06 Plant Water Relation - part 04 - Absorption of water by roots from soil

Absorption of water by roots from soil:

Root hair absorbs water by employing three physical processes that occur sequentially- viz.

1. Imbibition:

Imbibition is swelling up of hydrophillic colloids due to adsorption of water.
Substance that adsorbs water / liquid, is called as imbibant and water/ liquid, that gets imbibed is called as imbibate.
The root hair cell wall is made up of pectic compounds and cellulose which are hydrophillic colloids.
During Imbibition, water molecules get tightly adsorbed without the formation of solution.
Imbibition continues till the equilibrium is reached. In other words, water moves along the concentration gradient.
Imbibition is significant in -

2. Diffusion:

Diffusion means to disperse.
Diffusion can be defined as the movement of ions/ atoms/ molecules of a substance from the region of their higher concentration to the region of their lower concentration.
The movement is due to the kinetic energy of the molecules.
Diffusion continues till an equilibrium is reached. Thus, water passes into the cell by diffusion through a freely permeable cell wall.
Water is now at the interface of cell wall and plasma membrane.
Diffusion results in the diffusion pressure (D. P.) which is directly proportional to the number of diffusing particles.
Diffusion pressure of pure solvent (pure water) is always more than the diffusion pressure of solvent in a solution.
The difference in the diffusion pressures of pure solvent and the solvent in a solution is called Diffusion Pressure Deficit (DPD) or Suction Pressure (SP).
The term was coined by B.S. Meyer (1938). Now a days, term water potential is used for DPD.
In colloquial language, the term DPD is actually the thirst of a cell with which it absorbs water from the surroundings.
Water arround cell wall has more diffusion pressure than cell sap. Due to this, water moves in the cell by diffusion.
Diffusion is significant in plants -

3. Osmosis :

It is a process by which water enters into the cytoplasm of the root hair cell.
Osmosis is a special type of diffusion of solvent through a semipermeable membrane.
The cytoplasm of root hair cell contains minerals, sugars, etc.
In other words, solution inside the cell is more concentrated (stronger) than outside the cell (weaker).
Therefore, solvent from weaker solution enters into cytoplasm (i.e. to stronger solution) of cell through a semipermiable plasma membrane.This migration of solvent is called Osmosis.
Thus, water at the interface of cell wall and plasma membrane, enters into the cytoplasm of the root hair cell due to osmosis.
With respect to the concentration and osmotic migration, three types of solutions are recognized viz -

i. Hypotonic (weak solution or strong solvent) :

ii. Hypertonic (strong solution or weak solvent) :

iii. Isotonic :

Having such a concentration of solution where there is neither gain nor loss of water in an osmotic system.
In other words, concentration outside and inside the cell is same.

Types of Osmosis :

Exo-osmosis :

Endo-osmosis :

It is the migration of the solvent into the cell.
It causes turgidity of cell i.e. cytoplasm becomes turgid. Turgidity increases the turgor pressure (T. P.) of the cell.

Turgor pressure:

T. P. is the pressure exerted by turgid cell sap on to the cell membrane and cell wall.
In a fully turgid cell, DPD is zero.

Wall pressure :

Cell wall being thick and rigid, exerts a counter pressure on the cell sap. This is called Wall pressure (W. P.).
In a fully turgid cell, T. P. = W. P. but operating in opposite direction.

Osmotic pressure (O. P.) :

The pressure exerted due to osmosis is osmotic pressure.
Osmotic pressure is a pressure of the solution, which is required in opposite direction, so as to stop the entry of solvent molecules into the cell.
More simply, osmotic pressure of a solution is equivalant to the pressure which must be exerted upon it to prevent flow of solvent across a semipermeable membrane.
Therefore, D. P. D. = O. P. - T. P. ( T. P. = W. P.)

D. P. D. = O. P. - W. P.

Facilitated diffusion :

The passive absorption of solutes when mediated by a carrier, is called Facilitated diffusion.
Particles that are lipid soluble can easily diffuse through lipoproteinous cell membrane.
The diffusion of hydrophilic solutes has to be facilitated because their diffusion across the membrane is difficult.
Membrane proteins provide such sites for facilitated diffusion. These proteins are aquaporins and ion- channels.
These proteins help move substances across membranes without the expenditure of energy.
Concentration gradient must be present for the molecules to be diffused through facilitated diffusion.

Do you know ?
Improtance of T. P. :

Improtance of Osmosis :

It is responsible for absorption of water into root.
maintains turgidity of cell
facilitates cell to cell movement of water.
offers resistance to drought, frost, etc.
also helps in the drooping of leaflets and leaves in vicinity of "touch me not" plant.

Yogy's biology