Distinguish physical from chemical digestion
Explain subdivision without changing food molecules.
Section B7.3 · Physical digestion
Mapped to the named 0654 section using teaching from the corresponding single-science course. A 0654-specific review of every Core and Supplement objective is still in progress. Written practice is self-checked; it does not demonstrate supervised laboratory competence or award an official Cambridge grade.
Before you begin
- Recall large and small nutrient molecules, cell membranes and enzyme specificity.
- Use the stated diagram keys and evidence; models are not to scale.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What does physical digestion change directly?
What you will learn
- Explain subdivision without changing food molecules.
- Match a practical-model claim to the evidence recorded.
Change piece size
Physical digestion breaks food into smaller pieces without chemical change to the food molecules. Teeth cutting and grinding and stomach churning can make pieces smaller and mix them with digestive juices.
A smaller starch-containing piece still contains starch before enzyme action. Physical digestion does not itself turn every large molecule into a small soluble product.
Compare a second mechanism
Chemical digestion changes nutrient molecules through enzyme-catalysed breakdown. Physical subdivision can support that chemical process by exposing more surface, but the two mechanisms are not identical.
A model bag that squeezes cracker pieces can demonstrate mixing and smaller pieces. It does not show molecular products unless separate suitable evidence of chemical change is provided.
Sources for this lesson
Pause and explain
Which event is physical digestion?
Worked example
A cracker is crushed into crumbs, while a separate enzyme treatment produces reducing sugars from starch. Classify each.
Show the worked solution
- Crushing changes piece size.
- The enzyme treatment changes nutrient molecules.
- Name physical and chemical digestion separately.
Answer Crushing is physical digestion; starch breakdown to sugars is chemical digestion.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
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.
Define physical digestion.
Give me a hint
Include the limit on chemical change.
Compare my reasoning
- Smaller food pieces.
- No chemical change to the food molecules.
Breakdown of food into smaller pieces without chemical change to its molecules.
Look for these in your work
- Smaller pieces.
- No molecular chemical change.
Does a starch-containing crumb become glucose just because it is smaller? Explain.
Give me a hint
Physical subdivision is not enzyme action.
Compare my reasoning
- Starch remains starch before chemical digestion.
- Amylase is needed for its breakdown pathway.
No; physical size reduction alone does not turn starch molecules into glucose.
Look for these in your work
- Preserve molecule identity.
- Separate enzyme action.
A practical model only records smaller food pieces after squeezing. What claim is supported, and what needs further evidence?
Give me a hint
Match claim to observation.
Compare my reasoning
- Supported: physical subdivision/mixing.
- Unproven: chemical products from digestion.
The model supports physical subdivision; chemical digestion needs evidence of molecular breakdown.
Look for these in your work
- Supported observation.
- Clear evidence limit.
Common mistakes
- Piece size is not molecular identity.
- A squeezed bag does not by itself prove enzyme activity.
What can you explain now?
Which event is physical digestion?
Compare with the explanation
Grinding food between teeth.
This makes smaller pieces.
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Make it stick
Explain why smaller food pieces can contain unchanged large molecules.
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.