L4 - osmosis

• Osmosis:

  -the diffusion of water molecules moving from an area of high water potential to an area of low water potential, down a concentration gradient across a partially permeable membrane

  -a passive process
• What is water potential:

  -the pressure exerted by water molecules as they collide with a membrane or container

  -measured in kilopascals (kPa), the unit of pressure

  -symbol is ψ

  -pure water has the highest water potential and a value of 0kPa

  -solutions have a lower water potential than pure water, a negative water potential
• Equilibrium: equal water concentration on each side of a partially permeable membrane, no net movement of water
• Solute potential:

  -the pressure produced by solute molecules in a solution

  -symbol is ψs
• When you add a solute, you reduce the water potential

  -the water potential becomes negative

  -known as the solute potential (this is negative)

  -the more solute is added, the more negative it gets
• Types of solution:

  -isotonic

  -hypertonic

  -hypotonic
• Isotonic: a region where there are equal water potentials on either side of a membrane

  
• Isotonic solutions have the same solute concentratio. If they are separate by a partially permeable membrane the water potential will be the same on either side. There will be no net movement
• Hypertonic: a region of lower water potential and higher solute concentration

  
• Hypertonic solutions have a lower water potential and higher solute concentration. If they are separate by a partially permeable membrane there will be a higher water potential in the cytoplasm so water will move out of the cell by osmosis. Therefore the cell will shrink (crenation)
• Hypotonic: a region of higher water potential and lower solute concentration

  
• Hypotonic solutions have a higher water potential and lower solute concentration. If they are separated by a partially permeable membrane there will be a higher water potential outside the cell so water will move into the cell by osmosis. Therefore the cell will burst (cytolsis)
• Plant and animals cells in different solution types:

  
• What water does in the body:

  -moistens tissues for mouth, eyes and nose

  -regulates body temperature

  -lubricates joints

  -protects body organs and tissues

  -helps prevent constipation

  -reduces the burden on kidneys and liver by flushing out waste products

  -helps dissolve minerals and other nutrients to make them accessible to the body

  -carries nutrients and oxygen to cells