Plant Physiology
LECTURE 2
Water and its uniqueness
*Water makes up the major constituent of plant cells: lettuce 95%, carrots 85%,
* Water is a reactant , photosynthesis, hydrolysis of stored food etc.
* water is a solvent
* water is a coolant
**Plants will continuously absorb and lose water (TRANSPIRATION)- up to 100% every hour
Water bonds: H-bonding (20 kJ/mol)
O covalently bonds to 2 H.
O is strongly electronegative and it pulls e- away from the H, :. Partial charges exist, asymmetric electron distribution = polar molecule
This is responsible for the unique properties of water!!! (Cohesion, high melting and boiling points) Need a lot of energy to break such strong intermolecular force.
Physical Properties
1.
Wide thermal range for its size :. Good for most life
2.
Heat stable due to high specific heat (4.184 J/g C), water can absorb and redistribute heat E without large changes in Temp ie. Ocean
3.
High specific heat of vaporization: liquid

gas at 100 C = 40.7 kJ/mol, highest HOV for any known liquid. High because of H-bonds (How a plant sweats!!! At 25 C each mole of water evaporated requires 44.9 kJ !)
4.
Cohesion and surface tension – how water sticks to itself, why you get a drop of water out of a pipet
5.
Capilllary rise: Adhesion – how water sticks to solid stuff (cell walls), if attraction between the water and the solid is substantial then the solid is said to be wettable and water will rise in a capillary. How water travels up a plant (Increase diameter of vessel Decrease distance traveled)
6.
Tensile strentgth – maximum tension that an uninterrupted column can withstand without breaking
Water is the Universal Solvent

Both cations and anions will attract to water due to partial charge

It transports ions, nutrients, metablites

Incompressible = cell expansion

Source of Oxygen (photosynthesis)

Source of Hydrogen for CO2 reduction

Transparent = light can penatrate cells

Temperature regulation -- high heat of vaporization, high specific heat on a mass basis, high thermal conductivity

LECTURE 2
Water Transport – How does water travel from the soil into the seed, to the embryo, and through an adult plant?

Water movement is passive (energetically downhill)

Water movement is down an concentration gradient and a pressure gradient (Transport = P + conc)
3 Ways to move water
Diffusion – movement of molecules from a high to low concentration; From a region of high chemical potential to a region of low chemical potential. rate is rapid over short distances but slow over long distances
Chemical potential of a species (WATER) indicates the free energy associated with it available to do work
Chemical potential = activity : depends upon concentration , temperature and pressure
1.
Bulk flow or Mass flow – movement of molecules (water and dissolved materials) in response to pressure, rate depend on the pressure gradient and radius of tube and viscosity; good for long distances like up the xylem, within phloem and in soil
2.
Osmosis – water moves across a membrane (diffusion and bulk flow), downhill, depends on the difference in the water potential of the cell and the the environment.
Water potential = pressure potential + osmotic potential
Or as symbols

=
 p +
 s

= Water Potential

Equal to 0 for pure water at standard state

Expressed in Pressure units (Pascals or Mpa)

It is usually less than 0 (negative!) because pure water is 0 so non-pure water is –

Ie. Soil water at field capacity = -0.03 Mpa
 p = hydrostatic pressure potential = P

The rigid cell walls allow cells to build up a large internal pressure, this turgor causes cells to expand during growth

When water is under such positive pressure (push against cell walls) Water Potential increases

When water is under tension (as a long column of water being pulled by transpiration) a negative pressure forms and Water Potential decreases

 s= osmotic pressure but has a negative value. Depends on the concentration of the solute

When solutes added to the water the water concentration is lowered and so is the Water
Potential (that is why OP is negative)
Water moves from high to low water potential until

in =

out
*** Now do examples of problems (see hand written problems!) ***
How does water become absorbed by the root and transported to the leaves?
Into the root --
Two Pathways exist for water and ions to enter the root:
1.
Apoplast –
 network of connected cell walls and intercellular spaces
 water moves fast here because of its properties
2.
Symplast –

The entire network of cell cytoplasm interconnected by the PD and membranes

Slow because water is moving through an aqueous environment and must cross membranes
How does it actually enter the root and move through the plant?
1.
Crosses the epidermis (root hairs) and enters the apoplastic spaces.
2.
Enters the cytoplasm (symplastic) at the endodermis which prevents water and ion loss.
[ endodermis is impregnated by a suberin layer (Caspian strip) which is a hydrophobic fatty acid area (water proof) found at the transverse and radial walls. This barrier forces water to enter the cytoplasm of the endodermis ] Endodermis prevents water and ion loss from the stele.
3.
Once in the stele of the root, water is transported up the leaves by bulk flow
Xylem: tracheids, dead cells that form a continuous tube
The pressures involved in moving water
Root pressure: when transpiration is prevented or slow, ions accumulate in the xylem and can generate a positive hydrostatic pressure.
Active energy-dependent ion transport into the xylem builds up solute concentration and lowers

. Water will build up P due to osmotic movement.
Negative pressure: evapotranspiration generates it
:. Water leaves through the stomata because of the concentration gradient of water vapor between the leaf and the atmosphere….this gives low water potential and a move negative P = sucking on a straw

If you break the column of water you could cause bubbles in the pathway which will slow water transport down
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