Osmosis

• Osmosis is the movement of WATER from an area of higher water potential (lots of free water, little solute) to an area of lower water potential, down a water potential gradient across the phospholipid bilayer OR via protein channels (aquaporins). 
• Water potential is the 'measure of the ability of water molecules to move freely in solution'.
• A water potential gradient is required (water moves from an area of higher to an area of lower water potential).
• Osmosis can occur through special channel proteins or by diffusion directly through the bilayer (a partially permeable membrane) because water is small and polar.

  The special channel proteins are called aquaporins.
• • Water potential is measured in kilopascals (KPa).
  • Pure water has a water potential of 0KPa.
  • Adding solutes lowers the water potential, making it negative.
• Only water can diffuse by osmosis.
• Hypertonic solutions – have a lower water potential, for example a lower concentration of free water molecules than in the cell cytoplasm. 
• Isotonic solutions – have the same water potential as the cell so there is no net movement of water molecules.
• Hypotonic solutions – have a higher water potential, for example a higher concentration of free water molecules than in the cell cytoplasm. 
• In hypotonic solutions, animal cells undergo haemolysis.

  Water molecules move from a high water potential in the solution to a lower water potential in the cell down a water potential gradient.
• In hypotonic solutions, plant cells become turgid.

  Water moves from a high water potential outside the cell to a lower water potential inside the cell down a water potential gradient by osmosis.
• In hypertonic solutions, animal cells undergo crenation.

  Water moves from a higher water potential inside the cell to a lower water potential in the solution down a water potential gradent.
• In hypertonic solutions, plant cells become flaccid/ undergo plasmolysis.

  water molecules move from a higher water potential inside the cell to a lower water potential in the solution down a water potential gradient.