Practical - Investigating Osmosis (OCR Gateway GCSE Biology: Combined Science)

Revision Note

Test Yourself




Biology Lead

Osmosis & Microscopy

Observation of osmosis in plant cells using a light microscope

  • Evidence of osmosis occurring in plant cells can be shown when plant cells undergo plasmolysis:
    • If a plant cell is placed in a solution with a lower water potential than the plant cell (such as a concentrated sucrose solution), water will leave the plant cell through it partially permeable cell surface membrane by osmosis
    • As water leaves the vacuole of the plant cell, the volume of the plant cell decreases
    • The protoplast (living part of the cell inside the cell wall) gradually shrinks and no longer exerts pressure on the cell wall
    • As the protoplast continues to shrink, it begins to pull away from the cell wall
    • This process is known as plasmolysis – the plant cell is plasmolysed
  • This process can be observed using epidermal strips (sections of the very thin outer layer of tissue in plants)
  • Plants with coloured sap (such as red onion bulbs, rhubarb petioles and red cabbage) make observations easier
  • The epidermal strips are placed in a range of molarities of sucrose solution or sodium chloride solutions
  • The strips are then viewed under a light microscope
  • Plasmolysis may take several minutes to occur

Plasmolysis of red onion cells, downloadable AS & A Level Biology revision notes

Light micrograph of normal red onion cells alongside those that have plasmolysed (artistic impression)

Osmosis & its Effects

  • We can investigate osmosis by using cylinders of potato and placing them into distilled water and sucrose solutions of increasing concentration


  • Potatoes
  • Cork borer
  • Knife
  • Sucrose solutions (from 0 Mol/dm3 to 1 mol/dm3)
  • Test tubes
  • Balance
  • Paper towels
  • Ruler
  • Test tube rack


  • Prepare a range of sucrose (sugar) solutions ranging from 0 mol dm⁻³ (distilled water) to 1 mol dm⁻³
  • Set up 6 labelled test tubes with 10cm³ of each of the sucrose solutions
  • Using the knife, cork borer and ruler, cut 6 equally-sized cylinders of potato
  • Blot each one with a paper towel and weigh on the balance
  • Put 1 piece into each concentration of sucrose solution
  • After 4 hours, remove them, blot with paper towels and reweigh them

Osmosis Method_1, downloadable IGCSE & GCSE Biology revision notes Osmosis Method_2, downloadable IGCSE & GCSE Biology revision notes Osmosis Method_3, downloadable IGCSE & GCSE Biology revision notes

Experimental method for investigating osmosis in potato cylinders

Results and analysis

  • The percentage change in mass can be calculated for each piece of potato

Osmosis Analysis_1, downloadable IGCSE & GCSE Biology revision notes

Calculating percentage change in mass

  • The potato cylinder in the distilled water will have increased its mass the most as there is a greater concentration gradient in this tube between the distilled water (high water potential) and the potato cells (lower water potential)
  • This means more water molecules will move into the potato cells by osmosis, pushing the cell membrane against the cell wall and so increasing the turgor pressure in the cells which makes them turgid - the potato cylinders will feel hard
  • The potato cylinder in the strongest sucrose concentration will have decreased its mass the most as there is a greater concentration gradient in this tube between the potato cells (higher water potential) and the sucrose solution (lower water potential)
  • This means more water molecules will move out of the potato cells by osmosis, making them flaccid and decreasing the mass of the cylinder - the potato cylinders will feel floppy
  • If looked at underneath the microscope, cells from this potato cylinder might be plasmolysed, meaning the cell membrane has pulled away from the cell wall
  • If there is a potato cylinder that has not increased or decreased in mass, it means there was no overall net movement of water into or out of the potato cells
  • This is because the solution that the cylinder was in was the same concentration as the solution found in the cytoplasm of the potato cells, so there was no concentration gradient


  • Slight differences in the potato cylinders may mean that the results aren't reliable or comparable. A possible solution to this limitation could be:
    • For each sucrose concentration, repeat the investigation with several potato cylinders. Making a series of repeat experiments means that any anomalous results can be identified and ignored when a mean is calculated

Exam Tip

Questions involving osmosis experiments are common and you should be able to use your knowledge of these processes to explain the results. Don’t worry if it is an experiment you haven’t done. Simply figure out where the higher concentration of water molecules is (this is the solution with the higher water potential) and explain which way the molecules move due to the differences in water potential.

You've read 0 of your 0 free revision notes

Get unlimited access

to absolutely everything:

  • Downloadable PDFs
  • Unlimited Revision Notes
  • Topic Questions
  • Past Papers
  • Model Answers
  • Videos (Maths and Science)

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

the (exam) results speak for themselves:

Did this page help you?


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.