Plan diffusion and osmosis investigations
Plant tissue, agar and dialysis tubing can model transport questions when their limits and controls are made explicit.
AS Level · Cambridge International Biology 9700, 2028–2030. These lessons explain membrane transport and investigation reasoning. Written practice is self-checked; it does not demonstrate supervised practical competence or replace teacher-reviewed assessment.
Before you begin
- Relate phospholipid and protein structure to their roles, and distinguish a concentration gradient from the direction of net movement.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What is dialysis tubing in a transport investigation?
What you will learn
- Describe investigations of simple diffusion and osmosis using plant tissue and non-living materials
- Use dialysis tubing and agar appropriately as models and identify limitations
Choose a model for the question
Plant tissue can be placed in solutions to investigate osmosis by measuring change in mass or length. Dialysis (Visking) tubing is a non-living selectively permeable model that can show movement of water or small solutes across a membrane-like barrier.
Agar can contain an indicator or solute to show diffusion through a non-living block. These models support investigation of transport principles but are not living cells and do not reproduce all membrane proteins or cellular controls.
Control the method and describe limits
Use equal sample sizes, solution volumes and exposure times; control temperature and concentration. For plant tissue, use similar cylinders, remove surface liquid consistently and repeat measurements. For tubing, check for leaks and use tied ends consistently.
State what the observation demonstrates. A colour boundary or mass change is evidence in that model under those conditions; it does not establish every detail of transport in living tissue. Follow supervised laboratory safety and disposal procedures.
Pause and explain
Why should similar potato cylinders be blotted consistently before weighing?
Worked example
A student compares osmosis in potato cylinders but uses different exposure times and blots one cylinder while the other remains wet. Explain two flaws.
Show the worked solution
- Time differs, so the samples are not comparable.
- Unequal surface water changes measured mass.
Answer Exposure time should be the same because it affects how much water moves. Both cylinders should be blotted in the same consistent way before weighing, otherwise external solution adds to the recorded mass. Use similar tissue dimensions and repeat trials.
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.
Describe one investigation using Visking tubing to study diffusion or osmosis.
Give me a hint
Specify what crosses, what is measured and a control.
Compare my reasoning
- Fill or surround tubing with defined solutions and check for leaks.
- Keep time, temperature and volumes controlled; measure a stated change.
Use tied, leak-free Visking tubing with a defined solution inside and outside. Allow a set time at controlled temperature, then measure a change such as tubing mass or a solute test in the surrounding solution. Include a control and repeat trials. The tubing is a non-living model.
Look for these in your work
- Model and setup described
- Measurement and control included
Agar blocks of different sizes are placed in the same diffusing solution for the same time. State one useful measurement and one model limitation.
Give me a hint
Measure the extent of penetration and explain what agar lacks.
Compare my reasoning
- Measure the depth or fraction of block reached by diffusion.
- Agar does not have living membranes or active transport.
Measure the depth of penetration or the proportion of each block that changes colour. Agar is non-living and does not reproduce a cell membrane’s selective proteins or active transport.
Look for these in your work
- Comparable measurement stated
- Limitation stated
Two plant-tissue samples have the same starting mass but very different lengths and diameters. Explain why the comparison may be unfair.
Give me a hint
Relate geometry to surface area and water exchange.
Compare my reasoning
- Different surface area-to-volume ratios alter exchange.
- Use cylinders with matched dimensions and surface preparation.
Different dimensions produce different surface-area-to-volume ratios and can alter water exchange. Use similar tissue cylinders with the same starting dimensions and blot them consistently.
Look for these in your work
- Geometry linked to exchange
- Method improvement given
Common mistakes
- Calling Visking tubing a living plasma membrane
- Comparing samples with different dimensions or exposure times
- Treating a single mass change as proof of a full cell mechanism
What can you explain now?
What is dialysis tubing in a transport investigation?
Compare with the explanation
A non-living selectively permeable model
It can model some membrane transport properties.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
What can plant tissue, Visking tubing and agar model, and what do these models not establish?
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.