Practical - Surface Area: Volume Ratio (OCR Gateway GCSE Biology: Combined Science)

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Surface Area & Volume of Cells

  • The effect of changing surface area to volume ratio on diffusion can be investigated by timing the diffusion of ions through cubes of agar of different sizes

Apparatus

  • Coloured agar
  • Ruler
  • Boiling tubes
  • Dilute hydrochloric acid
  • Stopwatch

Method

  • Coloured agar is made up and 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 surface area, volume and surface area to volume ratio of these cubes is calculated and recorded
  • The cubes are then placed into boiling tubes containing a diffusion solution (such as dilute hydrochloric acid)
  • 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 (eg. 5 minutes)

_Investigating SA with Agar Method (1), downloadable AS & A Level Biology revision notesInvestigating SA with Agar Method (2), downloadable AS & A Level Biology revision notes

The steps used to investigate the effect of changing the surface area to volume ratio on diffusion

Analysis

  • If the time taken for the acid to completely change the colour of the indicator in the agar blocks is recorded, these times can be converted to rates
  • A graph could be drawn showing how the rate of diffusion (rate of colour change) changes with the surface area: volume ratio of the agar cubes

Investigating SA with Agar Analysis (1), downloadable AS & A Level Biology revision notesInvestigating SA with Agar Analysis (2), downloadable AS & A Level Biology revision notes

To analyse the results of the investigation, calculate the rates of diffusion before drawing a graph for rate of diffusion against surface area : volume ratio

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Lára

Author: Lára

Lára graduated from Oxford University in Biological Sciences and has now been a science tutor working in the UK for several years. Lára has a particular interest in the area of infectious disease and epidemiology, and enjoys creating original educational materials that develop confidence and facilitate learning.