Water Flashcards
Features of Water
- universal solvent
- responsible for movement of cellular constituents
- participate in biochemical reactions
Transpiration: how water flows through plant
- water often limits plant productivity
Properties of water
- forms H-bonds
- high specific heat (amount of energy required to raise the solvent by 1 degree of temp) (4.2)
- -> minimizes temperature fluctuations
- high heat of vaporization (amount of energy it takes to go from liquid to gas) (2452)
- cohesion - H bonding of water to itself
- adhesion - bonding of water to other molecules
Water Potential
symbol: Ψw
FORMULA:
Ψw = Ψp + Ψs
Water potential = pressure potential + osmotic potential
Water potential quantifies the tendency of water to move from one area to another due to osmosis, gravity, mechanical pressure, or matrix effects such as capillary action
Diffusion
- random movement (eg. Drop of dye in water)
- solute movement proportional to concentration gradient
Js = - Ds ΔCs / ΔX
where
Js = Flux (mass/area/time)
Ds = Diffusion coefficient
ΔCs =Difference in conc.
gradient
ΔX = Distance of conc.
gradient - rapid for short distances;
- slow for long distances
- time for conc. to decrease to 1/2 is proportional to (distance)^2 across cell in seconds; across plant in years
–> Water does NOT move by diffusion in plants (slow process)
bulk flow
- Water moves by bulk flow
- water moves in response to pressure gradient
volume flow rate = ( π r^4 Δ ΨP) / (8 n ΔX)
-> n is the viscosity of the
fluid
Flow is sensitive to radius
-> if you double r and raise it to 4th power, volume flow rate increases by 16
–> water transport in the plant (through xylem) occurs by bulk flow
Osmosis
movement of water through a differentially permeable membrane to equalize the water concentration on both sides of the membrane
Water moves from: HIGH to LOW
high water conc. –> low
water conc.
Equilibrium: [water] is the same on both sides of membrane
—> Cell membrane is differentially permeable
Water Potential Ψw
the tendency (what the system wants to do) for water to move from an area of HIGH [water] to an area of LOW [water]
- pure water: Ψw (water potential) = 0
- solutions (at atmospheric pressure)
Ψw (Water potential) is always NEGATIVE because the water concentration is less than pure water - water moves from less negative Ψw to a more negative Ψw
- Ψw is potential for water movement but “barriers” may reduce or prevent water movementat equilibrium ΔΨw = 0
(no difference in Ψw )
Osmometer (desired case)
- beaker of pure water
- tube with differentially permeable membrane
- tube contains solute (blue dots – salts)
–> water moves into tube,
solution becomes dilute,
Ψ becomes less negative.
ideally, all water enters
tube to attempt
ΔΨw = 0
–>system cannot achieve equal [water]
Actual case: osmometer
- not all water enters tube
- column of water exerts downward pressure on solution
–> turgor pressure - water no longer enters tube when turgor pressure equals pressure created by tendency of water to enter tube.
–> at equilibrium Δ [water] cannot equal 0 (difference in water concentration is not 0)
but, ΔΨw = 0 (system is at equilibrium)
External pressure
- apply external pressure with piston inside the tube
- Water wants to come in but cannot because of the presence of external pressure
- -> no net water movement when applied pressure equals pressure for water to enter tube.
–> Δ [water] cannot equal to 0
but, ΔΨw = 0 (system at equilibrium)
Components of Water Potential
- pressure potential (ΨP)
- osmotic potential (ΨS)
- minor potentials (matrix, gravitational)
FORMULA: w = p + s
Ψw = Ψp + Ψs
Water potential = pressure potential + osmotic potential
Pressure Potential (Ψp)
Ψp caused by build up of pressure on one side of membrane
–> it can be + (pos) , - (neg) , or 0 (zero)
Osmotic Potential (Ψs)
Ψs caused by presence of solutes, always negative
Formula: Ψs= - R T Cs
R = gas constant
T = absolute temp (K)
Cs = osmolality (moles of
total dissolved solutes per
kilogram of
water)
Ψs = 0 for pure water
Ψs = always NEGATIVE
for a solution
Examples: Ψw = Ψp + Ψs
pure water at atmospheric pressure
Ψs = 0 in pure water
Ψp = 0
Ψw = 0
solution at atmospheric pressure
Ψs = always - for sol.
Ψp = -
Ψw = 0
solution at positive pressure
Ψs = always - for sol.
Ψp = +
Ψw = -, 0, +
(turgid plant cell)
solution at negative pressure
Ψs = always - for sol.
Ψp = -
Ψw = -
(water transport in xylem
Example: Solution containing 0.1 M sucrose
Ψs = -0.244 Mpa
Ψp = 0 Mpa
Ψw = Ψp + Ψs
= 0 + (-0.244) Mpa
= -0.244 Mpa
