Plant Physiology
Water and Plant Cells
Water and plant cells
I. Background on water in plants
II. The properties of water
III. Understanding the direction of
water movement: Water potential
I. Water
• Plant cells are mostly water; 80 - 95%
of the mass of growing cells,
Wood (Sapwood 35-75%)
Seeds 5-15%
• Living cells must maintain a positive
water pressure, or “turgor” to grow
and function properly.
Cell walls
build Internal Hydrostatic Pressure
Turgor Pressure
Cell enlargement
Gas Exchange in Leaves
Transport in Phloem
Transport across membrane
Rigidity and Mechanical
stability to nonlignified plant tissues
I. Water
•
Plants lose large quantities of water in transpiration, the
evaporation from the interior of leaves through the stomata is
referred to as Transpiration.
•
•
•
•
•
•
CO2
H2O
100% water exchange in one hour
Water loss through leaf surface
Heat dissipation by leaves through transpiration
Half of the heat by sunlight is dissipated by
transpiration
Latent Heat of Vaporization
Plant - Water Relations
Corn yield as a function of water
availability
Plant - Water Relations
Productivity of various ecosystems as a function
of annual precipitation
Water passes easily through biological membranes,
particularly through Aquaporins - low resistance pores.
Formed by Aquaporins
Aquaporins:
are proteins embedded in
cell membrane that regulate
flow of water.
Prof Peter Agre and
discovery of Aquaporins?
II. The properties of water
Polar molecule that forms hydrogen bonds.
1) good solvent
2) cohesive properties - attraction to like molecules
3) adhesive properties - attraction to unlike molecules
The polarity of water molecules
results in hydrogen bonding
Covalent bond
Properties of water, continued
•Cohesion is the attraction of like molecules (H2O
here) that gives water its tensile strength.
•Adhesion is the attraction of unlike molecules.
Water adheres to cell walls, soil particles, glass
tubes, etc.
Adhesion explains capillarity & surface tension.
• Surface Tension is caused by cohesive forces
within liquid molecules.
All of above forces give rise to a phenomenon
called Capillarity (the movement of water along a
capillary tube)
III. What factors determine the direction of water
movement (through the soil, between cells, from roots to
leaves, from leaves into air)?
1. Gravity
2. Pressure
3. Concentration
100
80
water flows upward
in trees.
70
How does this work?
90
60 Height,
meters
50
40
30
20
10
Gravity causes water to move
downward unless it is opposed
by an equal and opposite force.
0
Pressure
Water moves from regions of higher to lower pressure
garden hose
straw
through xylem of plants
Water moves from higher to lower pressure
Water pressures in plant cells can be positive (turgor),
or negative, (tension).
Living cells ≥ 0 MPa to ≈ +3 MPa)
Dead xylem cells ≤ 0 MPa, to as low as -12 MPa.
3) Concentration
Water moves by diffusion from regions of higher to lower
water concentration.
Solutes added to pure water dilute the water concentration.
Osmosis is the diffusion of water across a selectively
permeable membrane from a region of higher to lower
water concentration.
How does reverse osmosis purify water?
A process by which a solvent passes through a porous membrane in the
direction opposite to that for natural osmosis.
The concept of water potential, Y, brings together the
influences of gravity, pressure, and concentration (solutes)
in describing the energy state of water and the direction
of water movement.
The water potential equation:
YW = Y S + Y P + Yg
YW = total water potential
YS = solute potential
YP = pressure potential
Yg = gravitational potential
All units will be pressure, pascals, Pa.
MPa is megapascal, 106 Pa
Change in water status causes physiological changes
Measure of the rate of passage of
CO2 entering or water vapors
exiting through stomata
Further readings
• Water and Plant Cells, Chapter 3, Plant Physiology by
Taiz and Zeiger