Osmosis (Edexcel GCSE Biology: Combined Science)

• Osmosis is:

The net movement of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution) through a partially permeable membrane

• Like, diffusion, osmosis is a form of passive transport (does not require energy) but it only applies to water
• The cell membrane is partially permeable which means it allows small molecules (like water) through but not larger molecules (like solute molecules)
• Water can move in and out of cells by osmosis
• It will move down its concentration gradient

Osmosis and the partially permeable membrane

Osmosis in cells

• It can get a little confusing to talk about the 'concentration of water' when we also talk about solutions being ‘concentrated’ (having a lot of solute in them)

• Instead, we can say that a concentrated solution has a low water potential (the left-hand side of the diagram below) and a dilute solution has a high water potential (the right-hand side of the diagram below)

How osmosis works

Osmosis in animal cells

• Animal cells lose and gain water as a result of osmosis
• As animal cells do not have a supporting cell wall, the results of osmosis can be severe
• If an animal cell is placed into a strong sugar solution (with a lower water potential than the cell), it will lose water by osmosis and become crenated (shrivelled up)
• If an animal cell is placed into distilled water (with a higher water potential than the cell), it will gain water by osmosis as it has no cell wall to create turgor pressure
• It will continue to gain water until the cell membrane is stretched too far and it bursts

Effect of osmosis on animal cells

Osmosis in plant cells

• Plant cells lose or gain water as a result of osmosis
  • Water entering the cell by osmosis makes the cell rigid and firm
  • This is important for plants as the effect of all the cells in a plant being firm is to provide support and strength for the plant - making the plant stand upright with its leaves held out to catch sunlight
  • If plants do not receive enough water the cells cannot remain rigid and firm (turgid) and the plant wilts

• As plant cells have a supporting cell wall, they are protected from cell lysis
• If a plant cell is placed into a strong sugar solution (with a lower water potential than the cell), it will lose water by osmosis
  • The vacuole gets smaller and the cell membrane shrivels away from the cell wall
  • It becomes flaccid or plasmolysed (shrivelled up)

• If a plant cell is placed into distilled water (with a higher water potential than the cell), it will gain water by osmosis
  • The vacuole gets bigger, pushing the cell membrane against the cell wall
  • The plant cell is described as being turgid or as containing a high turgor pressure (the pressure of the cytoplasm pushing against the cell wall)

The effect of osmosis on plant cells

• 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

Plasmolysed red onion cells

• 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

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