Practical: Investigating the Rate of Diffusion (OCR A Level Biology)

• It is possible to investigate the effect of certain factors on the rate of diffusion
• Different apparatus can be used to do this, such as Visking tubing and cubes of agar

Practical 1: Investigating the rate of diffusion using visking tubing

• Visking tubing (sometimes referred to as dialysis tubing) is a non-living partially permeable membrane made from cellulose
• Pores in this membrane are small enough to prevent the passage of large molecules (such as starch and sucrose) but allow smaller molecules (such as glucose) to pass through by diffusion
• This can be demonstrated by:
  • Filling a section of Visking tubing with a mixture of starch and glucose solutions
  • Suspending the tubing in a boiling tube of water for a set period of time
  • Testing the water outside of the visking tubing at regular intervals for the presence of starch and glucose to monitor whether the diffusion of either substance out of the tubing has occurred
  • The results should indicate that glucose, but not starch, diffuses out of the tubing

An example of how to set up an experiment to investigate diffusion

• The effect of concentration gradient on the rate of diffusion can be investigated more quantitatively by:
  • Estimating the concentration of glucose that has diffused into the water surrounding the Visking tubing at each time interval (separate boiling tubes are set up for each time interval) using the semi-quantitative Benedict’s test
    • Comparisons of the glucose concentration between the time intervals can be made using a set of colour standards (produced by known glucose concentrations) or a colorimeter to give a more quantitative set of results
    • A graph could be drawn showing how the rate of diffusion changes with the concentration gradient between the inside and outside of the tubing

Practical 2: Investigating the rate of diffusion using agar

• The effect of surface area to volume ratio on the rate of diffusion can be investigated by timing the diffusion of ions through different sized cubes of agar
• Coloured agar is cut into cubes of the required dimensions (eg. 0.5cm x 0.5cm x 0.5cm, 1cm x 1cm x 1cm and 2cm x 2cm x 2cm)
  • Purple agar can be created if it is made up with very dilute sodium hydroxide solution and Universal Indicator
  • Alternatively, the agar can be made up with Universal Indicator only

• The cubes are then placed into boiling tubes containing a diffusion solution (such as dilute hydrochloric acid)
  • The acid should have a higher molarity than the sodium hydroxide so that its diffusion can be monitored by a change in colour of the indicator in the agar blocks

• Measurements can be taken of either:
  • The time taken for the acid to completely change the colour of the indicator in the agar blocks
  • The distance travelled into the block by the acid (shown by the change in colour of the indicator) in a given time period (eg. 5 minutes)

• These times can be converted to rates (1 ÷ time taken)
• A graph could be drawn showing how the rate of diffusion (rate of colour change) changes with the surface area to volume ratio of the agar cubes

An example of how to set up an experiment to investigate the effect of changing surface area to volume ratio on the rate of diffusion