(ii) practical investigations into the factors affecting diffusion rates…

The idea

Model cells let you measure diffusion without living tissue. A common method uses cubes of agar jelly containing a pH indicator such as phenolphthalein, set with dilute alkali so the jelly is pink. When acid diffuses in from the surrounding solution, the indicator turns colourless, so the distance the colourless zone has moved into the cube is a measure of diffusion. You can vary surface area to volume ratio by cutting cubes of different side length, vary the concentration gradient by changing the acid concentration, or vary temperature. For each factor, change only that variable, keep the others constant, and measure the colourless depth from the outside of the cube inwards, for example with a ruler after slicing the cube. Repeat and calculate a mean. A larger surface area to volume ratio, a steeper concentration gradient or a higher temperature each increases the rate of diffusion.

What earns the marks

• Agar cubes containing indicator (for example phenolphthalein in alkali) act as model cells; acid diffusing in changes the indicator colour, giving a measurable colourless depth.
• Surface area to volume ratio is varied by using cubes of different side length; smaller cubes have a larger surface area to volume ratio and the colourless zone penetrates proportionally further.
• Concentration gradient is varied by changing the acid concentration while keeping cube size, temperature and time constant.
• Temperature is varied as an independent variable while cube size, acid concentration and time are controlled.
• The dependent variable is the depth of the colourless zone measured from the cube surface inwards, ideally after cutting the cube open, with repeats and a mean.
• Control variables include time in the solution, volume of solution, cube dimensions and temperature (when not the variable being tested).
• Conclusions link a faster rate of diffusion to a larger surface area to volume ratio, a steeper concentration gradient or a higher temperature.

Common mistakes and how to put them right

• Measuring the total mass change of the cube instead of the depth of the colourless zone.

  Put it right: The indicator colour change gives a direct measure of how far the substance has diffused.

• Changing several variables at once, such as cube size and acid concentration together.

  Put it right: Change only the independent variable and control the rest.

• Assuming the colourless zone reaches the centre of every cube.

  Put it right: In larger cubes the centre may stay coloured within the time allowed, which itself shows the effect of surface area to volume ratio.

Exam tips

• State clearly which variable is independent, which is dependent and which are controlled before describing the method.
• Explain why the agar cube is a model rather than a living cell, and what that limits.
• Link each result to the underlying cause, such as a shorter diffusion distance or a steeper gradient, rather than just saying 'it was faster'.