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 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 Nat and Ch 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, v) but while osmotic potential has a negative value, osmotic pressure ( , pi) has a positive value, (Vs -m) 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 10-30
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 Entry of soil water into root.
(i) Cell to cell movement of water.
li) Living cells remain distended or turgid only by the osmotic entry of water into th
(iv)Various cell organelles like mitochondria and chloroplasts will collapse, if they are not able to maintain a proper osmotic concentration 70%
of cell water is held in vacuoles. It enters through endosmosis, due to osmotic concentration of solutes dissol
in it.
(vi) The soft organs like leav
es, flowers, fruits and young stems are able to keep themselves stretched and swollen, due todey role in the growth of radicle and plumule during the of germination seeds turgidity of their cell, which is dependent on osmosis Many play.
(ix) The stomata open and
nt movements like the folding and drooping of leaves in Mimosa are brought about by osmosis close, only in response to increase or decrease of the osmotic pressure of the guard cells, inrelation to nearby epidermal cells.
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.
