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Detailed notes on Animal Nutrition for Cambridge IGCSE Coordinated Science, covering key concepts, explanations, examples, and exam-focused revision points.
Digestion breaks large, insoluble food molecules into small, soluble ones that can be absorbed into the blood. Know all digestive enzymes, where they are made, their substrates, and their products. The enzyme table is a guaranteed exam question.
Mapped to the Cambridge IGCSE 0654 syllabus (2025-2027).
Know source, substrate, and product for every enzyme. This table appears in nearly every exam.
Why digestion is necessary: Large food molecules (starch, protein, lipids) are:
The complete enzyme table:
| Enzyme | Where Made | Substrate | Product(s) | Optimum pH |
|---|---|---|---|---|
| Salivary amylase | Salivary glands | Starch | Maltose | ~7 |
| Pancreatic amylase | Pancreas | Starch | Maltose | ~7 |
| Pepsin (protease) | Stomach | Proteins | Peptides | ~2 |
| Trypsin (protease) | Pancreas | Proteins/peptides | Peptides/amino acids | ~8 |
| Lipase | Pancreas | Lipids (triglycerides) | Fatty acids + glycerol | ~7–8 |
| Maltase | Small intestine (ileum) | Maltose | Glucose | ~7 |
| Peptidases | Small intestine | Peptides | Amino acids | ~7–8 |
Hydrolysis: All digestion reactions involve HYDROLYSIS — breaking chemical bonds by adding water.
Why pepsin works at pH 2: The stomach secretes HCl, creating an acidic environment that is the optimum for pepsin. When food moves to the small intestine, bile neutralises the acid (raising pH to ~7–8), deactivating pepsin but providing the optimum for pancreatic enzymes.
Verbatim phrases and definitions Cambridge mark schemes credit.
Exam questions ask you to complete tables of enzyme name, source, substrate, and product. Always give the specific enzyme name (not just 'digestive enzyme'). State WHERE it is made and WHERE it acts (these are different for pancreatic enzymes). The hydrolysis/condensation distinction is often a 1-mark question.
Sources: Cambridge IGCSE Coordinated Sciences 0654 syllabus 2025-2027 (B7). Last reviewed 2026-05-14.
Step-by-step solutions to past-paper-style questions on digestion , written exactly the way a tutor would explain them at the board.
Question
Explain the role of bile in the digestion of lipids.
Step-by-step solution
Step 1
Bile is produced by the liver, stored in the gallbladder, and released into the duodenum (first part of small intestine).
Step 2
Bile emulsifies lipids: it breaks large lipid globules into many smaller droplets. This is a physical process, not a chemical one.
Step 3
Emulsification greatly increases the surface area of lipid exposed to lipase, allowing the enzyme to break down lipids much faster.
Step 4
Bile is also alkaline: it neutralises the acidic chyme from the stomach, creating a neutral/slightly alkaline pH in the duodenum — the optimum pH for pancreatic enzymes including lipase.
Answer
Bile emulsifies lipids (breaks large globules into small droplets), increasing surface area for lipase to act on. Bile is NOT an enzyme. It also neutralises stomach acid, providing the optimum pH for lipase and other intestinal enzymes.
Examiner tip
Examiners frequently penalise candidates who describe bile as an enzyme. Always state it emulsifies lipids and explain the surface area benefit.
Question
State where amylase is produced and describe its action.
Step-by-step solution
Step 1
Amylase is produced by the salivary glands (in the mouth) and the pancreas (released into the duodenum via the pancreatic duct).
Step 2
Amylase catalyses the hydrolysis of starch into maltose (a disaccharide). Maltose is subsequently broken down to glucose by maltase.
Answer
Amylase is produced by the salivary glands and pancreas. It breaks down starch into maltose by hydrolysis.
Question
A student eats a meal containing protein. Describe the complete digestion of the protein from mouth to small intestine.
Step-by-step solution
Step 1
Mouth: No proteases are present in saliva. Physical breakdown by teeth increases surface area.
Step 2
Stomach: Pepsin (a protease) is secreted by the stomach wall. It works at pH 2 (provided by hydrochloric acid). Pepsin begins breaking proteins into shorter polypeptides. Stomach churning aids physical digestion.
Step 3
Small intestine (duodenum and ileum): Pancreatic proteases (e.g. trypsin) further hydrolyse polypeptides. Peptidases on the surface of epithelial cells complete digestion to amino acids. Amino acids are absorbed into blood capillaries in the villi.
Answer
Mouth: physical breakdown only (no protease). Stomach: pepsin breaks proteins to polypeptides at pH 2. Small intestine: pancreatic proteases complete digestion to amino acids, which are absorbed into blood capillaries via villi.
Question
Explain why chewing food aids chemical digestion.
Step-by-step solution
Step 1
Chewing (mastication) physically breaks food into smaller pieces, greatly increasing the total surface area of food.
Step 2
A larger surface area means more food molecules are exposed and accessible to digestive enzymes.
Step 3
Enzyme molecules collide with substrate more frequently → rate of chemical digestion increases.
Answer
Chewing physically breaks food into smaller pieces, increasing surface area. More substrate is exposed to digestive enzymes, increasing the rate of chemical digestion.
Question
A person has their gallbladder removed. Suggest how this might affect the digestion of lipids.
Step-by-step solution
Step 1
The gallbladder stores and concentrates bile produced by the liver. After removal, bile is still produced by the liver but drips continuously into the duodenum rather than being released in concentrated bursts after a meal.
Step 2
Less concentrated bile reaches the duodenum when a fatty meal is consumed → less effective emulsification of lipids.
Step 3
Reduced emulsification → smaller surface area for lipase → slower lipid digestion. The person may experience difficulty digesting fatty meals and possible fatty stools (steatorrhoea).
Answer
Without the gallbladder, bile is still produced by the liver but is no longer stored and concentrated; less bile reaches the duodenum after a fatty meal. Emulsification of lipids is less effective, reducing surface area for lipase and slowing lipid digestion.
Definitions to memorise and the exact keywords mark schemes credit for digestion answers — sharpened from recent examiner reports for the 2026 0654 sitting.
The breaking of food into smaller pieces by physical means (e.g. chewing by teeth, churning in the stomach) to increase surface area for enzyme action. No chemical bonds are broken.
The breakdown of large insoluble food molecules into small soluble ones by enzymes through hydrolysis reactions.
A carbohydrase enzyme produced in the salivary glands and pancreas that catalyses the hydrolysis of starch into maltose.
An alkaline fluid produced by the liver and stored in the gallbladder; released into the duodenum. It emulsifies lipids (physical process) and neutralises stomach acid. Bile is NOT an enzyme.
The physical breakdown of large lipid globules into smaller droplets by bile, increasing the surface area exposed to lipase.
The breakdown of a large molecule into smaller ones by the addition of water. All digestive enzyme reactions are hydrolysis reactions.
The traps other students keep falling into on digestion questions — taken from recent Cambridge IGCSE 0654 examiner reports and mark schemes — and how to avoid them.
0654 Examiner Report 2022
Why it happens
Bile breaks down fat, so students classify it as an enzyme.
How to avoid it
Bile is NOT an enzyme. It emulsifies (physically breaks up) fat droplets. The enzyme that chemically digests fats is lipase.
Why it happens
Students assume all enzymes have an optimum pH of 7.
How to avoid it
Pepsin is a stomach enzyme adapted to work at pH 2 (acidic conditions provided by HCl). At neutral pH, pepsin is relatively inactive.
Why it happens
Digestion in general begins in the mouth, so students generalise.
How to avoid it
Only starch digestion begins in the mouth (salivary amylase). Protein digestion begins in the stomach (pepsin, pH 2). No protease is present in saliva.
Why it happens
Students know the end product of carbohydrate digestion is glucose.
How to avoid it
Amylase converts starch → maltose (not glucose). Maltase then converts maltose → glucose.
The things students keep getting wrong in this sub-topic, answered.