1. III. Thermodynamics of water
Free energy(自由能）
Chemical potential（化学势）
Water potential（水势）
Water potential of a plant cell
（细胞水势）
5. Water movement among cells
（细胞间水分运动）

2. 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).” 

3. 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

4. 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!)

5. 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 !

6. 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

7. ψw = = J/m3 = N/m2 = Pa（帕） (1.5) ψw = (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：
ψw = = J/m3 = N/m2 = Pa（帕） (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。

8. 溶液 ψw（MPa） 纯水 Hoagland营养液 -0.05 海水 - 2.50 1mol ·L-1蔗糖 -2.69
表1.2几种常见化合物水溶液的水势 溶液
ψw（MPa） 纯水
Hoagland营养液
-0.05 海水
- 2.50
1mol ·L-1蔗糖
-2.69
1mol·L-1KCl
- 4.50

9. 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.

10. 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.

11. 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.

12. 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

13. 5. Plant cell is an osmosis system
渗透作用是由膜两侧的水势差所驱动的。

14. 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

15. Changes of Ψw, Ψs and Ψp with cell volume during plasmolysis
Note: small changes in volume cause steep changes in turgor pressure

16. 6 How to measure water potential and its components
Measuring water potential:
Psychrometery(干湿测定法）
Pressure chamber（压力室法）

17. Isopiestic psychrometry

18. Pressure chamber

19. Measuring osmotic potential
Plasmolysis
Psychrometry
Cryoscopic osmometer (Freezing point osmometer )

22. Measuring turgor pressure---Pressure probe