach |
| Believing humans can digest cellulose | Humans lack the enzyme for cellulose digestion |
| Forgetting the role of hydrochloric acid | HCl in stomach kills bacteria and activates pepsin |
10. CBSE exam focus
| Question type | Marks | Frequency |
|---|---|---|
| Human digestive system diagram | 2-3 marks | 1 question |
| Enzyme functions and locations | 2 marks | 1 question |
| Digestion in ruminants | 3 marks | 1 question |
| Digestion in amoeba | 2-3 marks | 1 question |
| Villi structure and function | 2 marks | 1 question |
11. Self-test
- Draw a labelled diagram of the human digestive system.
- What is the function of saliva in digestion?
- Name the four compartments of a ruminant stomach.
- How does an amoeba obtain and digest its food?
- What is the role of the pancreas in digestion?
- Why do ruminants chew the cud?
12. Answer key
- Diagram should include: mouth, oesophagus, stomach, small intestine, large intestine, anus, liver, pancreas.
- Saliva contains salivary amylase that breaks down starch into sugar. It also moistens food for easy swallowing.
- Rumen, reticulum, omasum, abomasum.
- Amoeba uses pseudopodia to engulf food, forming a food vacuole. Enzymes digest the food, and nutrients are absorbed.
- Pancreas produces enzymes (amylase, trypsin, lipase) that digest carbohydrates, proteins, and fats.
- Ruminants eat grass quickly. When resting, they bring the cud back to the mouth and chew it thoroughly for better digestion.
13. Quick revision
- Digestion begins in the mouth (salivary amylase).
- Stomach: HCl + pepsin digest proteins.
- Small intestine: most digestion and absorption occurs.
- Villi: finger-like projections for absorption.
- Liver produces bile; pancreas produces digestive enzymes.
- Amoeba uses pseudopodia for phagocytosis.
- Ruminants have 4 stomach chambers for cellulose digestion.
- Humans cannot digest cellulose.
