What you'll learn
This topic examines how humans obtain and process nutrients through the digestive system. CIE IGCSE Biology papers frequently test your understanding of the alimentary canal structure, enzyme action at different pH levels, and the absorption of digested products. Questions range from labelling diagrams to explaining adaptations of the small intestine for absorption.
Key terms and definitions
Ingestion — the taking of food into the body through the mouth.
Digestion — the breakdown of large, insoluble food molecules into small, soluble molecules that can be absorbed into the bloodstream.
Absorption — the movement of digested food molecules through the wall of the intestine into the blood or lymph.
Assimilation — the movement of digested food molecules into the cells of the body where they are used, becoming part of the cells.
Egestion — the passing out of undigested food as faeces through the anus.
Peristalsis — waves of muscular contractions in the wall of the alimentary canal that push food along.
Enzymes — biological catalysts that speed up the rate of chemical reactions, including the breakdown of large food molecules during digestion.
Villi — finger-like projections in the small intestine that increase the surface area for absorption.
Core concepts
The human alimentary canal
The alimentary canal is a continuous tube running from mouth to anus, approximately 9 metres long in adults. Each region has specific functions tested in CIE IGCSE Biology examinations.
Mouth: Mechanical digestion occurs through chewing (mastication), breaking food into smaller pieces and increasing surface area for enzyme action. Saliva is secreted by salivary glands and contains the enzyme salivary amylase, which begins starch digestion. Saliva also contains mucus to lubricate food, forming a bolus for swallowing.
Oesophagus: A muscular tube connecting the mouth to the stomach. Food is moved by peristalsis — rhythmic contractions of circular and longitudinal muscles that create wave-like movements. This process does not depend on gravity.
Stomach: A muscular bag with several key functions:
- Churns food mechanically, mixing it with gastric juice
- Secretes gastric juice containing hydrochloric acid and the enzyme pepsin
- Hydrochloric acid provides an acidic pH (approximately pH 2) for pepsin to work optimally
- Acid also kills harmful bacteria in food
- Mucus lines the stomach wall, preventing the acid and pepsin from digesting the stomach itself
- Food remains in the stomach for 2-4 hours, forming a semi-liquid called chyme
Small intestine: Divided into two regions:
Duodenum (first 25 cm): Receives secretions from the pancreas and liver. Pancreatic juice contains three types of enzymes — pancreatic amylase, trypsin (a protease), and lipase. Bile from the liver (stored in the gall bladder) is alkaline and neutralises stomach acid, providing optimal pH for intestinal enzymes. Bile also emulsifies fats, breaking large fat droplets into smaller droplets, increasing surface area for lipase action.
Ileum (remaining 6 metres): The primary site of absorption. The inner surface is highly adapted with millions of villi and microvilli, creating an enormous surface area. Each villus contains a network of blood capillaries and a lacteal (lymph vessel).
Large intestine (colon): Water is reabsorbed from undigested material, along with minerals and vitamins produced by intestinal bacteria. This prevents excessive water loss. The remaining waste forms semi-solid faeces.
Rectum: Stores faeces temporarily until egestion.
Anus: Muscular opening controlled by sphincter muscles where egestion occurs.
Digestive enzymes and their roles
CIE IGCSE Biology examinations regularly test knowledge of specific enzymes, their substrates, products, and optimal conditions.
Carbohydrase enzymes:
- Amylase (salivary amylase and pancreatic amylase): breaks down starch into maltose
- Location: mouth and small intestine
- Optimal pH: neutral (pH 7-8)
- Maltase: breaks down maltose into glucose
- Location: small intestine
- This is produced in the intestinal wall
Protease enzymes:
- Pepsin: breaks down proteins into polypeptides
- Location: stomach
- Optimal pH: acidic (pH 2)
- Trypsin: continues protein digestion, breaking polypeptides into smaller peptides and amino acids
- Location: small intestine (secreted by pancreas)
- Optimal pH: alkaline (pH 8-9)
Lipase enzyme:
- Breaks down lipids (fats and oils) into fatty acids and glycerol
- Location: small intestine (secreted by pancreas)
- Optimal pH: alkaline (pH 8-9)
- Works more efficiently after bile has emulsified the fats
Physical versus chemical digestion
Physical (mechanical) digestion involves breaking food into smaller pieces without changing its chemical structure:
- Chewing with teeth
- Churning in the stomach
- Emulsification of fats by bile (breaking large droplets into smaller droplets)
This increases surface area for enzymes to act upon.
Chemical digestion involves enzymes breaking large molecules into smaller molecules:
- Starch → maltose → glucose
- Proteins → polypeptides → amino acids
- Lipids → fatty acids + glycerol
Both types work together for efficient digestion.
Adaptations of the small intestine for absorption
The ileum is highly specialised for absorption, and CIE IGCSE papers frequently ask students to explain these adaptations:
Villi and microvilli: Create an enormous surface area (approximately 200 m² in adults). Each epithelial cell on a villus has microvilli on its surface, further increasing surface area.
Thin epithelium: The wall of each villus is only one cell thick, providing a short diffusion distance for absorbed molecules to reach blood capillaries.
Rich blood supply: Each villus contains a dense network of capillaries. The constant blood flow maintains a concentration gradient, ensuring continuous absorption of glucose and amino acids by diffusion and active transport.
Lacteal: A lymph vessel in each villus that absorbs fatty acids and glycerol. These are absorbed into the lymphatic system rather than directly into blood.
Long length: The small intestine is approximately 6 metres long, providing extended time for absorption.
Absorption and assimilation of nutrients
Glucose and amino acids:
- Absorbed into blood capillaries in villi by diffusion and active transport
- Travel via the hepatic portal vein to the liver
- Glucose is used for respiration or stored as glycogen in liver and muscles
- Excess amino acids are deaminated in the liver (amino group removed to form urea)
- Amino acids are used for protein synthesis and growth
Fatty acids and glycerol:
- Absorbed into the lacteal in each villus
- Recombined to form fats inside the epithelial cells
- Transported in the lymphatic system, eventually entering the bloodstream
- Used for energy storage, cell membrane formation, and insulation
Vitamins and minerals:
- Water-soluble vitamins (B, C) and minerals absorbed with water
- Fat-soluble vitamins (A, D, E, K) absorbed with fats
Water:
- Absorbed mainly in the large intestine by osmosis
- Some absorption occurs in the small intestine
The role of the liver in nutrition
The liver receives all absorbed nutrients (except fats) via the hepatic portal vein:
- Regulates blood glucose: converts excess glucose to glycogen for storage; breaks down glycogen to glucose when needed
- Deamination: removes amino groups from excess amino acids, forming ammonia which is converted to urea (less toxic)
- Bile production: produces bile continuously, which is stored in the gall bladder
- Detoxification: breaks down alcohol and other toxins
- Storage: stores vitamins A and D, iron, and glycogen
Worked examples
Example 1: Explain why pepsin does not work in the small intestine, even though it is present. [3 marks]
Model answer: Pepsin requires acidic conditions to function optimally [1 mark]. The stomach has a pH of approximately 2, which is ideal for pepsin [1 mark]. The small intestine has an alkaline pH (approximately 8) due to bile and pancreatic juice neutralising stomach acid, so pepsin is denatured and cannot work [1 mark].
Example 2: A student investigated the effect of bile salts on fat digestion. They set up two test tubes containing milk (which contains fat), lipase enzyme, and an indicator that turns pink when fatty acids are produced. Tube A contained bile salts; tube B did not. Both were kept at 37°C. The indicator in tube A turned pink after 5 minutes, while tube B turned pink after 15 minutes. Explain these results. [4 marks]
Model answer: Bile salts emulsify fats [1 mark], breaking large fat droplets into smaller droplets [1 mark]. This increases the surface area for lipase to act on [1 mark]. Therefore, digestion occurs faster in tube A, producing fatty acids more quickly and turning the indicator pink sooner [1 mark].
Example 3: Describe the pathway of a protein molecule from ingestion to assimilation. [5 marks]
Model answer: Protein is ingested in the mouth and swallowed [1 mark]. In the stomach, pepsin breaks protein into polypeptides [1 mark]. In the small intestine, trypsin from the pancreas breaks polypeptides into amino acids [1 mark]. Amino acids are absorbed through the villi into blood capillaries [1 mark]. Amino acids travel to the liver and then to body cells where they are used for protein synthesis/growth/repair [1 mark].
Common mistakes and how to avoid them
Mistake: Confusing digestion with absorption. Students often write that "food is absorbed in the stomach" when describing digestion.
Correction: Digestion is the breakdown of large molecules into smaller ones. Absorption is the movement of these small molecules through the intestine wall into blood. Very little absorption occurs in the stomach (only water and alcohol).
Mistake: Stating that bile is an enzyme or that bile digests fats.
Correction: Bile is not an enzyme. It emulsifies fats (physical digestion), breaking large droplets into smaller ones. The enzyme lipase performs chemical digestion of fats. Bile also neutralises stomach acid.
Mistake: Writing that all nutrients are absorbed into the blood in the villi.
Correction: Glucose, amino acids, vitamins and minerals are absorbed into blood capillaries. Fatty acids and glycerol are absorbed into the lacteal (part of the lymphatic system), not directly into blood.
Mistake: Confusing egestion with excretion.
Correction: Egestion is the removal of undigested food (faeces) that has never been part of cells. Excretion is the removal of metabolic waste products that have been produced by cells, such as urea and carbon dioxide.
Mistake: Stating that enzymes are "used up" during digestion.
Correction: Enzymes are biological catalysts that are not consumed in reactions. They can be used repeatedly to break down substrate molecules. However, they can be denatured by extreme pH or temperature.
Mistake: Describing peristalsis as "food being pushed down by gravity."
Correction: Peristalsis is the contraction of circular and longitudinal muscles in the alimentary canal wall that pushes food along. This works independently of gravity — you can swallow while upside down.
Exam technique for Nutrition in Humans
Command words matter: "Describe" requires you to state what happens without explanation. "Explain" requires you to give reasons why something happens, often using "because" or "therefore." For a 3-mark explain question on enzyme action, provide substrate, enzyme name, and product.
Diagram questions: When labelling the alimentary canal, use precise terms (oesophagus, duodenum, ileum, colon — not just "intestine"). If asked to label where an enzyme is produced, stomach and small intestine are too vague; use stomach lining or pancreas.
Adaptation questions: Questions asking "How is structure X adapted for function Y?" require you to name a feature and link it to advantage. Use the pattern: feature → consequence → benefit. Example: "Villi have a large surface area → more space for absorption → faster/more efficient nutrient uptake."
Enzyme questions: Always state the enzyme name, substrate, product, and optimal pH when asked about digestive enzymes. CIE mark schemes award separate marks for each component.
Quick revision summary
Digestion breaks down large insoluble molecules into small soluble ones through mechanical and chemical processes. The alimentary canal comprises mouth, oesophagus, stomach, small intestine (duodenum and ileum), and large intestine. Key enzymes include amylase (starch to maltose), proteases like pepsin and trypsin (proteins to amino acids), and lipase (fats to fatty acids and glycerol). The small intestine is adapted with villi for efficient absorption. Glucose and amino acids enter blood capillaries; fats enter lacteals. Assimilation occurs when absorbed nutrients are used by cells. Undigested material is egested as faeces.