Osmosis

• Osmosis is the net movement of water molecules from a region of higher water potential to a region of lower water potential through a partially-permeable membrane.
• The higher the water potential the lower the solute concentration of the solution.
• Water potential is the likelihood (potential) of water molecules to diffuse out of/into a solution.
• The symbol for water potential is the letter ψ (psi) and is a measure of pressure; its units are kilopascals (kPa).
• Pure water has the highest water potential (0kPa). Adding solutes to a solution lowers the water potential (more negative).
• A hypertonic solution is a solution with a lower water potential (ψ) than the cell.
• An isotonic solution is a solution with the same water potential (ψ) as the cell.
• A hypotonic solution is a solution with a higher water potential (ψ) than the cell.
• Red blood cells:

  • shrivel in hypertonic solutions
  • isotonic solutions are ideal (maintained by homeostatic processes)
  • burst/lyse in hypotonic solutions
• Plant cells:

  • vacuole and cytoplasm shrink, causing the cell membrane to move away from the cell wall in hypertonic solutions
  • the cell is flaccid in isotonic solutions
  • the cell swells to become turgid in hypotonic solutions (ideal for structure)
• Osmosis occurs through a partially-permeable membrane. It occurs down the concentration gradient of water molecules, from a lower solute concentration to a higher solute concentration.
• The rate of osmosis depends on the surface area available for diffusion, temperature, and pressure difference across the membrane.
• The letter ψ (psi) is used to represent water potential.
• The water potential of pure water is 0kPa.
• If the concentration of a solution is increased, what would happen to the water potential?

  It would decrease and become more negative.
• If the water potential on each side of a cell membrane is equal, what will be reached?

  Dynamic equilibrium.
• Turgor pressure is the force exerted by plant cells against their walls due to osmotic uptake of water.
• When plants are placed in distilled water with no solutes, they take up water until they reach turgidity.
• Water moves into the vacuole through aquaporins, which are channels made of proteins embedded in the tonoplast.
• Plant cells have rigid cellulose cell walls that prevent them from bursting when they swell with water.
• Aquaporins allow water molecules to pass through quickly without being affected by other substances or particles.
• Plant cells have a large central vacuole that contains sugars, amino acids, salts, pigments, and other substances dissolved in water.
• The movement of water across the plasma membrane can cause it to stretch or even break if there is too much pressure.
• Aquaporins allow water molecules to passively move from an area of low water potential (outside) to high water potential (vacuole).
• What term refers to a solution with the same water potential as the cell cytoplasm?

  Isotonic.
• If an animal cell is placed in pure water, what happens?

  Water enters the cell until the cell bursts/undergoes lysis.
• 𝙁𝘼𝘾𝙏𝙊𝙍𝙎 𝘼𝙁𝙁𝙀𝘾𝙏𝙄𝙉𝙂 𝙊𝙎𝙈𝙊𝙎𝙄𝙎:

  • the available surface area of the exchange surface
  • the number of available aquaporins
  • the diffusion distance (thickness of the exchange surface)
  • the water potential gradient
• The term 'net movement' refers to the number of molecules moving in one direction minus the number of molecules moving in the opposite direction.
• Which direction will the water move in?

  Water will move from cell 1 into both cell 2 and cell 3.

  Water in cell 2 will only move into cell 3.

  At -4kPA water potential, equilibrium will be reached.

  
• Aquaporins allow greater quantities of water to pass through the bilayer.
• Osmosis is explained in terms of water potential.
• Osmosis is a type of diffusion defined by the movement of water molecules from a higher water potential to a lower water potential through a selectively permeable membrane.
• Pure water has a water potential of zero, and as more solute is added the water potential of the solution becomes more negative.
• Water molecules diffuse from the less negative region to the more negative region down a water potential gradient.