Required Practical: Osmosis (AQA GCSE Biology: Combined Science)

• Aim:
  • To investigate the range of concentrations of salt or sugar solutions on the mass of plant tissue
• Procedure:
  • Prepare samples of potatoes (or other plant tissue) and place in different concentrations of sugar or sodium chloride (salt) solution
  • Make measurements of mass and length of your samples before and after soaking them in solutions
  • Calculate the percentage change in mass of plant tissue
  • Plot, draw and interpret appropriate graphs
• In this practical, you should take care to prepare your samples of potato carefully and record your measurements accurately
• This practical can be carried out with either salt or sucrose solutions of at least five different concentrations
• The length of time that the potato cylinders are left will vary
  • E.g. This experiment can be carried out in a water bath at 30 °C in 30 minutes 

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Measure out the volumes of solutions and potato lengths carefully using appropriate equipment; all measurements should be recorded

• Independent variable:
  • Concentration of salt or sucrose solution in mol dm³
• Dependent variable:
  • Mass and the length of each potato cylinder before and after it has been submerged in solution should be measured
  • Percentage change in mass and length will be calculated using these measurements
• Important control variables are:
  • type and volume of solute in solution
  • temperature
  • time

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A positive percentage change in mass indicates that the potato has gained water by osmosis (net movement of water from the solution into the potato) meaning the solution is more dilute, a negative percentage change suggests the opposite

Analysing osmosis graph results

• Where results lie above 0 on the X-axis, this means the potato has increased in mass and taken water in from the surrounding solution, via osmosis
• Where results lie below 0 on the X-axis, this means the potato has decreased in mass and water has been lost from the potato, via osmosis
• Where the line crosses the X-axis, this is the concentration of sugar/salt inside the potato and no water has been lost or gained at this point