Learning focus
Develop transferable AO3 skills: plan, measure, record, process, interpret and evaluate biological investigations accurately and safely.
Agar-cube model
Indicator agar cubes can be placed in acid or alkali. The reagent diffuses inward and changes colour. After a fixed time, cut the cube and measure the unchanged centre or penetration depth.
Fair comparison
Use cubes cut from the same agar, the same solution volume and concentration, temperature and immersion time. Different cube size is the independent variable. Percentage volume reached is often a fairer dependent variable than penetration depth alone.
Biological interpretation
Small organisms or thin exchange surfaces have a greater surface-area-to-volume ratio and shorter diffusion distance. This increases exchange relative to volume. Large organisms need specialised surfaces and transport systems.
Evaluation
Cubes may be cut inaccurately, colour boundaries may be unclear and the solution may warm. Use a template or sharp blade, repeat sizes and photograph sections against a ruler. State that agar is a model and lacks metabolism or circulation.
Practical or data skill
Plan an agar-cube investigation comparing three cube sizes. Calculate surface area, volume and SA:V ratio, then propose how penetration will be quantified.
Examination tip
Keep immersion time constant and compare percentage penetrated, not just colour depth.
Review questions and suggested answers
Question 1
Why do small cubes change colour more completely?
Suggested answer
They have a larger surface-area-to-volume ratio and shorter diffusion distance.
Question 2
Name one control variable.
Suggested answer
Solution concentration, volume, temperature, agar composition or immersion time.
Question 3
Why is agar only a model?
Suggested answer
It is not living tissue and lacks metabolism and transport systems.
