4.2.64 · Cell Membranes and Transport

Investigating Diffusion — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions

5 min read 10 parts Lesson 13 of 20 1 quiz

📚 Cambridge AS & A Level Biology 9700 Hub / Topical Revision

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Lesson summary

4.2.6 Investigating Diffusion — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Investigate how surface area, concentration, temperature and molecular size affect diffusion using agar, dye and Visking tubing models, and evaluate the methods.

Why models
Indicators
Explanation
Depth reached
Concentration gradient
Dye in agar

Part 1 of 8Models of diffusion

Why models

Why models diffusion cannot be seen directly,
- so agar jelly is used,
- because it is mostly water and dissolved substances diffuse through it like cytoplasm.

Indicators

Indicators they show how far a substance has travelled.

Set-up

Set-up the substance diffuses from a source into a block that mimics a cell.
- (the surrounding liquid)

Links

Links the work illustrates Fick's law and the surface area to volume ratio, and shows why cells are small.

Part 2 of 8Agar blocks containing indicator

Make agar jelly containing phenolphthalein and a little sodium hydroxide so that it is pink, pour it into a shallow dish and let it set.

Cut blocks of different sizes, for example 1, 2 and 3 cm cubes, using a scalpel and ruler on a tile.

Place the blocks in equal volumes of dilute hydrochloric acid (for example 0.1 mol dm^-3) at a constant temperature.

Time how long each block takes to become completely colourless as acid reaches the centre.

Alternatively, remove blocks at a fixed time, cut them in half and measure the depth of colour change with a ruler.

Results

Results smaller blocks with a larger surface area to volume ratio decolourise faster.

Explanation

Explanation the diffusion distance is shorter and the surface area is large compared with the volume to be reached.

Depth reached

Depth reached acid diffuses to about the same depth in a given time in any block, but it reaches a larger fraction of a small block.

Safety

Safety dilute acid and alkali irritate skin and eyes, so wear eye protection and cut away from the hand.

Part 3 of 8Other variables to investigate

Temperature

Temperature repeat with acid at 20, 30, 40 and 50 °C in water baths, where higher temperature gives more kinetic energy and faster diffusion.

Concentration gradient

Concentration gradient use different acid concentrations, where a steeper gradient gives a faster rate.

Dye in agar

Dye in agar place a drop of potassium permanganate or a dye crystal on agar in a petri dish and measure the diameter of the coloured circle at timed intervals.
Dyes with a larger molecular mass spread more slowly.

Rate

Rate calculate it as 1 ÷ time, or as distance diffused ÷ time.

Part 4 of 8Visking tubing

Visking tubing

Fill a section of Visking (dialysis) tubing with starch and glucose solutions and place it in a tube of water.

After a set time, test the water outside with iodine solution for starch and Benedict's reagent for glucose.

Repeat at intervals to see whether either substance has diffused out.

Result

Result glucose passes through the pores but starch does not.

Conclusion

Conclusion a non-living partially permeable membrane can select by molecular size.

Part 5 of 8Calculating and presenting results

1Recording

Recording record each time to the nearest second and repeat at least three times for each block size.

2Processing

Processing calculate a mean and then the SA:V of each block from its dimensions.

3Graph

Graph plot time or rate against SA:V or temperature, and draw a line of best fit.

4Anomalies

Anomalies check for anomalous results and decide whether to repeat them.

Part 6 of 8Evaluating the investigation

Limitation	Improvement
End point judged by eye	Use the same observer, a colour standard or a colorimeter
Blocks cut unevenly	Use a sharper cutter or a template
Acid concentration falls as it is used	Use a larger volume of acid
Few repeats	Do more repeats and use a digital timer

Limit of the model

Limit of the model an agar block has no membrane with transport proteins, active transport or metabolism, which a real cell has.

Link to cells

Link to cells a 10 micrometre cell has a very high SA:V and is supplied within a fraction of a second,
but the centre of a 10 cm block would take hours to receive oxygen by diffusion alone.

Part 7 of 8More from the specification

Facilitated diffusion

Passive movement through transport proteins.

Channel protein

Hydrophilic pore for specific ions or water.

Animal cell

A cell with a nucleus, cytoplasm, cell membrane and mitochondria but no cell wall, chloroplasts or large vacuole.

Osmosis

The net movement of water molecules from a region of higher water potential to a region of lower water potential through a partially permeable membrane.

Movement

An action by an organism or part of it causing a change of position or place.

Carrier

A female with one recessive sex-linked allele who is unaffected.

Sucrose

The sugar mainly transported in phloem.

Endocytosis

Bulk transport into a cell using a membrane vesicle.

Bilayer

A double layer of phospholipids with tails facing inwards.

Water potential

A measure of the tendency of water to move; pure water has the highest.

Selectively permeable

Allowing only some substances through.

Size effect

Size effect a larger block has more volume but proportionally less surface area, so diffusion to its centre takes longer and is less effective.

Part 8 of 8More on this topic

Make the agar with very dilute sodium hydroxide and Universal Indicator (purple), or use phenolphthalein.

Key terms

Term	Meaning
Agar	A water-based jelly used as a model of cytoplasm.
Indicator	A chemical that changes colour depending on pH.
Visking tubing	Partially permeable dialysis membrane used to model cell membranes.
Surface area to volume ratio	Surface area divided by volume.
Diffusion distance	The distance particles must move to cross a surface or enter a cell.
End point	The point at which a visible change shows a reaction is complete.
Rate	Change in a quantity divided by time.

Avoid these mistakes

Saying the acid diffuses faster into small blocks — the speed is the same, but the time to reach the whole block is shorter.

Confusing diffusion distance with surface area — they are different factors.

Not controlling temperature, which directly affects diffusion rate.

Stating that Visking tubing is fully permeable — it is partially permeable by molecular size.

Last updated: 14 October 2026