Explain fibre as roughage
Connect plant-food fibre to movement of gut contents.
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
Which source often supplies fibre?
What you will learn
- Connect plant-food fibre to movement of gut contents.
- Separate egestion from metabolic-waste excretion.
Roughage is useful without the starch pathway
| Component | Examples of sources | Role |
|---|---|---|
| Fibre | Wholegrains, fruit, vegetables | Bulk and movement through the gut |
Recognise fibre sources
Wholegrain foods, vegetables, fruit and pulses can provide dietary fibre, also called roughage. Fibre includes components of plant material that human digestive enzymes do not fully digest. Its importance is not that it supplies the same usable glucose as starch.
Fibre adds bulk and helps material move through the gut, supporting normal bowel movement. Water and the rest of the diet also matter; fibre is not a universal treatment inferred from a school diagram.
Separate starch and cellulose
A plant food can contain both digestible starch and cellulose-rich fibre. Human amylase acts on starch, while humans do not produce the cellulose-digesting enzyme needed for the same pathway. Do not say all plant carbohydrates are digested identically.
Undigested material can contribute to faeces and is removed by egestion. Egestion is distinct from excretion of metabolic waste such as urea. Later lessons place these processes in their correct organs.
Sources for this lesson
Pause and explain
Why is fibre important here?
Worked example
A food contains both starch and cellulose. Explain why the two components need not follow the same digestion pathway.
Show the worked solution
- Recognise different carbohydrate structures.
- Amylase acts on starch.
- Human enzymes do not digest cellulose in the same way.
Answer Starch can supply absorbed sugars; cellulose-rich roughage contributes to gut bulk rather than the same amylase pathway.
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.
Give two fibre sources and its gut role.
Give me a hint
Use plant foods.
Compare my reasoning
- Name wholegrains and vegetables.
- State bulk and movement.
Wholegrain foods and vegetables; fibre adds bulk and supports gut movement.
Look for these in your work
- Two sources.
- Correct role.
Why does a negative starch digestion result for cellulose not make fibre useless?
Give me a hint
Function need not require glucose production.
Compare my reasoning
- Cellulose is not the same amylase substrate.
- Roughage supports bulk and movement.
Its roughage role does not depend on being digested by amylase into absorbed sugars.
Look for these in your work
- Enzyme distinction.
- Functional importance.
A student calls removal of undigested fibre “excretion of urea”. Correct both the material and process.
Give me a hint
Distinguish undigested food from metabolic waste.
Compare my reasoning
- The material is undigested food matter.
- Its removal in faeces is egestion.
Undigested fibre can leave in faeces by egestion; urea excretion is a different process.
Look for these in your work
- Correct material.
- Correct process.
Common mistakes
- Fibre is not useless because it is not fully digested.
- All carbohydrates do not have identical enzyme pathways.
What can you explain now?
Why is fibre important here?
Compare with the explanation
It adds bulk and supports movement through the gut.
This is the named roughage role.
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Make it stick
Explain why a food can supply both digestible carbohydrate and useful roughage.
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.