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

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Osmosis required practical

  • 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 

Osmosis Method_1, downloadable IGCSE & GCSE Biology revision notesOsmosis Method_2, downloadable IGCSE & GCSE Biology revision notes

Osmosis Method_3, downloadable IGCSE & GCSE Biology revision notes

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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

Analysing osmosis required practical

investigating-osmosis-results

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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

Examiner Tip

This is an extremely common exam question – you should be able to calculate the percentage change in mass and length and be able to plot a graph of the results.

You may also be presented with a graph to analyse so make sure you understand what this type of graph is showing. 

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Lára

Author: Lára

Expertise: Biology Lead

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.