By the end of this lesson
- Investigate diffusion using indicator agar as a non-living model.
- Investigate osmosis using potato tissue as a living system.
- Process percentage data and evaluate validity, reliability and model limits.
BIG QUESTION
How can cubes of indicator agar and cylinders of potato reveal different aspects of movement across biological boundaries?
A model makes diffusion visible; living tissue adds selectively permeable membranes, water potential and biological variability.
Non-living diffusion model: indicator agar
Cut phenolphthalein agar into cubes of different side lengths while keeping composition constant. Place them in the same concentration and volume of dilute acid for a fixed time. Acid diffuses in and removes the pink colour.
Cut each cube open and measure the colourless penetration distance or calculate the percentage volume decolourised. Compare this with cube SA:V ratio. Use prepared cubes or supervised cutting, eye protection and suitably dilute acid.
What the agar model can and cannot show
The model makes diffusion distance and SA:V visible. Smaller cubes should show a greater fraction penetrated in the same time because every internal point is closer to a surface.
Agar is non-living, has no selectively permeable cell membrane, metabolism, blood supply or active transport. It models physical diffusion, not a complete cell or organism.
Living osmosis system: potato cylinders
Cut equal-diameter potato cylinders to equal length, blot consistently and measure initial mass. Place them in equal volumes of a range of sucrose concentrations for the same time and temperature. Remove, blot using the same method and measure final mass.
Calculate percentage mass change = (final − initial) ÷ initial × 100. Positive values show net water gain; negative values show net water loss. Repeat each concentration using pieces from the same potato region where practical.
Estimating tissue water potential
Plot mean percentage mass change against sucrose concentration. The x-intercept, where percentage change is zero, estimates the isotonic concentration under those conditions.
Zero mass change means no net water movement; water molecules still cross both ways. The intercept is an estimate and should not be quoted more precisely than the concentration intervals and data justify.
Improve confidence in both investigations
Keep time, temperature, external solution concentration and volume, sample geometry and blotting the same (control variables). Repeat, calculate means and display spread. Randomise positions in shared baths or containers if gradients might exist.
For potato, biological variation and damaged cut cells add variability. For agar, irregular cubes and subjective colour boundaries limit measurement. Improvements should target the named limitation.
Graph skills: SLAPUK and choosing a scalePercentage mass change goes below zero: scales with negative values.
Two practical systems, two kinds of evidence
The systems are complementary rather than interchangeable.
| Feature | Indicator agar | Potato tissue |
|---|---|---|
| System | Non-living gel. | Living multicellular tissue. |
| Process emphasised | Acid diffusion through a volume. | Osmosis across cell membranes. |
| Independent variable | Cube size/SA:V or another defined diffusion factor. | External sucrose concentration. |
| Dependent variable | Penetration distance or percentage decolourised. | Percentage mass change. |
| Major limitation | No cell membrane or metabolism. | Biological variation, cut damage and blotting inconsistency. |
Language that earns precision
- percentage change
- (final value − initial value) ÷ initial value × 100.
- isotonic
- Having equal effective water potential across a membrane, causing no net water movement.
- x-intercept
- The point where a plotted relationship crosses the horizontal axis.
- model
- A simplified representation used to investigate or explain selected features.
- biological variation
- Natural differences among living samples.
Common misconceptions
Claim: Acid moves by osmosis into agar.
Correction: The acid particles diffuse; osmosis specifically describes water through a partially permeable membrane.
Claim: A potato cylinder with zero mass change has no water movement.
Correction: Water moves both ways at equal rates, producing no net change.
Claim: A larger potato piece can be compared using mass change alone.
Correction: Percentage mass change standardises for different initial masses.
Claim: Agar proves exactly how a cell behaves.
Correction: It lacks a cell membrane, metabolism and active transport, so it is a limited diffusion model.
Match the answer to the command word
Separate the two systems and specify variables (including control variables), measurements, repeats and safety.
Keep the sign in percentage change; it shows gain or loss.
Explain how a limitation affects the result and propose a matched improvement.
Recall, apply, then justify
Recall, apply, then justify (continued)
A potato cylinder changes from 2.40 g to 2.10 g. Calculate percentage mass change.
Hint
Use final minus initial and retain the negative sign.
Suggested marking points
- Change = 2.10 − 2.40 = −0.30 g.
- −0.30 ÷ 2.40 × 100.
- Percentage change = −12.5%.
Explain why potato cylinders should be blotted consistently before both mass measurements.
Hint
Distinguish tissue water from surface liquid.
Suggested marking points
- Surface solution would add to measured mass.
- Consistent blotting makes mass change more representative of water gained or lost by tissue.
Explain why smaller agar cubes have a greater percentage decolourised after the same time.
Hint
Use SA:V and maximum distance to the centre.
Suggested marking points
- Smaller cubes have a larger SA:V ratio.
- They provide more surface relative to volume.
- They also have a shorter diffusion distance to the centre, so a greater fraction is penetrated.
State what the x-intercept of percentage mass change against sucrose concentration estimates.
Hint
At the intercept, percentage mass change is zero.
Suggested marking points
- The external isotonic sucrose concentration.
- An estimate of the concentration corresponding to the potato tissue’s water potential under the conditions.
Design a combined analysis that separates the effects of SA:V ratio and maximum diffusion distance in agar cubes.
Planning hint
Ordinary cubes change both factors together; propose geometry or measurements that help distinguish them.
What a strong design includes
- Recognise that changing cube size changes both SA:V and distance to centre.
- Measure penetration distance directly as well as percentage volume penetrated.
- Use shapes or slab thicknesses that vary one geometric feature more independently.
- Keep agar composition, acid concentration, volume, temperature and exposure time constant.
- Use replicate accurately measured samples.
- Model expected penetrated volume from measured depth and compare with observations.
- State that complete separation may require a more controlled geometry or mathematical model.