D2.3 Water potential

solvation

combination of a solvent with molecules or ions of solute

• water molecules form a 3D shell around ions and charged molecules due to attraction, preventing precipitation by clumping together

movement of water molecules

• particles in liquid move, do not separate until turned into gas due to intermolecular attraction

• strong overall attraction between water molecules due to many bonds existing at a time despite some hydrogen bonds breaking and forming with changes in position

solute-water attractions

• stronger than attraction between water molecules

• restrict movement of water molecules, solutions are more viscous

osmotically active solutes

intermolecular attractions form betwen them and water (e.g. sodium, potassium, chloride ions, glucose)

water movement between 2 solutions

always movement in both directions, net movement of water from less concentrated to more concentrated

• hypertonic solution - higher concentration of osmotically active solutes

• hypotonic solution - lower concentration of osmotically active solutes

• isotonic solutions - no difference in concentrations of osmotically active solutes, no net movement of water between two isotonic solutions

osmosis

net movement of water from less concentrated to more concentrated solution across membrane

• passive movement

• speed altered by changing permeability of plasma membrane

• direction altered by changing concentration of osmotically active solutes inside cell

properties of plasma membrane vs cell walls

• plasma membranes: phospholipids as the main constituent; thin; liquid, allowing changes of position; easily torn; semi-permeable

• cell walls: cellulose as the main constituent; thicker; solid, limited changes of position; high tensile strength, very strong; freely permeable unless impregnated with waterproof material

animal cell bathed in hypotonic solution

water enters cell by osmosis, cell swells, bursts easily due to lack of support/lack of wall

effects of water movement on cells with a cell wall

• turgidity - normal state of plant cell where cell becomes pressurized/swollen by entry of water/osmosis

  • high pressures can build up due to strong cell wall

  • provides support, strength under compression

• wilting - caused by plant cells losing water

  • pressure of cytoplasm decreases

  • plasma membrane no longer pushes against cell wall

  • cell is not turgid, plant cells become flaccid

• plasmolysis - plasma membrane pulls away from cell wall

  • when plant cells bathed in hypertonic solution, cell wall does not move (permeable to water) but volume of cytoplasm decreases

  • plasma membrane pulls away, eventually causing cell death

medical applications of isotonic solutions

• medical procedures require isotonic solutions to prevent cell bursting or dehydrating

• normal saline - isotonic NaCl solution

  • safely introduced to patients’ blood system via intravenous drip

  • used to rinse wounds and skin abrasions

  • used to keep areas of damaged skin moistened prior to skin grafts

  • basis for eye drops

  • frozen to consistency of slush for cooling hearts, kidneys, donor organs to be transported to hospital