• Osmosis is the diffusion of water molecules from a dilute solution (high concentration of water) to a more concentrated solution (low concentration of water) across a partially permeable membrane

• We can investigate osmosis 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/dm³ (distilled water) to 1 mol/dm³
• 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

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 potato cylinders may mean that results aren't reliable or comparable
  • Solution: 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

   

Applying CORMS evaluation to practical work

• When working with practical investigations, remember to consider your CORMS evaluation

? CORMS evaluation

• In this investigation, your evaluation should look something like this:
  • C - We are changing the concentration of sucrose solution
  • O - The potato cylinders will all be taken from the same potato or potatoes of the same age
  • R - We will repeat the investigation several times to ensure our results are reliable
  • M1 - We will measure the change in mass of the potato cylinders
  • M2 - ...after 4 hours
  • S - We will control the volume of sucrose solution used, the dimensions of the potato cylinders and each cylinder must be blotted before it is weighed each time

   

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