Osmosis

• Osmosis is the net movement of water molecules across a partially permeable membrane from a region of high to low water concentration
• A partially permeable membrane is one that allows some small substances such as water to pass through it but not others such as glucose
• • A solution with higher water concentration is a dilute solution
  • A solution with lower water concentration is a concentrated solution
• Osmosis only considers the movement of water molecules, so even if there is a higher concentration of overall particles on one side, if the other side has more water molecules it has a higher water concentration
• • Water potential is the likelihood that water will diffuse into or out of a cell.
  • If it has a low water potential, then it has a low concentration of water molecules
• • Watering a plant increases the water potential around it. This mean the plant cells draw water by osmosis until they become turgid
  • If there is no water in soil, the plant starts to wilt. This is because the cells become flaccid
• When a plant becomes turgid, the contents of the cell push against the cell wall - this is called turgor pressure. This helps support the plants tissues
• If water moves into a cell, its mass will increase. It will lose mass if water leaves the cell
• Investigating the effect of a range of concentrations of sugar solutions on the mass of a plant

  1. Place six boiling tubes in a rack. Label them with different salt concentrations
  2. To boiling tubes add 20ml of appropriate solution
  3. Using balance, record mass of each potato cylinder, keeping track of which cylinder is which
  4. Place each potato cylinder in a boiling tube
  5. After 30 minutes, dry the potato and reweigh them
• If the same amount of particles are on both sides, there is no net movement
• Crenated cell (hypertonic concentration)


  When a cell is put in a solution with higher concentration than the cells cytoplasm, water in the cytoplasm will move out and and cell will shrink
• Lysed cell


  When a cell is put into a solution with a higher concentration than the cells cytoplasm, water molecules will rush into the cytoplasm, causing it to swell and burst
• Only animal cells can be crenated and lysed
• Hypertonic - high concentration

  Hypotonic - lower conentration

  Isotonic - same concentration
• Turgid (hypertonic) 


  Plant cells which have high water concentration due to absorbed water in the cytoplasm, so the cell membrane pushes against the cell wall, making the cell firm and swollen
• Flaccid (isotonic)


  Flaccid plant cells have lost water, resulting in low water concentration in the cytoplasm and reduced pressure against the cell wall. This causes the cell to become limp and wilted
• Plasmolyzed (hypotonic) 


  Cells which have lost so much water that the cell membrane pulls away from the cell, potentially damaging the cell