Water makes up ~95% of a plant’s volume, and in non-woody tissues, 99%.
Water is important for:
•dissolving and transport of all nutrients into and around the plant body (circulation)
• cooling the plant by evaporation from surfaces
• hydrostatic structure
plants ‘wilt’ when they
lose turgor pressure:
turgor develops when water
is attracted osmotically into
cells but cell walls push back
on entering water, creating
turgor
In plants: water moves passively down potential energy gradients.
Over short distances (cell-cell), water moves mainly by diffusion along water potential
gradients established by osmosis and resultant turgor.
Over longer distances (through xylem, for instance) water moves by bulk flow,
driven by pressure gradients.
For solutions separated by semi-permeable membranes:
Osmosis is the diffusion of water through a semi-permeable membrane. More
specifically, it is the movement of water across a semi-permeable membrane from an
area of high water potential (low solute concentration) to an area of low water
potential (high solute concentration). It is a physical process in which a solvent moves,
without input of energy, across a semi-permeable membrane (permeable to the
solvent, but not the solute) separating two solutions of different concentrations.
Osmosis releases energy, and can be made to do work.
For plant cells, cells are bound by cell walls that are rigid, and resist changes in
volume. As a result, water potential of a cell (Yw) is determined not only by solute
potential (Ys), but also by pressure (Yp)!
Yw = Ys + Yp
(a) Solute potential is the tendency of water to move
by osmosis.
Solute potential inside
cell and in surrounding
solution is the same.
No net movement of water.
Solute
Cell is placed in pure water.
The cell’s solute potential is
low relative to its surroundings.
Water moves into cell via osmosis.
Pure water
Water
movement
Cell
Isotonic solution
Hypotonic solution
(b) Pressure potential is the tendency of water to move in
response to pressure.
Turgor pressure is
an important source
of pressure on
water in cells
Inside of cell
Expanding volume of cell
pushes membrane out.
Turgor pressure
Plasma membrane
Cell wall
Wall pressure
Stiff cell wall pushes back with
equal and opposite force.
Outside of cell
(a) Solute potentials differ.
This is a diagram of an
osmometer. The two sides
are separated by a semipermeable membrane.
Plant cells operate like
osmometers with walls
pushing back on water trying
to enter
Pure water
  0 MPa
Solution
P 
0 MPa
 S  1.0 MPa
Pure water
  0 MPa
  1.0 MPa
Water moves left to right—
from area with high water
potential to area with low
water potential
Flaccid cell
P 
0 MPa
 S  1.0 MPa
  1.0 MPa
Water moves into cell—from area
with high water potential to area
with low water potential
(b) Solute and pressure potentials differ.
Pure water
  0 MPa
Solution
 P  1.0 MPa
 S  1.0 MPa
  0.0 MPa
Water potentials are equal—
no net movement
Pure water
  0 MPa
Turgid cell
 P  1.0 MPa
 S  1.0 MPa
 
0.0 MPa
Water potentials are equal—
no net movement
Stomata open when turgor pressure increases
in the guard cells.
• guard cells attract water into them by lowering
their Yw
• the ‘glue’ between guard cells has been
digested away, and the remaining wall is very
stiff, causing turgid guard cells to bow out,
opening the pore.
Please see “cell water potential
worksheet” posted on our web site
Factors causing stomata to open or close:
Open
Close
Light
Lack of light (dark)
Low CO2 level
High CO2 level
Circadian rhythm
Circadian rhythm
Toxins (e.g.
fusicoccin) Hormones
(e.g. auxin)
Abscisic Acid
Humidity
How?
for opening:
1 – proton pump (H+ ATPase)
hyperpolarizes cell (Em – 180 mV)
acidifies the cell wall space
2 – potassium (K+) is taken up passively
3 – anions are taken up by H+/A- symport
4 – cell Ys and Yw becomes more negative
5 – cell takes up water, turgor goes up
for closing:
inhibit pump
open anion channels
lose solutes
lose turgor
Water moves from soil through the plant to the atmosphere --- passively --- moving
down a water potential gradient.
Over short distances, water moves cell-cell by osmosis (across semipermeable membranes).
Much of the long-distance flow is by bulk flow (through volumes without membranes)
driven by pressure gradients.
The pathway of transpirational water flow is through
the xylem, a complex tissue making up part of the vascular
bundles (veins). Phloem is the other tissue in the veins.
Xylem is a complex tissue; it contains several other tissues, each comprised of specialized
cell types.
conducting tissue – tracheids or vessel elements
parenchyma – parenchyma cells
support tissue -- fibers
When plants are transpiring, the water in the
xylem is under tension.
When transpiration is not happening (at night,
or in very humid conditions) the vascular
parenchyma in roots and stems can develop
‘root pressure’ which appears as ‘guttation’
that is, droplets of water exuding from the
ends of veins in leaves.
A ‘pressure bomb’ will measure
how much pressure it takes to make
the sap return to the petiole surface;
this is called the balancing pressure, and
is opposite in sign to the tension that
existed before the leaf was cut.