by salting tennis lawns, the weeds can be killed due to permanent plasmolysis and consequent death of their cells.
Plants are not allowed to grow in the cracks of the walls by the method of salting. Salting of pickles, meat and fish
and sweetening of the jams and jellies with sugar, kill the spores of fungi and bacteria.The swelling up of a plasmolyzed protoplast, under the influence of hypotonic solution is called deplasmolysis. It is
possible only immediately after plasmolysis, due to endosmosis.
Osmotic pressure (OP)
Osmotic pressure of a solution is equivalent to the pressure, which must be exerted upon it, to prevent the flow of solvent
into it, across a semipermeable membrane. It is usually measured in pascals, Pa or bars or atmospheres.
The osmotic pressure of a solution largely depends upon the ratio between concentration of solute and solvent particles in
a given solution. for example, a molar solution of sucrose has an osmotic pressure of approximately 22.4 atmospheres at
0°C. However, a molar solution of sodium chloride has almost twice the osmotic pressure of a molar solution of sucrose.
This is because, sodium chloride is an electrolyte, so it dissociates almost completely into Na+ and Cl– ions (giving twice
the particle number), whereas sucrose or glucose molecules are nonelectrolytes, and do not dissociate in water.
Osmotic pressure is numerically equal to osmotic potential (= solute potential, ys) but while osmotic potential has a
negative value, osmotic pressure (p, pi) has a positive value, (ys = – p).The instrument used for measuring osmotic pressure is called osmometer, e.g., berkeley and Hartley’s osmometer,
Pfeffer’s osmometer.
aquatic plants have an osmotic pressure of 1-3 atm, mesophytes 5-15 atm, while in xerophytes, it lies between 10-
atm., but goes upto 60 atm. during drought conditions. Halophytes have the maximum osmotic pressure with Atriplex
confertifolia showing an OP of 202.4 atm.
Factors influencing osmosis
● Presence of imperfect semipermeable membrane disturbs osmosis.
● Concentration of dissolved solute on the both sides of semipermeable membrane also influences osmosis.
● Osmosis is also influenced by differences in pressure.
Importance of osmosis
Osmosis is responsible for:
(i) entry of soil water into root.
(ii) Cell to cell movement of water.
(iii) Living cells remain distended or turgid only by the osmotic entry of water into them.
(iv) Various cell organelles like mitochondria and chloroplasts will collapse, if they are not able to maintain a proper osmotic
concentration.
(v) 70% of cell water is held in vacuoles. It enters through endosmosis, due to osmotic concentration of solutes dissolved
in it.
(vi) The soft organs like leaves, flowers, fruits and young stems are able to keep themselves stretched and swollen, due to
turgidity of their cell, which is dependent on osmosis.
(vii) Osmosis plays a key role in the growth of radicle and plumule during the germination of seeds.
(viii) Many plant movements like the folding and drooping of leaves in Mimosa are brought about by osmosis.
(ix) The stomata open and close, only in response to increase or decrease of the osmotic pressure of the guard cells, in
relation to nearby epidermal cells.
(x) a high osmotic pressure has been found to protect the plants against drought and frost injury.
(xi) Seeds and spores are similarly able to pass through the unfavourable periods, due to high osmotic pressure.