2. H2O FUNCTIONS
• Each cell contains large water filled vacuole
• 5-10% of cell volume in only cytoplasm
• Constituent:
– 80-95% fresh weight herbaceous plants, (carrots and
lettuce.
– 35-75% woody plants (dead cells, sapwoods).
– Seeds (the driest plant tissues 5-15%)
• Solvent/Transport and influence structure of proteins,
nucleic acids, polysacchrides
• Substrate/environment for biochemical reactions
• Transpiration and Temperature regulation (due to
escape of high energy molecules), half of net heat input
is dissipated by transpiration, nutrients for root uptake
3. H2O
PROPERTIES
– Cohesion
– Adhesion
– Surface Tension
• Transparency, incompressibility, density
– Universal solvent
– Specific Heat: 4.184 J g-1
– Latent Heat of Vaporization: 44 kj mole-1 at
25°C
– Dissociation of water molecule
4. Properties of water
• Polarity of water
molecule give rise to
Hydrogen bonding
• Water molecules carries
no net charge but makes
water a polar molecule
11. Water Potential (w)
– Definition: A measure of the free energy of
water:
– Symbol: Greek letter psi ()
– Units: bar or Pascal (1 bar = 0.1 MPa).
– w of pure water: zero
– w decreases: by addition of Solutes
(w <0) (more negative)
12. Water Potential
Magnitude
w = 0 MPa Pure Water
w = 0 to -1 MPa
Plant/Cell in
good
condition
w < -2 MPa
Plant/Cell
under water
stress
w = -1 to -2 MPa
Plant/Cell under
mild water
stress
13. Water Potential
Components
• Matric Potential (m):
– Represents the effect of insoluble materials
(colloids or cell walls). It is negative.
• Osmotic Potential (s):
– Represents the effect of solutes. It is negative.
• Pressure Potential (p):
– Represents the effect of hydrostatic pressure. It is
positive.
• Gravitational Potential (g):
– Represents the effect of gravity. It is negative.
16. Classification of water in the soil
• Hydration water:
– chemically bound to soil particles. Not available to
plants
• Hygroscopic water:
– tightly held by the soil. Not available.
• Capillary water:
– fills soil micropores. Most of it is available.
• Gravitational water:
– That moving in the soil by gravity through
macropores. Available
17. WATER IN SOIL
• FIELD CAPACITY:
– Moisture left in soil after gravity has drained
macropores;
• PERMANENT WILTING POINT:
– Moisture content at which a plant wilts and
does not recover, even when under a humid
environment.
• AVAILABLE WATER: FC - PWP
18. Water movement within soil
• To the root
surface:
– Soil to root:
Diffusion and bulk
flow.
– Roots growing into
moist soil.
19. Water movement in the plant
due to w
• Inside the plant:
– Osmosis: Water movement across
membranes.
– Diffusion: Effective at cellular dimensions
• Bulk flow: Important for long distance
transport via xylem
• Examples: garden hose, a river flowing, and
rain falling.
20. Water movement in Roots
• Roots hairs
• Water enters the root most readily in the apical
part of the root that includes the root hair zone
• Water transport within root, a complex process
• Three pathways from root epidermis to
endodermis
21. Root Anatomy
Root hairs are
microscopic
extensions of
root epidermal cells
that greatly
increase the
surface area
of the root, thus
providing greater
capacity for
absorption
of ions and water
from the soil.
24. Apoplastic pathway
Movement of water and
solutes through the cell walls
and the intercellular spaces
No crossing of the plasma
membrane
more rapid
less resistance to the flow of
water
At endodermis water
transport is blocked due to
Casparian strips (hydrophobic
due to wax like suberin)
25. Symplastic movement
Movement of water and solutes through the continuous
connection of cytoplasm (though plasmodesmata)
No crossing of the plasma membrane (once it is in the symplast
however, if the solute was initially external to the cell, then it must
have crossed one plasma membrane to enter the symplast)
26. Transmembrane Pathway
•Water enters a cell on one side,
exits the cell on the other side
and so on.
•Water passes at least two
membranes for each cell in its
path i.e. plasma membrane and
tonoplast are involved
29. Root Pressure
• Develops due to build up of solutes into xylem
• occurs when soil potential are high and transpiration
rates are low
• Plants produce liquid droplets on the edge of their
leaves (Guttation)
• Positive roots pressure exudates xylem xap through
hydathodes (specialized pores)
• Dew drops on the tips in grass leaves in the morning
• Guttation occurs at night when transpiration is
suppressed and high humidity into atmosphere
31. Root Pressure
• Disappears when transpiration rate is high; and
due to root pressure plants can transport water
up to 10 m (0.2 MPa= 0.02 MPa m-1= 10 m
• Inadequate to move water up to a tall tree i.e.
redwood in North America or Eucalyptus in
Australia (100 m long)
32. Cohesion-Tension theory of
Transpiration
• -ve hydrostatic pressure (tension) pulls the water
through xylem
• Develops at the surface of the cell walls in the
mesophyll
• Evaporates into the air spaces into the leaf
• Water vapors then exits the leaf through stomata and
across the boundary layer by concentration gradient
• Requires cohesion properties of water to sustain large
tensions in the xylem water column (Cohesion-Tension
theory of ascent of sap)
33. Driving forces for water transport from soil-plant-
atomsphere
-ve hydrostatic
pressure within soil
-ve hydrostatic
pressure within leaf
35. Water Potential. Flux.
Water will flow from sites of high w (close
to zero) to sites of low w (more negative):
Water moves from a wet soil, through the
plant, and evaporates (via transpiration) into a
dry atmosphere.
Soil Root Stem Leaf Air
-0.3 MPa -1 MPa -2 MPa -30 MPa
36. Transport
• Passive Transport
– Diffusion
– Facilitated diffusion
– Osmosis
– Bulk transport
• Active transport
energy mediated transport across a membrane by hydrolysis of ATP.
– Primary direct energy consumption
– Secondary indirect energy consumption, takes place through
pumps
– there is no direct coupling of ATP;
– Three main forms
• Uniport
• Symport
• Antiport
