D2.3 water potential

describe what happens when polar/charged solutes are put in water

• partially positively charged hydrogen pole of water molecule is attracted to negatively charged solute

• partially negatively charged oxygen pole of water molecule is attracted to positively charged solute

• water forms HYDRATION SHELLS around solutes

• precipitation is prevented

why is solution more viscous than water

[explain hydration shell formation]

attraction between water and solute, the hydration shells formed, restricts water movement.

define osmosis

net movement of water across a membrane due to attractions between solutes and water

the 2 statements related to omosis

• net movement of water from hypotonic solution to hypertonic solution, because hypertonic solution has higher concentration of osmotically active solutes

• no net movement of water between isotonic solutions because there is no concentration difference of osmotically active solutes → equal number of water molecules move between them → dynamic equilibrium

purpose of plasma membrane/its property

separates cytoplasm from solution,

permeable to water,

less permeable solutes

how do cells control direction of movement of water

raising and lowering concentration of osmotically active solutes in cell

what happens when u put animal cell in pure water

• water enters cell

• cell becomes swollen

• no cell wall, no support provided

• cell bursts

what happens when u put animal cell in concentrated solution

• water moves out of cell

• cell becomes crenated

explain how plant cell gets turgid

• water enters plant cells

• high water pressure in cell

• cell wall present to prevent bursting

• cell that is pressurised = turgid (meaning it is swollen)

how does cell get flaccid

• cell loses water

• pressure of cytoplasm decreases

• when pressure drops to atmospheric pressure, plasma membrane no longer pushes against cell wall

• cell is not turgid, becomes flaccid

• leaves and stem bend downwards → wilting

• usually seen in plants with lots of water loss under hot temperatures

how does cell get plasmolysed

• cell in hypertonic solution

• water moves out of cell

• cell wall = permeable to water, does not move

  • volume of cytoplasm decreases, plasma membrane pulls away from cell wall → plasmolysis

medical applications of isotonic solution

• introduced to patient’s blood system via intravenous drip

• used to rinse wounds and skin abrasions

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

• used as the basis for eye drops

• frozen to slush for cooling donor organs when transported to hospital where transplant operation is to be done

why is isotonic solution used for medical application

human cells in hypertonic solution → causes damage to cell by dehydration

human cells in hypotonic solution → cells swell and burst

human cells in isotonic solution → rate of water moving in cell = rate of water moving out of cell

when does something have a high and low water potential

• in pure water, there are no solutes. water molecules can move more freely than water molecules in solutions where they are restricted. this is called high potential energy for water

• in solution, water needs to form hydration shells around solutes so their movements are restricted. this called low potential energy for water

  • water is more stable when they have low potential energy than when they have high potential energy

water potential equation

water potential = solute potential + pressure potential

why does pure water have a water potential of 0kPa

• has no solutes → 0 solute potential

• is at atm → 0 pressure potential

relationship between solute concentration and solute potential + hydrostatic pressure and pressure potential

high solute concentration = low solute potential

high hydrostatic pressure = high pressure potential

water goes from… to… in order to…

water moves from high water potential to low water potential in order to minimise potential energy. water is more stable with lower potential energy

greatest solute potential value

0

why is there a high pressure potential in the cytoplasm of a plant cell

• pressure of water against cell wall = pressure of cell wall against water

  • no rigid cell wall surrounding animal cell → water pushes plasma membrane more and more → bursting, instead of withstanding pressure

  • rigid cell wall in plant cell → more water entering cell → cell turns turgid → cell responses to water pushing against cell wall by having the cell wall pushing inwards

why can’t solute potential be positive

• when solutes dissolve → lowered potential energy

• no solutes dissolved → solute potential is 0

  • impossible for water to hold less than no solutes → only possible solute potentials are zero or negative.

when water moves in from hypotonic solution to plant cell, why does the water potential of plant cell increase

• solute potential less negative because more water is present to dilute solution in plant tissue

• pressure potential more positive because more water within same volume of space leads to higher pressure

what happens when you put plant cell in hypertonic solution

• solute potential of hypertonic solution = more negative than solute potential of tissue

• pressure potential of hypertonic solution is probably 0, but pressure potential of plant cells is probably initially above 0

  • there is pressure within plant cells because cell wall withstands the turgidity

• hence cells have higher water potential than solution → net movement of water from cells to hypertonic solution