What you'll learn
This revision guide covers all aspects of nutrition and digestion tested in WJEC GCSE Biology. You'll learn about the structure and function of the digestive system, how enzymes break down food molecules, and how nutrients are absorbed into the bloodstream. Understanding these processes is essential for tackling both short-answer and extended-response questions on Paper 1 and Paper 2.
Key terms and definitions
Digestion — the breakdown of large, insoluble food molecules into small, soluble molecules that can be absorbed into the bloodstream
Enzyme — a biological catalyst that speeds up chemical reactions in living organisms without being used up itself
Peristalsis — rhythmic contractions of smooth muscle in the gut wall that push food along the alimentary canal
Villi (singular: villus) — finger-like projections in the small intestine that increase surface area for absorption
Emulsification — the breakdown of large fat droplets into smaller droplets to increase surface area for enzyme action
Egestion — the removal of undigested food waste from the body through the anus
Active transport — the movement of molecules from a region of lower concentration to a region of higher concentration using energy from respiration
Assimilation — the uptake and use of digested food molecules by cells and tissues
Core concepts
The alimentary canal and associated organs
The digestive system consists of the alimentary canal (gut) and associated organs that produce digestive secretions.
Main structures of the alimentary canal (in order):
- Mouth — mechanical digestion by teeth; chemical digestion of starch begins with salivary amylase
- Oesophagus — muscular tube connecting mouth to stomach; moves food by peristalsis
- Stomach — muscular bag that churns food and secretes hydrochloric acid and protease enzymes
- Small intestine — consists of duodenum (where most digestion occurs) and ileum (where most absorption occurs)
- Large intestine — absorbs water from undigested food; forms faeces
- Rectum — stores faeces before egestion
- Anus — muscular ring controlling egestion of faeces
Associated organs:
- Salivary glands — produce saliva containing amylase and mucus
- Pancreas — produces pancreatic juice containing amylase, protease and lipase enzymes, plus sodium hydrogencarbonate to neutralise stomach acid
- Liver — produces bile, which emulsifies fats and neutralises stomach acid
- Gall bladder — stores bile before release into duodenum
Food tests and nutrients
The major food groups have specific chemical tests you must know:
Starch test:
- Add iodine solution to food sample
- Positive result: blue-black colour
- Negative result: brown-orange colour (unchanged iodine)
Reducing sugars test (Benedict's test):
- Add Benedict's solution to food sample
- Heat in water bath at 80-100°C for 5 minutes
- Positive result: colour change from blue → green → yellow → orange → brick-red (depends on concentration)
- Negative result: remains blue
Protein test (Biuret test):
- Add sodium hydroxide solution to food sample
- Add copper sulfate solution and mix
- Positive result: purple/lilac colour
- Negative result: remains blue
Lipid (fat) test:
- Add ethanol to food sample and shake
- Pour liquid into water
- Positive result: white/milky emulsion forms
- Negative result: clear solution remains
Enzymes and digestion
Digestive enzymes are specific — each type breaks down only one substrate into particular products.
Carbohydrase enzymes:
Amylase breaks down starch into maltose (a disaccharide sugar):
- Produced in: salivary glands, pancreas, small intestine
- Works in: mouth, small intestine
- Optimum pH: 7-8 (neutral to slightly alkaline)
Maltase breaks down maltose into glucose:
- Produced in: small intestine
- Works in: small intestine
- Optimum pH: 7-8
Protease enzymes:
Break down proteins into amino acids:
- Produced in: stomach (pepsin), pancreas, small intestine
- Works in: stomach, small intestine
- Optimum pH: pepsin works at pH 2 (acidic); other proteases at pH 7-8
Lipase enzymes:
Break down lipids (fats and oils) into glycerol and fatty acids:
- Produced in: pancreas, small intestine
- Works in: small intestine
- Optimum pH: 7-8
The role of bile:
Bile is not an enzyme but aids fat digestion in two ways:
- Emulsifies fats — breaks large fat droplets into smaller ones, increasing surface area for lipase action
- Neutralises stomach acid — provides alkaline conditions (pH 7-8) for enzymes in the small intestine to work efficiently
The stomach environment
The stomach is adapted for both mechanical and chemical digestion:
Structural adaptations:
- Thick muscular walls contract to churn food
- Inner lining produces gastric juice containing hydrochloric acid and pepsin
- Muscular sphincters at entrance and exit control food movement
Functions of stomach acid:
- Provides optimum pH (around pH 2) for pepsin to break down proteins
- Kills harmful bacteria in food
- Denatures (unfolds) proteins, making them easier for pepsin to digest
Protection from acid damage:
The stomach lining produces mucus that:
- Forms a protective layer preventing acid from damaging stomach wall
- Neutralises acid at the stomach surface
Absorption in the small intestine
The small intestine, particularly the ileum, is adapted for efficient absorption of digested food molecules.
Adaptations of the ileum:
- Very long (approximately 6 metres) — provides large surface area and time for absorption
- Inner surface covered in millions of villi — increase surface area
- Each villus covered in microvilli — further increase surface area
- Villi have thin walls (one cell thick) — short diffusion distance
- Rich blood supply in each villus — maintains concentration gradient for diffusion
- Lacteals (lymph vessels) in each villus — absorb fatty acids and glycerol
How different nutrients are absorbed:
- Glucose and amino acids — absorbed into blood capillaries by diffusion and active transport; transported to liver via hepatic portal vein
- Fatty acids and glycerol — absorbed into lacteals; transported in lymph system before entering bloodstream
- Water — absorbed in small intestine and large intestine by osmosis
- Vitamins and minerals — absorbed in small intestine by diffusion and active transport
Active transport in absorption:
When nutrient concentration in the ileum is lower than in the blood, active transport moves molecules against the concentration gradient. This requires:
- Energy from respiration
- Carrier proteins in cell membranes
- Functional mitochondria in intestinal epithelial cells
The large intestine and egestion
The large intestine performs final processing before egestion:
Functions:
- Absorbs water from undigested food back into bloodstream
- Absorbs vitamins produced by bacteria living in the colon
- Forms faeces from undigested material (mainly cellulose fibre, dead cells, bacteria)
Faeces composition:
Faeces is undigested food, not metabolic waste (which is removed by kidneys). It contains:
- Cellulose and other dietary fibre
- Dead gut lining cells
- Bacteria (approximately 50% of dry mass)
- Small amounts of water
Importance of fibre:
Dietary fibre (cellulose from plant cell walls):
- Cannot be digested by human enzymes
- Adds bulk to faeces
- Stimulates peristalsis in large intestine
- Prevents constipation and reduces risk of bowel cancer
Worked examples
Example 1: Enzyme specificity (3 marks)
Question: Explain why the enzyme amylase cannot digest protein molecules.
Answer:
- Amylase has an active site with a specific shape (1 mark)
- Only starch molecules fit into the active site of amylase (1 mark)
- Protein molecules have a different shape so cannot bind to amylase's active site (1 mark)
Examiner note: This tests understanding of the lock-and-key hypothesis. Use the term "active site" and explain complementary shapes for full marks.
Example 2: Adaptations for absorption (4 marks)
Question: Describe how the structure of a villus is adapted for efficient absorption of digested food.
Answer:
- Large surface area due to many villi/microvilli increases absorption rate (1 mark)
- Thin wall (one cell thick) provides short diffusion distance (1 mark)
- Good blood supply maintains concentration gradient for diffusion (1 mark)
- Contains lacteal for absorption of fatty acids and glycerol (1 mark)
Examiner note: Link each structural feature to its function. "Large surface area" alone is insufficient — explain why this increases absorption.
Example 3: Experimental design (5 marks)
Question: A student investigates the effect of pH on protease activity using gelatine (a protein). Describe how the student could carry out this investigation.
Answer:
- Prepare test tubes containing gelatine solution at different pH values (pH 2, 4, 6, 8, 10) using buffer solutions (1 mark)
- Add equal volumes/concentrations of protease enzyme to each tube (1 mark)
- Maintain at constant temperature (e.g., 30°C in water bath) (1 mark)
- Measure time taken for gelatine to become liquid/clear in each tube (1 mark)
- Repeat and calculate mean time; shortest time indicates optimum pH (1 mark)
Examiner note: Include control variables (temperature, enzyme concentration, substrate concentration) and quantitative measurements for full marks.
Common mistakes and how to avoid them
Confusing digestion with absorption — Digestion is breaking down large molecules; absorption is taking small molecules into the bloodstream. They occur in sequence but are different processes.
Saying bile is an enzyme — Bile emulsifies fats and neutralises acid but does not chemically break down molecules. Only state that lipase is the enzyme that digests lipids.
Confusing egestion with excretion — Egestion removes undigested food (faeces); excretion removes metabolic waste products (e.g., urea in urine, CO₂ in breath). Faeces has never been absorbed into body cells.
Stating "breaks down food" without specificity — Always name the substrate (what is broken down) and products (what is formed). For example: "amylase breaks down starch into maltose" not just "amylase breaks down food."
Forgetting that different proteases have different optimum pH values — Pepsin works in acidic conditions (pH 2) in the stomach; pancreatic protease works in alkaline conditions (pH 8) in the small intestine.
Not explaining why surface area matters — Don't just write "villi increase surface area." Explain that increased surface area allows more molecules to be absorbed at once, increasing the rate of absorption.
Exam technique for "Nutrition and the Digestive System"
Command words matter — "Describe" requires you to state features or processes; "Explain" requires you to give reasons or mechanisms. For example, describing villi = "finger-like projections"; explaining villi = "increase surface area so more molecules absorbed per unit time."
Use correct technical terms — Write "small intestine" not "gut"; "egestion" not "excretion"; "emulsification" not "breaking up." Examiners award marks for precision, and incorrect terminology can lose marks even if you understand the concept.
Link structure to function — Questions about adaptations require you to identify a feature AND explain how it helps. Use "so that" or "which allows" to make connections clear: "Villi have a good blood supply, which maintains a concentration gradient for diffusion."
Show your working in calculations — Questions involving rates, percentages or concentration changes require showing steps. Even if your final answer is wrong, you can gain method marks for correct formula or approach.
Quick revision summary
The digestive system breaks down large, insoluble food molecules into small, soluble ones through mechanical and chemical digestion. Enzymes (amylase, protease, lipase) are biological catalysts specific to particular substrates. The stomach provides acidic conditions for pepsin; bile emulsifies fats and neutralises acid in the small intestine. The ileum is adapted for absorption with villi increasing surface area. Nutrients enter the bloodstream by diffusion and active transport. The large intestine absorbs water, forming faeces for egestion. Master food tests, enzyme action, and absorption adaptations for exam success.