1.  WATER IS THE SINGLE MOST IMPORTANT MOLECULE IN BIOLOGY. WHY?

• Good Solvent
• Solvent for biochemical reactions
• Medium used for transport of all ions and metabolites
• Incompressible, so its uptake lead to cell expansion
• Transparent, so it enables light to penetrate cells
• Well suited for temperature regulation

Polarity of water leads to intermolecular H bonding
H-bonding results in cohesion and high surface tension
Cohesion and adhesion results in high tensile strength
Because of its polarity, it is a universal solvent

Water requirements for seed germination

• dissolve metabolites
• hydrate enzymes
• breakdown stored food reserves
• provide energy for synthesis
• provide turgor for cell expansion

Water movement is passive. i.e. energetically downhill.
Water movement is down conc. gradient and pressure gradient

a. Three ways water move:

1) Diffusion- movement down a conc. gradient
2) Bulk Flow: movement down a pressure gradient
3) Osmosis: movement across a membrane and down both conc. and pressure gradient

b) What determines the direction of water movement?
The direction depends on the driving force.
    Water moves from a region of high water potential to a region of low water potential until the water potential of the inside and outside are equal. (equilibrium)

    Water potential indicates the free energy of water and the potential to do work.  The total driving force of water, the water potential, depends on the pressure potential + the osmotic potential.
y = y_P + y_p
Examples.
How can one measure water potential?  In Lab exp. 3.

c) What determines the rate of water flow?      Flow rate = driving force x  hydraulic conductivity (Lp)
Lp = permeability of membrane to water.
Lp depends on the amount of water channels (aquaporins) in the membrane.

3.  CELL EXPANSION IS DRIVEN BY TURGOR PRESSURE
Turgor pressure = positive pressure exerted by cell against cell walls.

4.  HOW IS WATER ABSORBED INTO THE ROOT?

Water moves into root by two pathways:
1) Apoplastic pathway: movement through the network of cell walls and intercellular spaces
2) Symplastic pathway: movement via the network of interconnected cytoplasm

Water enters the cytoplasm at the endodermis due to the presence of a suberin barrier in the cell wall.  Endodermis prevents water and ion loss from the plant to the soil during a drought.

Root Pressure: positive pressure in root xylem

5.  HOW IS WATER TRANSPORTED UP THE STEM TO THE LEAVES?

Pressure-driven bulk flow in tube-like structures, xylem
Negative pressure generated by:
    a) Transpiration: loss of water vapor through stomatal pores.
    b) Growing cells that take up water.

Lecture Review: Exercises

1. This question tests your understanding of water movement and plant anatomy.
 a).  Trace the pathway of water from the soil into a leaf mesophyll cell of a bean plant.  Name in sequence each organ and the cell types that the water molecule passes (or enters) during its journey.

b) Imagine a tree.  Assume a root xylem cell  has an osmotic potential of -0.1 Mpa and a water potential of -0.6 Mpa.  A leaf xylem cell of the same plant also has an osmotic potential of -0.1 Mpa; however its water potential is -0.8 Mpa.
    What is the pressure potential of the root xylem?
    What is the pressure potential of the leaf xylem?
   In which direction will the water move?  Why?

(c)  A root cell of the same plant also has a water potential of -0.6 MPa, and an osmotic potential of -1.1 MPa.
    What is the pressure potential of this cell?
    Why is the osmotic potential of the root cortex more negative than that of root xylem?

d) What processes cause the water potential difference observed between leaf and root cells?

BSCI 442