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Detailed notes on Animal Nutrition for Cambridge IGCSE Coordinated Science, covering key concepts, explanations, examples, and exam-focused revision points.
The alimentary canal is a muscular tube from mouth to anus where food is mechanically and chemically digested, then absorbed. Each region has a specific role. The villi of the small intestine are a classic exam topic for structure-function questions.
Mapped to the Cambridge IGCSE 0654 syllabus (2025-2027).
Each region specialises in a different aspect of digestion or absorption.
| Region | Process | Key Secretions/Features |
|---|---|---|
| Mouth | Mechanical: chewing (teeth). Chemical: starch → maltose | Salivary amylase; mucus; tongue mixes food |
| Oesophagus | Transport via PERISTALSIS (muscle contractions) | Mucus (lubrication); no digestion |
| Stomach | Mechanical: churning. Chemical: proteins → peptides | Pepsin (protease), HCl (pH~2), mucus |
| Duodenum (small intestine) | Chemical digestion | Bile (from liver); pancreatic juice (amylase, protease, lipase); intestinal juice |
| Ileum (small intestine) | Absorption of nutrients | Villi and microvilli for absorption |
| Large intestine | Water reabsorption; formation of faeces | Bacteria (produce some vitamins); no digestion |
| Rectum | Storage of faeces | — |
| Anus | Egestion (defecation) | Sphincter muscles |
Peristalsis: wave-like contractions of circular and longitudinal muscles in the wall of the alimentary canal that push food along.
Role of HCl (stomach acid):
Role of bile (emulsification — NOT enzyme activity):
Villi multiply the surface area of the small intestine ~600-fold for maximum nutrient absorption.
Structure of villi and their adaptations:
| Adaptation | How it aids absorption |
|---|---|
| Finger-like projections (villi) | Greatly increase surface area → more absorption per unit time |
| Microvilli (brush border) on epithelial cells | Further increase SA by ~20× |
| One-cell-thick epithelium | Very short diffusion distance |
| Dense network of blood capillaries | Maintains steep concentration gradient; carries absorbed nutrients away quickly |
| Lacteal (lymph vessel) in each villus | Absorbs fatty acids and glycerol (reassembled as lipids → enter lymph system) |
| Good blood supply | Maintains concentration gradient for diffusion |
What is absorbed where:
Hepatic portal vein: carries blood from the small intestine directly to the liver — allowing the liver to process nutrients (store glucose as glycogen, deaminate excess amino acids) before they reach general circulation.
Verbatim phrases and definitions Cambridge mark schemes credit.
Paper 4: 'Explain how the small intestine is adapted for absorption' (4–6 marks — must give structural feature + function + how it increases absorption). 'Explain the role of bile in digestion' (must say emulsification, NOT enzyme; must link to surface area for lipase). Drawing/labelling the alimentary canal. Distinguish emulsification (physical) from digestion (chemical) — this distinction costs marks.
Sources: Cambridge IGCSE Coordinated Sciences 0654 syllabus 2025-2027 (B7); 0654/42 May/Jun 2023 — Q5 (digestion and absorption). Last reviewed 2026-05-14.
Step-by-step solutions to past-paper-style questions on alimentary canal , written exactly the way a tutor would explain them at the board.
Question
Describe two adaptations of the small intestine that increase the rate of absorption of digested food.
Step-by-step solution
Step 1
Villi and microvilli (brush border): finger-like projections from the inner wall of the small intestine, further covered by microvilli. These greatly increase the surface area available for absorption.
Step 2
Rich blood supply: each villus contains a dense capillary network. Blood flow maintains a steep concentration gradient between the intestinal lumen and the blood, driving rapid diffusion and active transport of nutrients.
Step 3
Additional adaptations (for full marks): epithelium is only one cell thick (short diffusion distance); lacteal in centre of villus absorbs fatty acids/glycerol.
Answer
Question
Explain why glucose enters the blood capillaries in the villi but fatty acids enter the lacteals.
Step-by-step solution
Step 1
Glucose and amino acids are water-soluble and small enough to enter blood capillaries directly by diffusion and active transport. They are transported in blood plasma via the hepatic portal vein to the liver.
Step 2
Fatty acids and glycerol are lipid-soluble and are recombined into triglycerides inside epithelial cells. These are packaged as chylomicrons.
Step 3
Chylomicrons are too large to enter capillaries; they enter the lacteals (lymph capillaries) and travel via the lymphatic system before entering the bloodstream.
Answer
Glucose and amino acids are water-soluble and enter blood capillaries directly. Fatty acids and glycerol are reformed into triglycerides (chylomicrons) which are too large for capillaries; they enter lacteals and are transported via the lymphatic system.
Question
State the role of peristalsis in digestion.
Step-by-step solution
Step 1
Peristalsis is the wave-like contraction of circular and longitudinal muscles in the gut wall.
Step 2
It moves food through the alimentary canal from oesophagus to rectum, and also mixes food with digestive juices.
Answer
Peristalsis is the wave-like muscular contraction of the gut wall that moves food along the alimentary canal and mixes it with digestive enzymes.
Question
Explain why the products of protein digestion are taken to the liver before entering the rest of the body.
Step-by-step solution
Step 1
Amino acids absorbed from the small intestine travel via the hepatic portal vein to the liver.
Step 2
The liver performs deamination — removal of excess amino acids that cannot be stored. The nitrogen-containing group is converted to urea (for excretion by kidneys), and the remainder is used for energy.
Step 3
The liver also converts excess glucose to glycogen for storage, and detoxifies harmful substances, making it essential that blood from the intestine passes through the liver before the systemic circulation.
Answer
Absorbed amino acids travel via the hepatic portal vein to the liver, where excess amino acids are deaminated (converted to urea). The liver also regulates blood glucose and detoxifies substances before they enter the main circulation.
Question
A student says the stomach is where all food is digested. Explain why this statement is incorrect.
Step-by-step solution
Step 1
Starch digestion begins in the mouth (salivary amylase converts starch to maltose), not the stomach. The stomach has acidic conditions (pH 2) that denature salivary amylase.
Step 2
Lipid (fat) digestion occurs mainly in the small intestine (lipase from the pancreas, aided by bile from the liver). No significant lipid digestion occurs in the stomach.
Step 3
Protein digestion does begin in the stomach (pepsin, pH 2) but is completed in the small intestine by pancreatic proteases.
Answer
The statement is incorrect. Starch digestion begins in the mouth (salivary amylase); lipid digestion occurs in the small intestine (pancreatic lipase); protein digestion starts in the stomach but is completed in the small intestine. The stomach is not the only site of digestion.
Definitions to memorise and the exact keywords mark schemes credit for alimentary canal answers — sharpened from recent examiner reports for the 2026 0654 sitting.
A finger-like projection of the inner wall of the small intestine that increases the surface area for absorption of digested nutrients.
Wave-like muscular contractions of the gut wall (circular and longitudinal muscle) that move food along the alimentary canal.
A lymph capillary in the centre of each villus that absorbs fatty acids and glycerol (as chylomicrons/triglycerides) from the small intestine.
The blood vessel that carries nutrient-rich blood from the small intestine directly to the liver for processing.
The movement of digested food molecules from the intestine into the blood (or lymph) across the gut wall.
The taking of food and drink into the body through the mouth.
The traps other students keep falling into on alimentary canal questions — taken from recent Cambridge IGCSE 0654 examiner reports and mark schemes — and how to avoid them.
Why it happens
Students know villi are involved in digestion and place them where most digestion occurs.
How to avoid it
Villi are found in the small intestine (specifically the ileum), not the stomach. The stomach's role is mixing and chemical digestion, not absorption.
Why it happens
Bile aids digestion, so students classify it with digestive enzymes.
How to avoid it
Bile is not an enzyme — it does not catalyse reactions. It emulsifies lipids (physical process) and neutralises stomach acid to create the optimum pH for intestinal enzymes.
Why it happens
Students apply the same absorption route to all nutrients.
How to avoid it
Fats are absorbed as chylomicrons into lacteals (lymph vessels), NOT blood capillaries. They enter the bloodstream later via the lymphatic system.
Why it happens
Students think any waste removal is excretion.
How to avoid it
Faeces are undigested material — egestion. Excretion removes metabolic waste (e.g. CO₂ via lungs, urea via kidneys).
The things students keep getting wrong in this sub-topic, answered.