2.1.8 Osmosis

• All cells are surrounded by a cell surface membrane which is partially permeable
• Water can move in and out of cells across the cell surface membrane by a process called osmosis
  
  • Osmosis is the net movement of water molecules from a region of lower solute concentration to a region of higher solute concentration through a partially permeable membrane
    • Water molecules move from a dilute solution to a concentrated solution
• In doing this, water is moving down its concentration gradient, so osmosis is a specialised form of diffusion
  • Water molecules will move from an area of more free water molecules to an area of fewer free water molecules
    • Water molecules are considered 'free' when they are not surrounding substances in a solution; when a substance dissolves it becomes surrounded by water molecules; such water molecules are no longer free and cannot move through a membrane readily
• Cell membranes are partially permeable, allowing small molecules like water through but not larger molecules such as solutes
  • Although water molecules are polar they can still pass through the bilayer because of their small size.

Osmosis is the movement of water molecules from a dilute to a concentrated solution through a partially permeable membrane

Osmosis is the movement of water molecules down their concentration gradient. Note that 'water potential' is a term used to describe the number of free water molecules present.

• Osmosis is important because it constantly affects the cells of living organisms
  • Cell cytoplasm consists of water and dissolved substances
  • Cells lose or take on water depending on the number of free water molecules in their surroundings in comparison to their cytoplasm
• When cells are placed in pure water, which has the highest possible number of free water molecules, water moves into the cells by osmosis and the cells swell
  • In animal cells this could lead to cell bursting
  • In plant cells the cell wall prevents bursting
• When cells are placed into a solution that has a smaller number of free water molecules than their cytoplasm, e.g. a concentrated glucose solution, water moves out of the cells by osmosis and the cells shrink
  • In animal cells the entire cell shrivels
  • In plant cells the vacuole and cytoplasm shrink but the cell wall maintains the overall shape of the cell