1.7 Osmosis

• Osmosis is the diffusion of water across a partially permeable membrane. Just like diffusion, the movement of water molecules is random and requires no energy from the cell.

  The water moves from a region of high water concentration (a dilute solution) to a region of lower water concentration (a more concentrated solution). The cell membrane is partially permeable.
• What type of membrane is the cell membrane?

  Partially permeable
• Special scientific terms are used to compare the concentrations of two solutions

  • If the two solutions have the same concentrations, they are isotonic
  • The solution that is more concentrated is hypertonic
  • The solution that is more dilute is hypotonic
• What term is used to refer to two solutions with identical concentrations?


  Isotonic
• Animal cells that are surrounded by a hypotonic solution will swell and possibly burst, because water moves into the cell by osmosis. If the solution around animal cells is hypertonic, then water moves out of the cells and they shrink. Animals need complex mechanisms to control the concentration of the solutions around their cells to avoid bursting or shrinking.
• A solution containing a higher solute concentration than another solution is said to be hypertonic. A solution containing a lower solute concentration than another solution is said to be hypotonic. Solutions with equal solute concentrations are said to be isotonic.
• When an animal cell is placed in a hypotonic solution, water enters the cell by osmosis until the cell becomes turgid. When an animal cell is placed in a hypertonic solution, water exits the cell by osmosis until the cell becomes flaccid.