Osmosis

• Define osmosis.

  The movement of water molecules across a selectively permeable membrane from a region of higher water concentration to a region of lower water concentration.
• A dilute solution contains a high concentration of water molecules, while a concentrated solution contains a low concentration of water molecules.
• Cells contain dilute solutions of ions, sugars and amino acids.
• The cell membrane is partially permeable.
• Water will move into and out of cells by osmosis.
• Isolated plant cells placed in a dilute solution or water will take in water by osmosis. Root hair cells, if the soil is wet or moist, will also take up water by osmosis.
• Leaf cells of land plants, unless it is raining or the humidity is high, will have a tendency to lose water.
• Plant cells have a strong cellulose cell wall outside the cell membrane. The cell wall is fully permeable to all molecules and supports the cell and stops it bursting when it gains water by osmosis.
• In pure water, the cell contents (the cytoplasm and vacuole) push against the cell wall and the cell becomes turgid.

  
  

  Fully turgid cells support the stems of non-woody plants.
• In a more concentrated solution, the cell contents lose water by osmosis. They shrink and pull away from the cell wall. The cell becomes flaccid. It is becoming plasmolysed.
• In a very concentrated solution, the cell undergoes full plasmolysis as the cells lose more water.
• Animal cells also take in and lose water by osmosis. They do not have a cell wall, so will change size and shape when put into solutions that are at a different concentration to the cell contents.

  
  

  For example, red blood cells could:

  • lose water and shrink
  • gain water, swell and burst in a more dilute solution
• In animals, the concentration of body fluids – blood plasma and tissue fluid – must be kept within strict limits – if cells lose or gain too much water by osmosis, they do not function efficiently.