Compare water absorption in small intestine and colon
State the main site while retaining the colon’s contribution.
Section B7.5 · Absorption
Cross-referenced to the 2028–2029 0653 section using original teaching from the related single-science courses. A 0653-specific objective-by-objective and Core/Supplement review is in progress. Written practice is self-checked and 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 region absorbs most water in the required statement?
What you will learn
- State the main site while retaining the colon’s contribution.
- Use a water balance that accounts for explicitly stated secretions.
Most occurs earlier
Most water is absorbed from the small intestine. Some water is also absorbed from the colon. Do not replace this statement with the common shortcut that all water absorption occurs in the large intestine.
Water in gut contents includes water from food/drinks and digestive secretions. A complete real-body balance needs both inputs and outputs; a simplified problem must state which additions are included.
Use a closed teaching balance
In a supplied model with no additions after entry, the decrease in water along a region is the amount absorbed there. Record volumes and use the same interval when comparing regions. Real secretions would change this simple interpretation if left unaccounted for.
The relative school statement does not promise the same exact percentage in every person or condition. A numeric example illustrates a balance without replacing the main-site requirement.
Sources for this lesson
Pause and explain
Why must a balance include secretions when they are present?
Worked example
A closed teaching model starts with 1000 mL, loses 800 mL across small intestine and 150 mL across colon. Find each percentage of the initial amount and the remainder.
Show the worked solution
- 800/1000 × 100 = 80%.
- 150/1000 × 100 = 15%.
- 1000 − 800 − 150 = 50 mL.
Answer 80% small intestine; 15% colon; 50 mL remains in this supplied model.
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.
State the required comparison of water absorption sites.
Give me a hint
Use most and some.
Compare my reasoning
- Most in small intestine.
- Some in colon.
Most water is absorbed in the small intestine, with some also absorbed in the colon.
Look for these in your work
- Both sites.
- Correct relative terms.
A closed model has 600 mL initially, absorbs 480 mL in small intestine and 90 mL in colon. Calculate the remainder.
Give me a hint
Subtract both losses.
Compare my reasoning
- 480 + 90 = 570.
- 600 − 570 = 30.
30 mL remains; the small intestine absorbed the larger amount.
Look for these in your work
- Correct subtraction.
- Correct comparison.
An actual interval has 200 mL entering a region, 60 mL secreted into it and 100 mL leaving. Calculate the absorbed amount in this supplied balance.
Give me a hint
Input plus secretion minus output.
Compare my reasoning
- Total entering/added: 260.
- 260 − 100 = 160.
160 mL absorbed; ignoring secretion would incorrectly give 100 mL.
Look for these in your work
- Account for addition.
- Correct result and units.
Common mistakes
- The colon absorbs some water, not none.
- Percentages depend on the stated denominator and model conditions.
What can you explain now?
Why must a balance include secretions when they are present?
Compare with the explanation
They add water to gut contents.
A change cannot be assigned solely to absorption without accounting for inputs.
After trying it yourself, choose your next review. This is your self-assessment.
Your review choice appears on Today. Sign in to sync it across devices.
Make it stick
Explain why the colon’s contribution does not make it the main water-absorption site.
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.