Interrelationship of OP, TP (wP) and DPD (SP)
The difference in the concentration of solutions, on two sides of a semipermeable membrane, results in the flow of
water from solution of low concentration to solution of higher concentration. When a cell is placed in a hypotonic
solution, water enters into the cell and the turgor pressure of the cell increases. as a result of increase in TP, the wall
pressure of the cell also increases (equal in magnitude but opposite in direction).
Here, the value of diffusion pressure deficit or suction pressure (actual force responsible for entry of water into cell)
will be equal to osmotic pressure minus wall pressure.
i.e., DPD (SP) = OP – WP (as WP = TP) hence, DPD = OP – TP
In a flaccid cell, the TP is zero, therefore, DPD = OP – 0 = OP
Thus, DPD will be equal to OP in case of flaccid cell, and the water will enter the cell with a force equal to the OP of
the cell. When a flaccid cell is placed in water, endosmosis starts as a result of which, OP of the cell starts decreasing,
while TP starts increasing. a stage comes, when OP and TP become equal, so DPD becomes zero.
Now, the cell is said to be in fully turgid condition.
In a turgid cell, the value of OP = TP i.e., DPD = OP – TP
So, DPD = 0
Thus, there will be no absorption water by the cell in a fully turgid condition.
DPD or SP or suction pressure determines the direction of movement of water (water always moves from a cell of lower
DPD to a cell of higher DPD), e.g., if two cells, a and b have DPD 5 and 6 atm., water will flow from a to b.
Reverse osmosis
reverse osmosis is the expulsion of pure water from osmotically active solution, through a semipermeable membrane,
under the influence of pressure higher than the osmotic pressure of the solution. It is used in removing salts from
saline water as well as in greater purification of water.AbSORPTION OF wATER
Land plants absorb water mainly from the soil. The soil plays important role in water storage, and serves as a
medium for root growth. It is also a reservoir of mineral nutrients and provides anchorage for plants. The ultimate
source of water for the soil is rain or irrigation. a part of rainwater does not enter the soil, but is drained away from
soil-surface, along the slope and is called run-away water or run-off.
Water in the soil is known as soil water or soil solution. The total amount of water present in the soil is called holard.
The amount of water available to the plants is called chresard (available water). The amount of water which cannot
be absorbed by the plant is called echard (unavailable water).
Water that occurs freely in the pervious rocky matter is called ground water. The upper layer of ground water is
called water table, where soil is completely saturated with water and air is excluded. Very few plants can send their
roots upto the fringe of the water table, due to deficiency of air. The plants which do so, are called phreatophytes,
e.g., Populus deltoides.
Water is present in the soil in five forms– capillary water, gravitational water, hygroscopic water, combined
water and water vapours.
Capillary water is present in soil narrow spaces or micropores of soil (diameter ≤ 20 mm), held by capillary forces.
Only capillary water is available to plant roots for absorption. The optimum or maximum amount of water retained,
per unit dry weight of soil, after the stoppage of gravitational flow is called field capacity. Soil moisture, beyond
field capacity, produces water logging.
If soil water is not replenished from time to time, a stage is reached when the plants growing in it become permanently
wilted and die. It is known as permanent wilting percentage (PWP) or permanent wilting coefficient (PWC).
after a heavy rain, the surface layer of soil is temporarily saturated by water, which gradually percolates down, under
the influence of gravity. Thus, gravitational water is not readily available to the plants and penetrates soil, below
upto the reach of the roots.