Unit 3 :- Chapter 2

Osmosis

• All cells are surrounded by a cell membrane which is partially permeable

• Water can move in and out of cells by osmosis

• 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

• In doing this, water is moving down its concentration gradient

• The cell membrane is partially permeable which means it allows small molecules (like water) through but not larger molecules (like solute molecules)

Osmosis in Animals & Plants: Extended

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 

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

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 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 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

Effect of osmosis on animal cells

Osmosis: Extended

• 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)

How osmosis works