Syllabus Edition

First teaching 2023

First exams 2025

|

Investigating Diffusion (CIE AS Biology)

Revision Note

Test yourself
Phil

Author

Phil

Last updated

Investigating Diffusion

Investigating Diffusion using Visking tubing

  • Visking tubing (sometimes referred to as dialysis tubing) is a non-living, selectively 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 time
    • Testing the water outside of the visking tubing at regular intervals for the presence of starch and glucose to monitor whether diffusion of either substance out of the tubing has occurred
    • The results should indicate that glucose, but not starch, diffuses out of the tubing

Visking Tubing Experimental Setup Diagram

_Investigating diffusion using visking tubing

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

  • This 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 between the time intervals can be made with a set of colour standards (known glucose concentrations) or a colorimeter to give a more quantitative set of results
      • A graph could be plotted showing how the rate of diffusion changes with the concentration gradient between the inside and outside of the tubing

Using agar to investigate the effect of changing surface area to volume ratio on diffusion

  • The effect of size (surface area to volume ratio) on diffusion can be investigated by timing the diffusion of ions through different-sized cubes of agar
  • Agar, coloured with an indicator, is cut into cubes of the required dimensions (eg. 0.5cm × 0.5cm × 0.5cm, 1cm × 1cm × 1cm and 2cm × 2cm × 2cm)
    • Purple agar can be created if it is made up of very dilute sodium hydroxide solution and Universal Indicator
    • Alternatively, the agar can be made up with Universal Indicator only
    • Another method is to use sodium hydroxide and phenolphthalein to colour agar pink (this will turn colourless in the presence of acid)
  • The cubes are then placed into boiling tubes containing a diffusion solution (such as dilute hydrochloric acid)
    • The acid should have a higher concentration than the sodium hydroxide so that a change in the colour of the indicator in the agar blocks can be used to monitor diffusion
  • 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 plotted showing how the rate of diffusion (rate of colour change) changes with the surface area : volume ratio of the agar cubes

Using Coloured Agar Cubes to Demonstrate Diffusion Diagram

Investigating diffusion using agar

An example of how to set up an experiment to investigate the effect of changing surface area to volume ratio on diffusion, here phenolphthalein has been used as the indicator

Examiner Tip

When an agar cube (or for example a biological cell or organism) increases in size, the volume increases faster than the surface area, because the volume is cubed whereas the surface area is squared. When an agar cube (or biological cell / organism) has more volume but proportionately less surface area, diffusion takes longer and is less effective. In more precise scientific terms, the greater the surface area to volume ratio, the faster the rate of diffusion!

You've read 0 of your 10 free revision notes

Unlock more, it's free!

Join the 100,000+ Students that ❤️ Save My Exams

the (exam) results speak for themselves:

Did this page help you?

Phil

Author: Phil

Expertise: Biology

Phil has a BSc in Biochemistry from the University of Birmingham, followed by an MBA from Manchester Business School. He has 15 years of teaching and tutoring experience, teaching Biology in schools before becoming director of a growing tuition agency. He has also examined Biology for one of the leading UK exam boards. Phil has a particular passion for empowering students to overcome their fear of numbers in a scientific context.