P. 1
2007 Plant water relations
Osmosis and Plant Water Relations
Osmosis
I Definition
Osmosis can be defined as the net movement of water molecules from pure water or a weaker solution
(dilute solution, with more water proportion) to a stronger solution (concentrated solution, with less
water proportion) through a selectively ( / partially) permeable membrane which separates the two
region.
Therefore osmosis may be thought of as a special type of diffusion.
A partially permeable membrane is present. Partially permeable membrane is a unit membrane that
allowing some substance to pass freely, others to pass slowly, and still others to pass hardly at all. Note
that the terms “semi-permeable” and “differentially permeable” are no longer in use.
(During osmosis, water molecules, which are polar in nature, move across the membrane via hydrophilic pores found
within intrinsic protein molecules.)
II
Free energy and water potential (, psi )
a. Free energy
Free energy includes the velocity or kinetic motions of the particles and their rotational and
vibrational energies at a certain temperature.
b. Chemical potential
Chemical potential is the free energy per mole of molecules. It is a measure of the energy content
of particles in a system and therefore of their tendency to diffuse spontaneously from one place
to another.
c. Water potential (, psi )
The chemical potential of water is referred to as water potential. For practical purposes, the water
potential of a system is defined as the differences in chemical potential of water in this system
and that of pure water at the same temperature and pressure. It is usually expressed in pressure
units (kPa or bars) by biologist.
Plant physiologists now use the term water potential when describing the tendency of water
molecules to move from one place to another. Water potential is a measure of the tendency of
water to leave a solution. A higher water potential implies a greater tendency to leave.
2007 Plant water relations
P. 2
III Components of water potential in a plant cell
In plant cells, water potential has two main components: Osmotic potential () / solute potential (s)
and Pressure potential (p).
a. Osmotic potential ( ) / Solute Potential (s)
Osmotic potential is defined as the component of water potential that is due to the presence of solute
i.e. sucrose, solute particles decrease the free energy and hence the chemical potential of the solvent
molecules. Thus in a cell system an increase in the solute concentration would lower its water
potential. It is a measure of the tendency of a solution to pull water into it, it always has a negative
value. The term “solute potential” is the new name for “osmotic potential”. Thus solute potential is
always negative (-Ve) in sign.
The osmotic potential of a plant cell is defined as the component of the cell’s water potential that is
due to the presence of solutes.
Another term osmotic pressure (OP) which is measured in a non-living system using osmometer, which contains an
artificial partially permeable membrane (see figure below). The osmotic pressure referred to the potential (theoretical)
pressure is given a positive (+Ve) sign and has a magnitude equal to solute potential. It is the pressure that must be
applied to prevent entry of water into the glucose solution. The opposite force, the solute potential, is the tendency of the
glucose solution to gain water from pure water across the membrane . The term “osmotic pressure” is no longer used in
plant studies. Instead, descriptions of solutions should be given in term of water potential. However, the term is still used
in animal studies.
The solute potential of a plant cell has following characteristics:
( i ) It is always -Ve .
( ii ) The negative value of solute potential of a solution means if a solution is separated from pure
water by a selectively permeable membrane. Water will always enter the solution as the water
potential of a pure water is higher (pure water = 0, which is greater than a negative value.)
( iii ) The value of solute potential of a solution is determined by the concentration of the solution. The
more concentrated a solution is, the lower is its solute potential (i.e. more negative) and vice
versa.
P. 3
2007 Plant water relations
b. Pressure potential (p)
When a plant cell is immersed in pure water or hypotonic solution, there is a net influx of water. As a
result, the cell swells. However unlike animal cells, the plant cell does not burst. This is because the
cell wall is stretched and develops a tension, resisting further uptake of water into the cell and
therefore further expansion of the cell.
As water enters the cell the protoplast enlarges and creates a pressure against the cell wall (the turgor
pressure, TP). In turn, a hydrostatic pressure (the wall pressure , WP) is act against on the expanding
protoplast by the cell wall. This is called the pressure potential.
(Turgor pressure is a real pressure rather than a potential one, and can only develop to any extent if a cell wall is present.)
This hydrostatic pressure is used to force water out from the expanding cell. And pressure potential is
regarded as the capacity of the existing hydrostatic pressure of a cell to drive water out of it.
The pressure potential of a plant cell is defined as the component of the cell’s water potential that is
due to hydrostatic pressure. In most cases, pressure potential is usually a positive force, then it is
positive ( +Ve ) in sign.
II. The Vaculated cells
cell
water potential
of the cell
=
s
+
solute potential
of the cell
p
pressure potential
of the cell
P. 4
2007 Plant water relations
1. plasmolysed cell
The volume of cell decreases as water flows out of the vacuole by osmosis. When the plasma
membrane starts to pull away from the cell wall. The space between the cell wall and plasma
membrane will be filled with the outside solution. This withdrawal of the plasma membrane from the
cell wall is called plasmolysis.
let
then
p
=
s
=
cell =
=
=
zero
-X bars
s+ p
(-X + 0) bars
-X bars
The point when the plasma membrane starts pulling away from the cell wall (or the plasma
membrane just touching the cell wal without acting any force on it) is called incipient plasmolysis.
Full plasmolysis is reached when the plasma membrane has completely withdrawn from the cell wall.
2. turgid cell
- since a hydrostatic pressure exerted by membrane against cell wall and the plant cell wall is
permeable to water, and is rigid and slightly elastic.
---> and equal and opposite pressure exerted by the cell wall on water molecules in vacuole (i.e.
p > 0)
cell
= s (-ve in value) + p (+ve in value)
The pressure potential reaches its maximum when the cell wall is stretched as much as it can be and
the cell cannot take in any more water. At this point, the cell is described as fully turgid, i.e. full
turgor is achieved.
3. in xylem vessels
- transpiration is pulling water up the plant
---> p < 0
cell
= s (-ve in value) + p (-ve in value)
4. cells in different solutions
- since water tends to diffuse from high cell
i.
to low cell region :
in hypotonic solution
cell <  of bathing solution around cell
---> water rush into cell by osmosis ---> swell of protoplast ---> turgid
ii. in hypertontic solution
cell >  of bathing solution around cell
---> water out the cell by osmosis ---> shrinks of protoplast ---> plasmolysis
(flaccid)
iii. in isotonic solution
cell =  of bathing solution around cell
---> no osmosis occurs