Movement of Materials in
Plants
Diffusion (random, short distance)
Water Potential, Osmosis, Turgor Pressure, Transpiration
•
Facilitated Diffusion & Active Transport (directed, short distance)
Stomatal opening, Root pressure, Soil Acidification/mineral uptake
•
Bulk Flow (directed, long distance)
Cohesion-Tension Theory (Xylem, Water)
Pressure Flow Hypothesis (Phloem, sugars etc.)
BIO 241, February 18-23. 2015
Review of Plants and Water
 Why do plants need water?
 What’s special about the structure of water?
 Movement of water through cell walls and membranes
 The concept of “water potential” (the energy of water”
Why plants need water:
 Solvent for enzymes and their substrates
 Medium in which biochemical reactions take place
 Water is required for stomatal opening
 Water provides turgor pressure
 In some plants, water is used for reproduction
 Yes, e--donor for oxygenic photosynthesis
Special Properties of Water
 Inter-molecule H-bonding
 Cohesion
 Surface Tension
 High latent heat (energy that
must be released or
absorbed during phase
change)
 Liquid at the temperatures
that prevail over most of the
Earth.
 Evaporative cooling- many
H bonds must be broken for
evaporation. This requires
energy (heat). That heat is
then carried away by the
“sped-up” evaporating water
molecules.
(–)
(+)
Can dissolve many things!
H2O molecules form “shells” around
charged solute atoms or polar parts of
molecules, carrying them into solution.
Universal Solvent?
Due to their polarity, water molecules form “shells” around charged particles
or polar portions of molecules, “carrying” them into solution.
-completely non-polar substances don’t dissolve well.
Surface Tension and
Cohesion
Thanks to the strong affinity of water molecules for each other:
Water will “flow”
down an energy
gradient.
Waterfall: high gravimetric potential
to lower gravimetric potential
Osmosis: special case of diffusion;
water diffuses across membrane from
high water potential to low water
potential. Membrane is required
(othwerwise the solutes would move).
For purposes of figuring out which way
water will move across a membrane,
Water Potential can be thought of as
analagous to “water concentration.”
+ΨG
ΨG = gravitimetric potential (= weight)
Maximum = 0 (pure water)
Most of the contributing factors REDUCE the energy of water
--- reduce its potential to “do work”
-- reduce its ability to diffuse
-- Ψp may be positive (pressurized/turgid cells)
or negative (in plasmolysed cells and xylem)
Values given are measured relative to pure water. “How much
tension would the solution in question put on pure water?”
(1 atmosphere = 0.1 megapascal = 1 bar)
Measured with a tensiometer.
• Membranes often have aquaporins (protein
channels for water movement).
• Osmosis= diffusion of water through
membrane, down energy gradient
Pure water
Cell, soil, air…
Change in pressure can be used
as an indicator of the
direction/magnitude of water
movement.
Pressure equalizes water potential
In a turgid cell.
Measuring Water Column
Tension in Xylem
“Pressure Bomb”
1. Cleanly sever a small branch with
leaves.
• The water column in the xylem,
under tension, ‘snaps’ and is
withdrawn to some interior point in
the stem. Replaced with air.
2. Seal gasket around cut stem, insert
branch into pressure chamber.
3. Pressurize chamber with inert gas.
When water appears at the cut surface,
the pressure in the chamber is equal in
magnitude to the tension on the water
column when cut.
Common method used by ecologists studying
xylem tension (water potential gradient)
typically experienced/tolerated by plants in
different environments.
Osmosis
Possible routes: Apoplast and Symplast (or a combination of the two)
Apoplast – cell walls and intercellular spaces.
-- unless specifically modified, cell walls are very permeable to water and
dissolved X.
-- passive process; osmosis/diffusion. passive = occurs without additional energy
input.
-- water moves from high water potential  low
-- Dissolved X molecules tend to move from high X concentration  low
Symplast – interconnected cytoplasm of cells.
-- to move through the symplast, water and solutes must enter the
cytoplasm by passing through a plasma membrane.
** the plasma membrane is differentially and selectively permeable **
Turgor Pressure

Support for plant leaves.

High concentration of salts and other solutes in
cytoplasm (vacuole)  cells tend to absorb water
by osmosis.

Hydrostatic pressure builds up inside plant cell.

Outward-pushing turgor pressure is equal to innerdirected wall pressure. (equal and opposite forces)

Gives a degree of rigidity to leaves and non-woody
stems.

Some plants have leaves with more
sclerenchymatous tissues for additional toughness.
(slerophyllous vegetation)

Loss of turgor pressure: occurs when gradient of
water potential reverses (higher inside cell, lower
outside cell)

 water leaves the cell  reduction in turgor
pressure  shrinkage of protoplast and wilting
Turgor Pressure & Plasmolysis
When cells are hypertonic (greater solute conc. than surroundings)
 ψcell < surrounding soln. So water diffuses into cell (osmosis)
 builds positive pressure, increasing ψcell
When cells hypotonic (lesser solute conc.), or if ψsurrounding is for some reason very
very negative, reversing ψ gradient, cells undergo plasmolysis.
Plasmolysis
Under normal “well hydrated” conditions, which has the higher water potential?
The cytoplasm, or the water in the cell wall?
Plant Water
Relations
• CO2 is necessary for photosynthesis
• When stomata are open, so that CO2 can
enter the leaf, H2O exits (diffusion of water
vapor). This is Transpiration.
• For cells to maintain turgor pressure,
transpiration must be balanced by
water uptake.
`
Concept of Water
Potential
• Pressure potential
A Water Potential Gradient exists
between soil and atmosphere.
Atmosphere ~ –500 bars (1 bar = 1 atm)
Soil ~ -0.3 – -15 bars
–500 bars
Plants (land plants, anyway) straddle the
interface between Atmosphere and soil.
Ψ gradient provides the energy for bulk flow
movement of water from roots to leaves, to
replace water lost through transpiration.
–15 bars
to
–30 bars
–3 bars
to
–20 bars
–0.3 bars
to
–15 bars
As soil water content decreases, remaining water is tightly held by soil particles
(matric potential). Surface tension increases.
 Overall water potential goes down (more negative).
 To maintain water flow, plants must maintain lower water potential in roots
(solute potential? Other force??)
In Roots: water diffuses from soil into the root as long as Ψsoil >
Ψroot . Water flows down the energy gradient…
What about mineral nutrients? Can’t just flow into plants in
solution (blocked by membrane, plus concentration gradient opposes
diffusion into roots, PLUS + ions are strongly bound to soil particles)
Modes of Transmembrane Transport
Simple diffusion
Facilitated diffusion
Active Transport
- Solute moves down its - down conc. Grad.
-against conc. or
concentration gradient
-carriers or channels
electrochem grad.
- Water moves down Water
- Aquaporins (water and
- pump (ATP-ase)
potential gradient (osmosis) neutral mols).
- ions (K+, Na+, Cl-, Ca2+)
Soil acidification: energy used to pump H+ ions out of root hairs, into soil,
against electrochemical gradient.
H+ displace other anions from negatively-charged soil particles.
Anions can then be transported into root cells by active transport, facilitated
diffusion, etc.
Cells of endodermis actively
transport ions into the
vascular tissue.
Water follows the water
potential gradient…
From the root hairs, water follows one of two main paths to the xylem: (a)
Apoplastic pathway or (b) Symplastic pathway (or a combination of the two
(transcellular).
Root pressure is the result
of an active process:
“secretion” of ions into the
vascular tissue of the root.
- Root pressure disappears
if plants kept in dark for
long period, or poisoned.
Why “secretion?”
During transpiration, water
absorption by roots is a
passive process, driven by
transpiration.
Transpiration  negative
pressure in tracheary
elements in leaves 
tension on water column
Root pressure ranges from 0.3-0.5 megapascal
(1 atmosphere = 0.1 megapascal = 1 bar)
Absorption of water and ions by roots.
• Facilitated by
root hairs.
(extension of
root epidermal
cells)
Root pressure and Guttation
Not condensation (dew), but Guttation droplets.
Guttation = water loss due to root
pressure. Water is forced out of leaves
through special openings (hydathodes)
located along the leaf margin.
Hydathode structure
Radial micellation: radial
orientation of cellulose
microfibrils.
– guard cells can increase
in length, not width.
Phototropins: blue light sensing pigments
Demonstration of Cohesion-Tension theory
Cells of the Xylem
 Conducting cells (Tracheary elements; sclerenchyma)
 Imperforate: tracheids (found in most vascular plants)
 Perforate: vessel elements (found in Angiosperms)
 Fibers (sclerenchyma, mechanical/structural support)
 Xylem parenchyma (storage of water/nutrients)
Plant cell walls have evolved features that protect the water column from
distruption by cavitation or injury.
2° wall
Perforation plates at ends of
vessel elements prevent
embolism from expanding.
Surface tension prevents water
movement through the small holes in
the pit membrane (1° wall)
Pit Membrane in conifer tracheids bordered pit pairs.
Hydraulic Lifting
Ion uptake is an active process (requires energy and cellular activity)
Inorganic nutrients are
exchanged between xylem and
phloem.
A= absorbtion
U=unloading
I=interchange
Water and assimilate pathways through a leaf.
Assimilate transport
 Translocation: movement of substances through phloem.
 Direction: from sources to sinks.
 Sources: exporters of assimilate solutes (aka
“photosynthate,” sugars)
 Photsynthesizing leaves, storage tissues.
 Sinks: importers/consumers of assimilate.
 Actively growing meristematic regions, root tissues,
developing fruits, or any plant part unable to meet
nutritional needs (remember respiration occurs in all living
cells)
Patterns of assimilate transport:
Cells in Phloem
Sieve Tube Elements (members) with protein-contianing
plastids (corn, a) and P-protein (squash, b)
Sieve tube elements and
companion cells
Note sieve plates and P-protein bodies in STEs
Sieve tube element and companion cell development.
Derived from same mother cell.
Sieve plate = cell wall with large plasmodesmata (pores).
Most contents of STE’s autodigested, but cell still living (maintained by CC)
Phloem cell morphology
Sieve tubes in transverse and long. section.
(microradiographs, film exposed to tissues of
plant that had assimilated 14CO2)
Aphids are useful!
- provide what technology can not:
Aphid stylet mouthpart penetrates phloem sieve
tube without disrupting translocation process.
Normally if sieve tube is ruptured, pressure
fluctuation dislodges P-proteins along STM
wall, which plug the sieve plate (end wall).
Aphids can be anesthetized and severed from
their stylet ()…by analyzing phloem sap
obtained through the stylet, we know:
Assimilate contents: 10-25 % “dry matter”
• sucrose (90% of dry wt.)
• amino acids
• proteins
• RNA
• hormones
• inorganic ions (Mg++, PO43-, K+, Cl- )
Translocation rate ~ 100 cm/hour
Pressure Flow
Hypothesis
Osmitically-generated turgor pressure
gradient in sieve tubes.
- Phloem loading at source: Assimilate
sucrose actively transported into sieve
tube member (STM).
-  lower water potential
-  water moves into STM from xylem
-  increased turgor pressure in STM
-  bulk flow through sieve tube in
direction of lower pressure (toward sink)
- @ sink, active transport unloading of
- Assimilate from STM, increases water
potential of STM relative to surrounding
cells. Water moves out…lower turgor
pressure in STM at sink.
Sugar beet (Beta vulgaris): apoplastic loading
Fuchsia triphylla: Symplastic Loading