Osmosis in Animals & Plants (Cambridge O Level Biology)

• When water moves into a plant cell, the vacuole gets bigger, pushing the cell membrane against the cell wall
• 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
• The pressure created by the cell wall stops too much water entering and prevents the cell from bursting
• If plants do not receive enough water the cells cannot remain rigid and firm (turgid) and the plant wilts

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

Osmosis and Water Potential Diagram

How osmosis works

Plant cells in solutions of different concentrations

• When plant cells are placed in a solution that has a higher water potential (dilute solution) than inside the cells (e.g. distilled water) then water moves into the plant cells via osmosis
• These water molecules push the cell membrane against the cell wall, increasing the turgor pressure in the cells which makes them turgid 

Turgid Plant Cell Diagram

A turgid plant cell

• When plant cells are placed in a concentrated solution (with a lower water potential than inside the cells) water molecules will move out of the plant cells by osmosis, making them flaccid 
  • If plant cells become flaccid it can negatively affect the plant's ability to support itself
• If looked at underneath the microscope, the plant cells might be plasmolysed, meaning the cell membrane has pulled away from the cell wall

Plasmolysed Plant Cell Diagram

A plasmolysed plant cell

Animal cells in solutions of different concentrations

• Animal cells also lose and gain water as a result of osmosis
• As animal cells do not have a supporting cell wall, the results on the cell are more severe
• If an animal cell is placed into a concentrated 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 and, as it has no cell wall to create turgor pressure, will continue to do so until the cell membrane is stretched too far and it bursts

Water Potential and Animal Cells Diagram

Effect of osmosis on animal cells (hypertonic solution is a concentrated solution, hypotonic solution is a dilute solution, isotonic solution has the same solute concentration and water concentration compared to the cells within it)