Digestion, enzymes and absorption
Trace food through the mouth, stomach, duodenum and ileum, and explain enzymes, bile, absorption and the ileum’s structure.
Section 3 · Digestion, enzymes and absorption
Exam scope and assessment notes
This original StudyForA lesson maps Cambridge IGCSE Food & Nutrition 0648 topic 3 for examinations in 2026–2029. These are not Cambridge questions, mark schemes or official grades. The practical examination is a real, centre-administered cooking test with a planning session and assessed preparation, time management, results and quality. These screen-based lessons can teach the theory and help rehearse planning and evaluation, but they cannot provide a kitchen, verify safe food handling or practical skill, replace teacher-supervised cooking, award the practical mark, or guarantee an exam grade. Follow your centre’s instructions, local food-safety rules, medical advice and emergency procedures. Any meal examples are for syllabus practice, not individual dietary or medical advice.
Before you begin
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Before you start: Why must large food molecules be digested?
What you will learn
- Sequence the main digestive stages named in the syllabus
- Explain enzyme action with suitable examples
- Distinguish bile emulsification from enzyme digestion
- Relate villi and microvilli to absorption
Where digestion occurs
Digestion breaks large or insoluble food molecules into smaller soluble molecules that can be absorbed. In the mouth, chewing increases surface area and saliva begins starch digestion. In the stomach, muscular mixing and acidic gastric conditions support digestion; pepsin acts on protein. The duodenum receives bile and digestive secretions. The ileum completes digestion and absorbs soluble products.
Enzymes are biological catalysts: each works on particular substrates under suitable conditions. Amylase acts on starch; maltase, lactase and invertase act on their respective disaccharides; lipase acts on fats; pepsin and erepsin act on proteins or peptides. Rennin is named in the syllabus in relation to milk digestion, while trypsinogen is an inactive precursor that can be activated to trypsin. Do not claim enzymes all act in one organ or on every food molecule.
Bile and the ileum
Bile is made by the liver and stored in the gall bladder. It is alkaline and emulsifies fat into smaller droplets, increasing the surface area available to lipase; it is not itself a digestive enzyme. The products of digestion, such as glucose, amino acids, fatty acids and glycerol, can then be absorbed.
The ileum is long and its lining has many villi and microvilli, giving a large surface area. Each villus has a thin surface and a blood supply; a lacteal receives many fat digestion products. These features support rapid absorption and transport. Explain structure in relation to function, rather than merely naming a villus.
Sources for this lesson
Pause and explain
What is the main effect of bile on fat?
Worked example
A learner says that bile digests fat because fat droplets become smaller. Correct the explanation.
Show the worked solution
- Bile emulsifies fat rather than hydrolysing it.
- Smaller droplets create a larger surface area for lipase.
- Lipase carries out chemical digestion of fat.
Answer Bile emulsifies fat into smaller droplets but does not digest it chemically. The greater surface area allows lipase to act more effectively and break fat molecules into smaller products.
Try a topic test
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Trace how starch in a meal can be digested and absorbed, naming the relevant enzyme stages.
Give me a hint
Start in the mouth, then connect starch digestion to a soluble sugar and the absorptive small intestine.
Compare my reasoning
- State that salivary amylase begins starch digestion in the mouth.
- Explain that digestion continues in the small intestine and maltase acts on maltose.
- State that glucose is absorbed through the ileum into the blood.
Chewing mixes food with saliva, where amylase begins breaking starch into smaller carbohydrates including maltose. In the small intestine, maltase converts maltose to glucose. Soluble glucose is absorbed through the ileum’s villous lining into the blood; the process is not simply food passing unchanged into the body.
Look for these in your work
- Names amylase and maltase correctly
- Shows a sequence from starch to glucose
- Connects ileum structure to absorption
Explain why bile and lipase are both useful in fat digestion but perform different jobs.
Give me a hint
Compare a physical change in droplet size with chemical breakdown by an enzyme.
Compare my reasoning
- Describe bile’s emulsifying action.
- Explain the surface-area benefit.
- Describe lipase’s chemical action on fat.
Bile emulsifies fat into smaller droplets, increasing the surface area exposed to digestive enzyme. Lipase then catalyses the breakdown of fat molecules into fatty acids and glycerol. Emulsification is not the same as chemical digestion.
Look for these in your work
- Distinguishes physical emulsification
- Explains increased surface area
- States lipase’s chemical role
A villus drawing shows a thin lining, many folds and blood vessels. Explain how two features support absorption.
Give me a hint
Connect surface area, diffusion distance and transport to the blood.
Compare my reasoning
- Link folds and microvilli to large surface area.
- Link a thin epithelial surface to a short diffusion distance.
- Explain that blood flow carries absorbed soluble nutrients away.
The folds and microvilli provide a large surface area for absorption. A thin lining shortens the diffusion distance, while the blood supply carries many absorbed nutrients away and maintains a concentration gradient. A labelled drawing should show how each structure supports the process.
Look for these in your work
- Links surface area to absorption
- Links thin lining to diffusion
- Explains transport by blood
Common mistakes
- Calling bile an enzyme
- Saying villi make nutrients instead of absorbing them
- Mixing up the named enzymes and their substrates
What can you explain now?
What is the main effect of bile on fat?
Compare with the explanation
It emulsifies fat to increase surface area for lipase
Use the syllabus concept, connect it to the stated evidence or needs, and explain any limit on your conclusion.
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Make it stick
How is emulsification different from enzyme digestion, and why does the distinction matter?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.