Osmosis / Tonicity

• Osmosis is the net movement of water across a semi-permeable membrane from an area of lower solute concentration to an area of higher solute concentration.
• Diffusion is the movement of molecules from a region of higher concentration to a region of lower concentration.
• The cell wall provides structural support and protection against osmotic lysis.
• Active transport requires energy, while passive transport does not require energy.
• Osmolarity refers to the total concentration of solutes in a solution.
• Low osmolarity = fewer solute particles per litre of solution.
• High osmolarity = more solute particles per litre of solution.
• Adding solute molecules to a solution reduces the likelihood of water moving out of that area. The solute molecules bounce off the membrane and knock the water molecules backwards.
• The more solute water contains, the less likely it will be to move across a membrane.
• Solute molecules cannot move across a membrane.
• In osmosis, water moves from an area with low solute concentration (low osmolarity) to an area with high solute concentration (high osmolarity).
• If there is no difference in solute concentrations on either side of the cell membrane, then there is no net movement of water into or out of the cell (isotonic).
• When cells are placed in hypotonic solutions, they swell up as water rushes into them by osmosis.
• A solution with a higher osmolarity is said to be hyperosmotic to the other (hypoosmotic) solution. If two solutions have the same osmolarity they are isoosmotic.
• Cells can survive being placed in hypotonic solutions if they return to their original shape when removed from the solution.
• Plant cells do not recover from exposure to hypotonic solutions because they lack contractile vacuoles that could squeeze excess water out of the cell.
• Tonicity is the ability of an extracellular solution to make water move into or out of a cell by osmosis.
• Tonicity is different to osmolarity because it takes into account both the relative solute concentration and the cell membrane's permeability to the solutes.
• If a cell is placed in a hypertonic solution there will be a net flow of water out of the cell. The cell will therefore lose volume. This is because the solute concentration is higher in the solution than the cell.
• If a cell is placed in a hypotonic solution there will be a net flow of water into the cell. The cell will therefore gain volume. This is because the solution has a lower solute concentration than the cell.
• Isotonic means equal tonicity, where the solute concentrations are the same inside and outside the cell.
• If a cell is placed in an isotonic solution there will be no net flow of water into or out of the cell. The cell's volume will therefore remain stable. This is because the solute concentration is the same in the cell and solution.
• The movement of water across a selectively permeable membrane can cause changes in the shape of cells.
• Plasmolysis occurs when a plant cell loses so much water that its cytoplasm shrinks away from the plasma membrane.
• Red blood cells favour isotonic conditions in the blood. The body has homeostatic (stability-maintaining) systems to ensure these conditions stay constant.
• An animal cell placed in a hypertonic solution would lose water by osmosis until it shrivelled up and died.
• A red blood cell placed in a hypotonic solution would take on water by osmosis until it swelled up and burst.
• Plant cells favour hypotonic extracellular solutions. The cell wall prevents the cell from bursting / osmotic lysis as the plasma membrane can only expand up to the maximum turgor pressure, preventing an excess influx of water.
• In a hypotonic environment, the cell will gain water by osmosis until it becomes turgid.
• If a plant cell were placed in a hypertonic solution, it would lose water by osmosis until it became plasmolyzed.
• Turgidity is the state where the cell is full of water but not yet at risk of rupturing due to the presence of the cell wall.