D2.3: Water Potential Flashcards
What is solvation/solution?
Solvation:
Interaction between solvent and solute
Solution:
Solute dissolved in solvent
Why is water a good solvent?
It is dipolar:
Hydrogen side -> slightly positive
Oxygen side -> slightly negative
Allow for formation of H bonds with other polar solute/molecules/ions
Polar solvent orient themselves towards polar solutes/ions to form H bonds/ion-dipole forces
-> hydration shell made
Charged ions will dissolve in water - hydrophilic
Nonpolar moelcules will not dissolve - hydrophobic
What is osmolarity?
Total concentration of solutes in a solution
Determines the water potential of a solution (inverse of water potential)
What is osmosis?
The net movement of water molecules from a region of high water potential/low concentration to a region of low water potential/high concentration across a partially permeable membrane
-> down a concentration gradient
-> passive
-> partially permeable: allows small but not large molecules
What are the different types of solution?
Hypotonic:
Lower solute concentration than the cytoplasm of the cell
Net movement of water into the cell via osmosis
Hypertonic:
High solute concentration than the cytoplasm of the cell
Net movement of water out of the cell via osmosis
Isotonic:
Same/similar solute concentration as the cytoplasm of the cell
No net movement of water
What is an experiment that can be preformed to show the effects of bathing plant tissue in different concentrations?
POTATOES :D
-> osmotic concentration estimated
-> different concentrations used to determine isotonic point
Experiment condensed:
1. Same size potatoes
2. Record mass/length
3. Bath in different concentration of salt/sugar
4. Leave them there
5. Ré-measure weight and legnth
6. Calculate percentage change and make graph against ___ concentration
Positive percentage change -> lower concentration in solution -> water into cell -> increased turgidity
Negative percentage change -> higher concentration in solute -> water out cell -> flaccid/plasmolysed
No/little change -> same concentration (x-axis)
How does different concentration effect animals cells/cells without cell walls?
Animal cells -> no cellulose cell wall -> more severe results than plants
Hypotonic:
Gain water via osmosis -> continue until cell membrane stretched too far -> bursts
Hypertonic:
Lose water via osmosis -> becomes crenated
-> can lead to blood clots (RBCs get stuck while moving)
Isotonic:
No net movement of water/equal in/out
-> needed to prevent harmful changes in animals cells
Unicellular organisms and water movement
Ex: Amoeba: Freshwater aquatic habitat -> hypotonic -> constant net influx of water in -> increases internal pressure
Prevent organism from bursting -> contractile vacuole
-> excess water continuously collected and pumped out
-> maintain osmotic concentration of cytoplasm
-> ex: paramecium
How does being in a hypotonic solution effect plant cells/cells with a cell wall?
Lower solute concentration than plant cell -> water in vacuole -> volume increase -> expanding protoplast (living part of cell inside cell wall) -> increased pressure
-> turgor pressure
-> inelastic cell wall prevents bursting
-> pressure by cell wall -> prevents too much water rom entering -> also prevents bursting
Cell fully inflated and is rigid + firm -> fully turgid
Turgidity important for plant:
Being firm provides support/strength
-> helps plant stand upright with leaves held out (sunlight)
Not enough water -> cannot stay turgid -> plant wilts
How does being in a hypertonic solution effect plant cells/with a cell wall?
More concentrated -> water leave vacuole -> volume decreases -> protoplast gradually shrinks -> no longer exerts pressure on cell wall -> beings to pull away from cell wall
-> plasmolysis -> cell flaccid and plasmolysed
What are some medical uses of isotonic solutions?
Organs being prepared for transplant -> bathed in isotonic fluid -> prevent damage to cells
Any fluid given intravenously (IV) has to be isotonic in relation to blood plasma
IV drip: treat dehydration + deliver medicine to bloodstream
-> usually 0.9% sterile saline solution
Hypotonic solution: net movement of water into RBC -> burst
-> decrease in oxygen carrying ability
Hypertonic solution: net movement of water out of RBC -> shrivel and become created
-> increase risk of blood clots
What is water potential?
Potential energy of water / unit volume relative to pure water at sea level and room temp
ψw (psi like poseidon)
-> measured in units of pressure: kPa or MPa
-> ψw of pure water at sea level and room temp = 0 MPa
Always from high to low
Why can water potential be describe as ‘energy change when water is moved out of a solution into pure water’?
Solute added to solution -> ψw decreases into negative values
-> high solute concentration -> lower ψw
-> energy is stored in H bonds between solute molecules and water molecules -> less energy available as potential energy
-> water molecules in solution with higher solute concentration -> less energy as potential energy
Move water out of water -> no energy -> ψw of pure water is 0
Any added solutes -> increased amount of energy needed to take water out
Movement of water from higher to lower water potential (solute)
ψw -> tendency of water molecules to move from dilute - concentrated solution
High water potential -> contain water molecules with greater potential energy for movement -> greater tendency to move
Low water potential -> many H bonds between water + solute molecules -> reduce potential energy for movement -> less tendency to move
Go from area of high ψw/high potential energy/low solute concentration -> low ψw/lowpotential energy/high solute concentration
What is solute potential (ψs)?
ψs of a solution is proportional to the number of dissolved molecules
More solutes dissolved -> potential energy of water reduced -> ψw decrease
ψs always negative (if + imply you are removing solutes from pure water)
No solute -> ψs = 0
Provided pressure potential is constant -> decrease in ψs = decrease in ψw
Solutes effect on ψw:
Bind to water via H bonds as they dissolved
Potential energy -> transferred to H bonds
Reduction of potential energy -> ψw reduced
What is pressure potential (ψp)?
The physical pressure on a solution
-> Occurs in cells with a cell wall
-> Also known as turgor potential/pressure (pressure exerted by the plasma membrane against the cell wall)
—> + water -> + turgor pressure -> + ψp
Usually positive in plant cells:
-> cytoplasm exert pressure on the inside of cell wall -> turgor pressure provides support
Negative ψp can occur in xylem vessels:
Water + minerals dissolved -> transported under tension
Equation for water potential
Water potential: measurement that combines the effects of solute concentration and pressure
ψw = ψs + ψp
Water potential and movement in plant tissue:
Hypotonic solution
Plant cell cytoplasm has dissolved substances -> lower ψ s -> lower ψw
Water from surrounding solution -> into plant down gradient
Inward movement of water -> increase in volume, increase in ψp as cytoplasm pressures increases against cell wall
Eventually ψp reaches point where ψw equal inside and out of cell -> inward movement of water stops
Plant cell is turgid -> structural support to plant
Water potential and movement in plant tissue:
Hypertonic solution
Surrounding solution -> lower ψs than cell cytoplasm -> lower ψw of solution
Water moves out of cell into solution down gradient -> loss of water -> reduced volume, decreased ψp inside cell -> plant cell lose turgor pressure -> wilt
How does water potential work in plant roots?
Higher ψw in cell -> lower ψw in xylem (low due to transpiration pull)
ψw lowers as water goes up the tree
-> transpiration pull (diffusion out of leaves) keeps ψw in leaves very low
Exception:
Normally there would be a positive ψp
But in xylem -> evaporation from leaves -> transpiration -> negative pressure -> further reduced ψw
= increased uptake in water
