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 MpaFlaccid cell dropped into sucrose solution
Flaccid cell:
Ψs = -0.732 Mpa
Ψp = 0 Mpa
Ψw = Ψp + Ψs = -0.732 Mpa
Cell after equilibrium:
Ψs = -0.732 Mpa
Ψp = 0.488 Mpa
Ψw = -0.244 Mpa
Water moves from A to B (from a negative potential to a more negative potential)
Regulation of Water Potential Ψw
Ψw of cells is more negative than soil –> cells try to take up H2O, turgor pressure
Ψw can be different in different parts of the plant.
regulate Ψw by:
1) solute movement
- ions (K+); rapid
- ions make Ψs more
negative which
makes Ψw more
negative
- water moves in to that
region
2) metabolism of polymers
- polymer --> monomers
Ψw becomes more
negative (slow)
Aquaporins
- facilitate movement of water across membranes
- integral membrane proteins that form water selective channels
- aquaporin activity can be regulated by pH and Ca
How does water move quickly across membranes? Aquapotins
Functions of Water Potential
1) plant rigidity
- when Ψw is more negative than soil, water moves INTO plant -> plant becomes stiff
- when Ψw is less negative than soil, water moves OUT of plant -> plant wilts
2) movement of plant parts
- stomata (open at light,
close at night)
- Venus fly trap
3) plant growth
turgor pressure –> cell
enlargement
How does cell enlarge if cell wall is so strong?
- Turgor pressure stimulates auxin release
- Auxin stimulates ATPase that triggers secretion of H+ into apoplast
( turgor pressure –> auxin release –> ATPase stimulated –> H+ secreted into apoplast)
Evidence: What can you do to prove this?
Disrupt/ break it by:
- preventing the release of
auxin
- inhibit ATPase to stop the secretion of H+
- acidic buffers (
How do Protons loosen cell wall?
- Break H bonds (probably not)
- Stimulate enzymes (transglycolase, expansins)
-> sensitive to enzyme
denaturing conditions:
boiling methanol
pronase
8M urea
-> high Q10
-> transglycolase known to be
pH regulated
rapidly reversible
break/resynthesize
glycosidic bonds
process of plant growth
Turgor pressure ↓ Auxin release ↓ Acidification of apoplast ↓ Activation of transglycolases ↓ Cell expands ↓ Plant grows!
Water transport
water movement:
roots →stems → leaves → air
movement depends on water potential
- air can have extremely negative Ψ
- therefore, water moves from soil → plant → air
Water movement
From roots to trunks to leaves: water potential becomes more negative
Vaporization: liquid to gas – plant gives off heat
Transpiration
evaporation of water from leaves through the stomata
Heat absorbed from leaf -> temp raised -> vaporization
Transpiration is the process by which moisture is carried through plants from roots to small pores on the underside of leaves, where it changes to vapor and is released to the atmosphere. Transpiration is essentially evaporation of water from plant leaves.
Transpiration and cohesion of water molecules drives water transport.
Water is pulled up (not pushed) the plant due to transpiration.
99% H2O lost by
transpiration
0.9% retained in tissue
0.1% metabolic reactions
ex: 47 foot maple tree
220 liters of water/ hour
Transpiration pulls the water up the plant
Factors that affect transpiration
Transpiration affected by: (LTWH)
1) light (stomata open)
increases transpiration
2) temperature
increases transpiration
3) wind
increases transpiration
4) humidity
decreases transpiration
–> Transpiration provides evaporative cooling, due to high specific heat and high heat of vaporization of water.
- -> Wind increases transpiration
- -> Water vapor concentration in saturated air increases with temperature.
Relative Humidity
What effect does 100% RH have on a plant? Plant will die unless water moves inside the plant and cool it off
Vascular Tissue
- same in roots and stems
- transports water and minerals
- tracheids & vessel elements have massive secondary cell walls (cells are dead)
- cells have perforations in end walls
The primary components of vascular tissue are the xylem and phloem. These two tissues transport fluid and nutrients internally.
Three pathways of water uptake:
apolpast
symplast
transmembrane
Cohesion-Tension Theory
properties of water
-> cohesion: cohere to each other (H-bonds)
-> adhesion: adhere to walls
of vascular tissue
model:
transpiration
leaf Ψw decreases
water flows from vascular tissue --> leaf down Ψw gradient water pulled up xylem water column must be unbroken
Cohesion-tension theory
- explains how how water is pulled up from the roots to the top of the plant. Evaporation from mesophyll cells in the leaves produces a negative water potential gradient that causes water and minerals to move upwards from the roots through the xylem.
Summary
Osmosis
Diffusion (NOT) vs. bulk flow (YES)
Water Potential Ψw = Ψp + Ψs Ψs= - R T Cs - Regulation and function of water potential - Transpiration - Cohesion - Tension Theory
