Nutrition in Animals — From Food to Energy
"'You are what you eat' — but more accurately, you are what your body ABSORBS from what you eat. Digestion is the process that makes food usable."
1. Why Do Animals Need Nutrition?
Unlike plants, animals CANNOT make their own food. They are heterotrophs — they must CONSUME other organisms for energy.
Steps of animal nutrition:
- Ingestion — Taking food into the body
- Digestion — Breaking down food into simpler, soluble substances
- Absorption — Absorbing digested food into the bloodstream
- Assimilation — Using absorbed food for energy, growth, repair
- Egestion — Eliminating undigested waste
2. The Human Digestive System — A Step-by-Step Journey
'The human digestive system is essentially a LONG TUBE (alimentary canal) from mouth to anus, with several accessory organs along the way.'
Overview
Mouth → Oesophagus → Stomach → Small Intestine → Large Intestine → Anus
Accessory organs: SALIVARY GLANDS, LIVER, PANCREAS.
Step 1: The Mouth (Buccal Cavity)
Mechanical digestion: CHEWING with teeth breaks food into smaller pieces. This INCREASES the surface area for enzyme action.
Types of teeth:
| Tooth Type | Function | Number (Adult) |
|---|---|---|
| Incisors | Cutting and biting | 8 |
| Canines | Tearing food | 4 |
| Premolars | Chewing and grinding | 8 |
| Molars | Grinding | 8 (including wisdom) |
Chemical digestion: Saliva (from salivary glands) contains salivary amylase (ptyalin) — an ENZYME that breaks down STARCH into SUGAR (maltose).
'Chew a piece of bread for 30 seconds — it starts tasting SWEET. That is salivary amylase breaking starch into sugar!'
The tongue MIXES food with saliva and pushes it towards the pharynx.
Step 2: The Oesophagus (Food Pipe)
A muscular tube connecting the mouth to the stomach.
Peristalsis: 'Rhythmic, wave-like CONTRACTIONS of the oesophagus muscles that PUSH food downward — even if you are standing on your head!'
The epiglottis (a flap of cartilage) closes the windpipe (trachea) during swallowing to prevent food from 'going down the wrong pipe.'
Step 3: The Stomach
A J-shaped, muscular bag that RECEIVES food from the oesophagus. It has three functions:
Mechanical: Churning — the stomach muscles CONTRACT and RELAX, mixing food with digestive juices.
Chemical:
- Hydrochloric acid (HCl) — KILLS bacteria in food. Creates an ACIDIC environment (pH ~2) needed for pepsin to work.
- Pepsin — an ENZYME that breaks down PROTEINS into smaller peptides.
- Mucus — PROTECTS the stomach wall from being digested by HCl and pepsin.
Storage: Food stays in the stomach for 3-4 hours. The partially digested food (now called chyme) is released into the small intestine.
'Why does the stomach NOT digest itself? Mucus forms a PROTECTIVE lining. When this fails — that is how ULCERS form.'
Step 4: The Small Intestine — The Main Site of Digestion and Absorption
The small intestine is about 6-7 metres long and has THREE sections: duodenum, jejunum, ileum.
Digestion in the duodenum:
- Bile (from liver) — EMULSIFIES fats (breaks them into tiny droplets). Bile does NOT contain enzymes — it is a PHYSICAL breakdown.
- Pancreatic juice (from pancreas) — contains enzymes that digest CARBOHYDRATES (amylase), PROTEINS (trypsin), and FATS (lipase).
Digestion completes here — all food is broken into SIMPLE, soluble forms:
- Carbohydrates → Glucose
- Proteins → Amino acids
- Fats → Fatty acids + Glycerol
Absorption in the ileum: The inner wall of the small intestine has MILLIONS of finger-like projections called VILLI (singular: villus).
| Feature of Villi | How It Helps Absorption |
|---|---|
| LARGE surface area | More area for absorption |
| THIN walls (one cell thick) | Rapid diffusion |
| Rich BLOOD supply | Carries away absorbed nutrients |
| Lacteals (lymph vessels) | Absorb fatty acids and glycerol |
'Villi are like TINY fingers waving in the intestine. Each one increases the surface area, making absorption HIGHLY efficient.'
Step 5: The Large Intestine (Colon)
About 1.5 metres long. It:
- ABSORBS water and minerals from the undigested material
- Houses BACTERIA that produce some vitamins (Vitamin K, B vitamins)
- Forms and stores faeces (undigested waste)
Step 6: Egestion (Defecation)
Faeces are expelled through the anus via rhythmic contractions. 'Regular egestion is essential for health — constipation means too MUCH water absorption, diarrhoea means too LITTLE.'
3. Digestion in Amoeba
Amoeba is a SINGLE-CELLED organism. Its nutrition is very different from humans.
Process (Phagocytosis)
- Ingestion: Amoeba extends pseudopodia (false feet) to ENGULF food particles, forming a food vacuole.
- Digestion: Digestive enzymes are SECRETED into the food vacuole. Food is broken down into simpler substances.
- Absorption: Digested food DIFFUSES from the vacuole into the cytoplasm.
- Assimilation: Food is used for energy and growth.
- Egestion: Undigested waste is EXPELLED when the food vacuole fuses with the cell membrane.
4. Summary Table
| Organ | Mechanical Action | Chemical Action / Secretions |
|---|---|---|
| Mouth | Chewing by teeth, mixing by tongue | Salivary amylase breaks starch |
| Oesophagus | Peristalsis pushes food down | — |
| Stomach | Churning, mixing | HCl, pepsin (protein digestion), mucus |
| Small intestine | Peristalsis | Bile (fat emulsification), pancreatic enzymes, intestinal enzymes |
| Large intestine | — | Water absorption, bacterial action |
5. Common Mistakes and Fixes
| Mistake | Why It Is Wrong | Correct Approach |
|---|---|---|
| Bile digests fats | Bile EMULSIFIES fat (physical breakdown), not digests | Lipase (from pancreas) does the CHEMICAL digestion of fats |
| Absorption occurs in the stomach | Stomach digests proteins; almost NO absorption occurs there | Absorption is mainly in the SMALL INTESTINE |
| Amoeba uses specialised organs | Amoeba is SINGLE-CELLED — no organs | It uses pseudopodia and food vacuoles |
| Villi are in the large intestine | Villi are in the small intestine ONLY | The large intestine has a smooth inner wall |
6. AP SSC Exam Focus
| Topic | Marks | Question Type |
|---|---|---|
| Human digestive system (diagram) | 4-5 | Label parts and explain |
| Enzymes and their functions | 2-3 | What breaks down what |
| Villi and absorption | 2-3 | Structure-function |
| Digestion in Amoeba | 2-3 | Describe process |
| Balanced diet and nutrition | 1-2 | Short notes |
Quick Self-Test
Q1. Name the enzyme in saliva and what it digests. A1. Salivary amylase — breaks down STARCH into sugar (maltose).
Q2. What is the function of hydrochloric acid in the stomach? A2. HCl kills harmful bacteria and creates an acidic medium for pepsin to work.
Q3. Which part of the alimentary canal absorbs the most nutrients? A3. The SMALL INTESTINE (through villi).
Q4. What is peristalsis? A4. Rhythmic wave-like muscle contractions that push food through the oesophagus and alimentary canal.
Q5. How does Amoeba ingest food? A5. By extending pseudopodia around the food particle to engulf it (phagocytosis), forming a food vacuole.
Q6. Why is bile important even though it has no enzymes? A6. Bile emulsifies fats (breaks them into tiny droplets), increasing surface area for lipase to act on.
Q7. Name the three sections of the small intestine. A7. Duodenum (first part), jejunum (middle part), ileum (last part).
