1. Chapter 1 Plant cells and Water
Contents in brief:
I: Roles of water in plant life
II: Structure and properties of Water
III: Water movement
Absorption of water
Transport of water
Loss of water(transpiration)

2. I. Roles of Water in Plant Life

3. Please give me water….

4. Water deficiency
is a principal limiting factor in crop production worldwide.

5. Plant Water Content Two indices：
Water content=(fresh weight-dry weight)/fresh weight ×100%
WC=(FW-DW)/FW ×100%
Relative water content=(fresh weight-dry weight)/(saturated fresh weight-dry weight) ×100%
RWC=(FW-DW)/(SFW-DW) ×100%
Some valuable data：
hydrophyte 90%；xerophyte : e.g.lichen 6%; mesophyte :70~90%
root tip, bud, young leaf: 70~90%
plant stem: 35~70%；dormant seed:5~15%

6. Roles of Water in Plant Life
70~90% of cytoplasm, its contents influences plant metabolism
Reaction and transport medium of Plant metabolism 。
Substrate / product of metabolism。
Keep turgid for both plant cell and plant itself
Keep plant in its optimal growth position。
Regulate plant micro climate: adjust humidity, temperature and air composition.

7. Irrigation and agriculture
Irriagtion is one of the main cultivation techniques for high yield.
Irrigation often leads to soil salinization.

8. II: Structure of H2O and its properties
Structure: H2O is a polar molecule

10. Hydrogen bonding is largely responsible for the many unique properties of water
Thermal properties such as higher specific heat, boiling point and heat of vaporization compared with other molecules with a similar molecule size (Table 1.1)
Water shells or called bound water: hydrogen bonding is the basis for hydrogen shells that form around biologically important macromolecules such as proteins

11. Two status of water in Plant Cells
Free water : The water that is far way from cytoplasm particles such as ions and macromolecules and has high free energy.
Bound water: the water that is bound to cytoplasm particles and not readily to take part in plant metabolism but to stabilize the particles. Bound water accounts for about 30% by weight of hydrated protein molecules.

12. These properties of water keep plant tissue or cells a little variation in temperature under natural condition of extreme change of temperature.

13. 2. Properties of water 1) High boiling point Compare: CH4 16 CH3CH3 30
CH3CH2CH2CH3 58
CH3(CH2)3CH boiling point 36℃
but : water boiling point 100℃
reason: n H2O≒(H2O) n +heat
gas

14. 2) Water has high specific heat
The polarity of water and the resulting hydrogen bonding among water molecules means that it takes much heat (calorie) to raise the temperature of 1 mL of water just 1°C. The hydrogen bonding has to be given a lot of energy to get them to vibrate and generate the temperature change. This property of water is called specific heat. It means that this liquid can absorb much heat from the various chemical reactions occurring in cells without temperature change; it is a heat buffer. It helps maintain an even body temperature.

15. 3) Water has a high latent heat of vaporization
Among liquids, water has the highest latent heat of vaporization (44 kJ·mol-1). This means that when water goes from liquid to gas it takes a lot of energy. This property can obviously be traced directly to hydrogen bonding again. As the highest energy molecules in the liquid achieve what it takes to move away as a gas, their energy is removed from the liquid and it gets cooler. We sometimes refer to this as evaporative cooling. This is a critical property in maintaining the temperature of dark green leaves essentially "parked" in sunshine.

16. 4) Water has high dielectric constant
Dielectric constant(介电常数） is used to demonstrate the polarity of molecules. Water has high dielectric constant (Table 1.2). Therefore, water is the excellent solvent for charged ions and organic molecules.
In the other hand, water can form hydration layer around polar molecules，and thus maintain the stability of the molecules and in the same time increases their solubility.

18. Water shells or bound water around ion or macromolecules

19. Hydration layer around glucose molecule

20. Hydration layer around a peptide molecule

21. 5) Water demonstrates cohesion and adhesion
Cohesion(内聚力）:The attraction between the same liquid molecules. The cohesion of pure water is up to 30MPa.
Adhesion（粘附力）:The attraction between liquid phase and solid phase is called adhesion.
Reason of cohesion and adhesion: water is a polar molecule and can readily form hydrogen bonds with itself and other polar molecules.

22. 6) Water has a high surface tension（表面张力）
It takes a lot of energy to break through the surface of water, because water molecules at the surface are attracted (cohesion) to others within the liquid much more than they are to air. Thus, water acts as though it has a skin.

24. 7) cohesion, adhesion and surface tension leads to capillarity
h=2Tcos/rg=14.9×10-6/r

25. 8) Water is incompressible
While gases can be compressed into smaller and smaller spaces, liquid water is not so compressible. Thus compression of water into a space surrounded by a cell wall produces turgor pressure(膨压）. This form of hydraulic pressure is critical for cell growth, for the opening and closing of stomata, flow processes in translocation in the phloem, exchange of materials within and between cell compartments, and for the rigidity and support for herbaceous (not supported by lignin in wood) plants. Turgor keeps petals and leaves extended into the air and prevents wilting.

26. 9)Water has high tensile strength
Tensile strength(抗张强度）is the amximum tension that an uninterrupted column of any material can withstand without breaking.
The cohesive property of water keeps the column of water in the xylem unbroken all the way up to the top of a tree. In very small capillary tubes, the backwards-pull (tension) of the weight of the column of water below a given point may reach -30 MPa without breaking the column. This means that such a narrow column of water is about 1/10 as strong as copper or aluminum wire of similar diameter!

27. 10) Water is highly transparent
Visible light and UV can penetrate through water, thus submerged plants can survive; Furthermore, the inner mesophyll cells can absorb the penetrated light to photosynthesize as well.

28. III: Movement of water Diffusion(扩散） Bulk flow（集流运输） Osmosis （渗透作用）
Water movement includes how water flows into and out of plant cells, from soil, through plant, into the atmosphere.
Mechanism of water movement of plant is passive process including the following processes:
Diffusion(扩散）
Bulk flow（集流运输）
Osmosis （渗透作用）

29. 1. Diffusion Definition：The passive movement of
any material from a region of higher concentration to a region of lower concentration is often called diffusion.
Basis：Random thermal motion of individual molecules. Diffusion is driven mainly by concentration difference.

31. -=£ Fick’s first law described diffusion quantitatively DADC l
Diffusion coefficient (how easily substances move through a particular medium
DADC l
-=£
Concentration differences
單位時間內物質通過單位面積的數量(mol m-2 s-1)
Diffusion path
cross-sectional area (A)
length (l)

32. Rate of diffusion-Fick’s Law (A. Fick, 1855)：
JB=-D C/ 
tc=½= ( 2/D)K
For a plant cell with a diameter of 50μm, the diffusion coefficient D=10-9m2S-1, then
for a distance of 1m，
So: Diffusion works only over short distances

33. 2. Bulk flow
Definition: Bulk flow means all of the molecules of the substance move in a mass, so also called mass flow.
Basis: driven by pressure or gravity, e.g. from an area of greater pressure to an area of lesser pressure.
In plant: water movement in bulk flow are found in xylem conducting cells, from soil into root apoplast.

34. Bulk flow (mass flow) pressure driven (gravity or pressure)
Solutes move in vascular tissue is mainly by bulk flow

35. 3. Osmosis Osmosis is the passive movement of water across a membrane.
In fact , The actual movement of water through a cell membrane is the result of two processes: diffusion (across lipid bilayers)
and bulk flow (aquaporins).
Aquaporins (Peter Agre) and ion channel (Roderick Mackinnon): 2003 Nobel Prize)

36. Aquaporins are a class of integral membrane proteins that form pores in the membrane of biological cells.
Aquaporins selectively conduct water molecules in and out, while preventing the passage of ions and other solutes. Also known as water channels

37. aquaporins are membrane pore proteins
aquaporins are membrane pore proteins. Aquaporins are commonly composed of four (typically) identical subunit proteins in plant cells, with each monomer acting as a water channel.
Aquaporins consist of six transmembrane α-helices arranged in a right-handed bundle, with the amino and the carboxyl termini located on the cytoplasmic surface of the membrane
Water molecules traverse through the pore of the channel in single file. The presence of water channels increases membrane permeability to water.

40. Generally the movement of water across the membrane is not treated by plant physiologists as either diffusion or bulk flow. Rather plant physiologists focus upon the driving force for osmosis...energy
Osmosis is driven by water potential (discussed later)