Download Water potential

III. Thermodynamics of water
1.Free energy(自由能）
2.Chemical potential（化学势）
3.Water potential（水势）
4.Water potential of a plant cell
（细胞水势）
5. Water movement among cells
（细胞间水分运动）
1. Free energy vs. bound energy
• “Total energy of a substance or a
system includes bound energy and free
energy.”
• Free energy is defined as the energy that
is used to doing work.
• Free energy = “total energy minus that
energy which is unavailable for doing work
(bound energy,束缚能).”
The Direction of Spontaneous
Processes for a system
• According to law of thermodynamics, the
direction of movement or chemical
reaction is always spontaneous from
higher energy state to lower one .
• For all spontaneous processes at constant
temperature and pressure, ΔG is negative.
– ΔG<0, take place spontaneously
– ΔG>0, can not take place
2. Chemical potential
• Under constant temperature and pressure, the
free energy of 1mol component contained in a
system (multiple components) is called chemical
potential.
• Unit: J/mol
• Symbol: μ
• For pure water, μ=0
• Chemical potential also predicts a spontaneous
response (from higher chemical potential to
lower one!)
For water
The chemical potential of water:
μw=μw* + RTlnαw+VwmP+mwgh
αw: mole fraction of water; Vwm: partial molal volume of water
However, absolute chemical potential of water is never used, usually, its
relative value (μw) is used for predicting direction of water movement!
μw= μw - *w
In general, *w is defined as 0 and the maximum value, therefore chemical
potential of water in a multiple system must be negative!
For plant physiology, chemical potential of water is not used for
predicting direction of water movement rather than water potential !
3. Water potential
• For a plant cell : Water potential is defined
as the chemical potential of water divided
by the partial molar volume of water (Vwm:
volume of 1 mol water equal to 18*10-6
m3/mol)
Ψw= (μw-μw*)/Vwm= (RTlnαw+VwmP+mwgh)/
Vwm
= RTlnαw／ Vwm+P+ρgh
= Ψs + Ψp + Ψg + Ψm
ψw
w  
=
Vw, m
0
w
 w

Vw, m
(1.4)
ψw 代表水势；Δμw 为化学势差，单位为J· mol －1 ，J＝N·m（牛
顿·米）；Vw，m为水的偏摩尔体积，单位为 m3·mol-1。则水势的
单位：
1
ψw =
Jmol
m  mol
3
3 = N/m2 = Pa（帕）
=
J/m
1
(1.5)
由此可见，水势单位为压力单位，一般用兆帕（MPa，1MPa＝
106Pa）来表示。过去曾用大气压（atm）或巴（bar）作为水势
单位，它们之间的换算关系是：lbar＝0.1MPa＝0.987 atm，1标
准atm＝1.013 × 105 Pa=1.013 bar。
表1.2几种常见化合物水溶液的水势
ψw（MPa）
溶液
纯水
0
Hoagland营养液
-0.05
海水
- 2.50
1mol ·L-1蔗糖
-2.69
1mol·L-1KCl
- 4.50
4. Components of water potential
Ψs is the solute potential（溶质势或渗透势）
The solute potential (ψs) is the effect of dissolved
substances on the potential energy of a solution.
It is defined as 0 MPa for pure water. For
solutions the solute potential is determined by
the Van't Hoff Equation:
Ψs = - CiRT
where C is the molar concentration of the solute, i
is the ionization constant for the solute, R is a
constant and T is the absolute temperature (°K).
The negative sign indicates that solutes
decrease the potential energy of a solution.
ψp is the pressure potential(压力势）
The pressure potential (ψp) is the effect of
hydrostatic pressure on the potential
energy of a solution. It is defined as 0 MPa
for STP (absolute pressure of 1 atm = 0.1
MPa).
Increasing the pressure of an area will
increase the water potential and water will
tend to leave that area.
ψg is the gravitational potential (重力势）
The gravitational potential (ψg) is the effect of
height of a system above sea level. It is defined
as 0 MPa at sea level.
Basically raising a system 10 meters will increase
its water potential energy by 0.1 MPa, water will
then tend to move down from there.
As most laboratory biology is done all at one level,,
in particular in the case of a cell, this component
is often considered negligible.
ψm is the matric potential(衬质势）
The matric potential (ψm) is the effect of
colloids (adhesion) in soil or as a result of
polymers in the cell wall.
For a mature cell containing central vacuole,
matric potential is negligible, and because
the height of the cell in the lab is negligible,
the water potential of a plant cell simplifies
to:
ψ = ψs + ψp
5. Plant cell is an osmosis system
• 渗透作用(osmosis)是指水透过半透膜的一种
迁移方式，事实上是一种特殊的扩散，但也
与集流(膜两侧的压力差)有关。
• 渗透作用是由膜两侧的水势差所驱动的。
Plasmolysis- osmosis of water
through cell membrane
Hypertonic
solution
Through plasmolysis, we can:
1 determine whether the cell is alive or dead
2 determine osmotic potential
Changes of Ψw, Ψs and Ψp with cell volume during plasmolysis
Note: small changes in volume cause steep changes in turgor pressure
6 How to measure water potential
and its components
• Measuring water potential:
– Psychrometery(干湿测定法）
– Pressure chamber（压力室法）
Isopiestic psychrometry
Pressure chamber
• Measuring osmotic potential
– Plasmolysis
– Psychrometry
– Cryoscopic osmometer (Freezing point
osmometer )
Measuring turgor pressure---Pressure probe