• 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