Skip to content
Dabour HubLEARN · PRACTICE · EXPLORESign inStudent

3.1.2Exchange and Transport

Agar Blocks Practical — AQA A Level Biology 7402 Revision Notes & Topical Questions

5 min read 9 parts Lesson 5 of 48 1 quiz

📚 AQA A Level Biology 7402 Hub / Topical Revision
🩺
100% Curriculum Specification Match Lesson Doctor Verified
The only platform with 100% curriculum specification match and verified interactive question banks.
0%read
Lesson summary

3.1.2 Agar Blocks Practical — AQA A Level Biology 7402 notes: Describe and evaluate the use of agar blocks containing an indicator to model the effect of size on diffusion, and process the data.The investigation uses blocks of agar jelly as a model of cells of different sizes to …

  • phenolphthalein indicator
  • hydrochloric acid
  • time taken to decolourise
  • SA:V ratio
  • rate of diffusion
  • time to reach the whole block

Part 1 of 7Aim and principle

The investigation uses blocks of agar jelly as a model of cells of different sizes to show how the surface area to volume ratio affects diffusion.

1Phenolphthalein indicator
The agar is made with phenolphthalein indicator and a small amount of dilute sodium hydroxide,
  • which makes it pink (purple-red) in alkaline conditions.
2Hydrochloric acid
When the blocks are placed in dilute hydrochloric acid, the acid diffuses in and neutralises the alkali so the indicator turns colourless.
3
The time taken for the pink colour to disappear shows how fast the acid reaches the centre; this is a model for how quickly substances reach the centre of a cell.
4
Alternatively, the depth of diffusion after a fixed time can be measured.

Part 2 of 7Method

1
Prepare agar containing phenolphthalein and sodium hydroxide, pour it into a tray and allow it to set.
2
Use a scalpel and ruler to cut cubes of different sizes, such as 1 cm, 2 cm and 3 cm sides, using a white tile. Cut cleanly with straight edges.
3
Measure the dimensions of each block and calculate the surface area, volume and SA:V ratio.
4
Place each cube into a beaker containing the same volume of dilute hydrochloric acid (e.g. 0.1 mol dm-3) at the same temperature. Start a stopwatch.
5
Time how long it takes for the pink colour to disappear completely. For larger blocks, it may be necessary to cut the block open to see the centre.
6
Repeat the experiment at least three times for each size, and calculate a mean time.

Safety

  • wear eye protection, since even dilute acid and alkali are irritants;
    • take care with scalpels.

Part 3 of 7Results and analysis

Calculate the SA:V ratio for each block, and plot a graph of time taken to decolourise against SA:V ratio (or against block side).

The smaller blocks, with the larger SA:V ratio, decolourise more quickly.
The rate of diffusion can be found as 1/time or as distance diffused / time.
Rate of diffusion
Note that the rate of diffusion (distance per unit time) into the block is the same for all sizes as it depends on the concentration gradient and temperature; it is the time to reach the whole block that differs.
A common analysis is the percentage of the cube that has changed colour:

Percentage = (volume that has become colourless / total volume) x 100.

For a cube of side 2 cm in which a 0.5 cm layer has been affected on each side, the unaffected core is 1 cm (2 - 2 x 0.5), with volume 1 cm3, so the colourless volume is 8 - 1 = 7 cm3, which is 87.5 percent of the whole.

Block side (cm)SA:VMean time (s)
16 : 190
23 : 1300
32 : 1620

Example data like this show that as the SA:V ratio decreases, the time increases.

Part 4 of 7Evaluation and limitations

1
Cutting inaccuracies change dimensions; use a sharp blade and measure each side, and use a template.
2
Judging the end point of the colour change is subjective; use a white background and the same observer, or use a colorimeter.
3
Blocks should be kept submerged and stirred gently so the acid concentration at the surface is constant; the volume of acid should be large relative to the block.
4
Blocks of different shapes can be used to show the effect of shape; flat or thin shapes have a larger ratio.
5
Agar is not a cell: it has no membrane, no active transport and no metabolism that uses the substance, so the model is simple.
  • Real cells use the substance as it arrives, which keeps the gradient steep.

Part 5 of 7Variables

Independent

  • size of the block (SA:V ratio). Dependent: time for colour to disappear. Controlled: concentration and volume of acid, temperature, composition of agar, shape of the block, observer and end point.
  • These are the usual items to give when asked to plan or evaluate the practical.

Part 6 of 7Improving the model

To make the model more realistic, use blocks of different shapes with the same volume, such as a cube and a thin slab, and compare times: the slab has the larger SA:V ratio and decolourises faster.

Improving the model
You could also vary the temperature or the acid concentration to show their effect on the rate of diffusion, as predicted by Fick law.
Plotting the time against the SA:V ratio gives a curve,
  • whereas plotting time against the square of the distance from surface to centre should give an approximately straight line,
  • because diffusion time increases with the square of distance.
This explains why even a modest increase in the size of an organism makes diffusion alone too slow.

Part 7 of 7Exam-style questions

  1. Describe how you would use agar blocks to investigate the effect of size on diffusion. [5]5 marks

  2. A 2 cm cube of agar was placed in acid. After 4 minutes the pink core was a cube of 1.2 cm side. Calculate the percentage of the volume that had changed colour. [3]3 marks

  3. Explain why the small cube changed colour faster than the large cube. [3]3 marks

Key terms

TermMeaning
PhenolphthaleinIndicator that is pink in alkaline conditions and colourless in acid
AgarJelly used as a model of cell material
DiffusionNet movement of particles from a higher to a lower concentration
SA:V ratioSurface area divided by volume
ModelSimplified representation used to investigate a biological idea
End pointThe stage at which the colour change is judged complete

Avoid these mistakes

1
Saying the acid diffuses faster into small blocks - the rate is the same; it reaches the centre sooner as the distance is shorter.
2
Mixing up colour changesthe agar goes from pink to colourless as acid neutralises the alkali.
3
Measuring the colour change at the surface instead of the centre.
4
Forgetting to calculate SA:V and plotting against block size without explanation.
5
Failing to control the temperature and concentration of acid.

Last updated: