7.1.10 Practical - Investigating Osmosis

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

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

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

Apparatus

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

Method

• 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

Experimental method for investigating osmosis in potato cylinders

Results and analysis

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

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

Limitations

• 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