1.1.    Diffusion:-
Movement of particles of a matter due to its kinetic energy is known as diffusion. Diffusion occurs always from the region of higher concentration of particles of a matter to the region of lower concentration of a matter. Diffusion of particles of one substance is independent of another substance.
1.1.1.    Factors affecting diffusion:-
1.    Temperature:- It is a directly proportional factor because temperature increases the kinetic energy of particles.
2.    Density according to Graham's law of diffusion, the rate of diffusion of a substance is inversely proportional to the sequence root of their densities.
3.    Concentration gradient:- More is the concentration gradient (difference in concentration on two sides) more will be the rate of diffusion.
4.    The concentration of medium through which diffusion occurs:- More dense is the medium, lesser will be the rate of diffusion.
Types of membrane:-
(i)    Permeable:- which allows all solutes and solvents to pass through them are called permeable membranes. eg. cellulosic cell wall, lignified cell wall.
(ii)    Impermeable:- Which do not allow any solute or solvent through them are called impermeable membranes. eg. - cutinized cell wall and suberized cell walls.
(iii)    Semipermeable:- Which allow all solvents but no solute to pass through them. eg. :- Colloidal membrane, parchment membrane, eg. membrane.
(iv)    Selectively permeable:- which allows all solvents and a few selected solutes to pass through them are called selectively or differentially permeable membranes. eg. :- all biological membranes, Plasma membrane, tonoplast.
1.2.    Osmosis:-
Diffusion of water through the semipermeable membrane is known as osmosis. Is a movement of the solvent (H2O)  from lower solute concentration to highs solute concentration, through a semi-permeable membrane. Osmosis is the movement of solvent from hypotonic solution to hypertonic solution. Its weaker solution through semi-permeable. Osmosis was discovered by Pfeffer.
Keeping in view that the membrane is semipermeable, it allows only solvent molecules, so solvent moves from its higher concentration to lower concentration.
1.2.1.    Osmotic Pressure:- Osmotic pressure is an external pressure required to stop the net movement of solvent across the semi-permeable membrane.
It is directly proportional to the amount of solute. It opposes water to enter the solution through the semipermeable membrane. The osmotic pressure of pure water is zero. Osmotic pressure is highest in leaves, lowest in roots. It is highest in halophytes, lowest in hydrophyte. 

Exosmosis and plasmolysis:-
When a cell is placed in a hypertonic solution (solution with greater concentration than another), the solution comes out from the cell into the water solution. This is exosmosis.
Due to exosmosis, the protoplasm shrinks and leaves the cell wall and thus cell becomes flaccid. This cell is called plasmolysis of the cell and this phenomenon is called plasmolysis thus exosmosis leads to plasmolysis.
Endosmosis and Deplasmolysis:-
If the plasmolysed cell (flaccid cell) is placed in hypotonic solution, water enters into the cell. Entry of water under influence of dilute solution is known as endosmosis. It makes the cell again turgid and this is known as deplasmolysis.

1.3.    Turgor Pressure (TP):-
Pressure applied by the cell content against cell wall is called as turgor pressure (TP). It is not applicable to a free solution. It is also called as Hydrostatic Pressure. The cell wall is rigid and elastic exert, an opposite pressure equal in magnitude to the turgor pressure. This pressure is called wall pressure.
WP  =  TP
Plant cell does not burst when placed in pure water due to wall pressure, but an animal cell burst when placed in pure water because wall pressure is absent due to the absence of cell wall in the animal cell.
Flaccid cell → Zero TP
Fully turgid cell → Turgor pressure  =  Osmotic pressure
The value of turgor pressure is assumed negative during plasmolysis of the cell.
1.4.    Diffusion Pressure Deficit (DPD) or Suction Pressure (SP):-
The diffusion of water in a solution depends upon the concentration of solute. When the solute concentration is high, the solvent (H2O) does not exist in to form i.e. the solvent is bounded with solute and this prevents the diffusion of water. The reduction in the diffusion pressure of water in the solution is cell over its pure state due to the presence of a solute in it is known as diffusion presence of a solute in it is known as diffusion pressure deficient. DPD is high when the concentration of the solution is high DPD and concentration of solute in the solution. 
–    The osmotic pressure also depends upon the concentration of solute and it is also directly proportional to it i.e., O. P. and concentration of solute i.e.
D.P.D.  =  O.P.  (of solution)
but however, for a cellular system DPD is affected by o.P. and wall pressure (Turgor pressure)
 DPD  =  OP  –  W.P.
for free solution DPD  =  O.P.  for the fully turgid cell, the Turgor pressure is equal to osmotic pressure. i.e.
DPD  =  OP  –  WP
DPD  =  O
Thus DPD is zero. entry of water will stop now the cell can no longer absorb water from outside. Thus DPD equates the water absorbing ability of a cell, i.e. it is called suction force or suction pressure.
–    For plasmolysed cell.
The water absorbing capability of the fully flaccid cell is maximum. It has a high concentration of solute inside the plasma membrane. Its suction pressure is maximum. i.e.
DPD  =  OP  –  (–TP)
DPD  =  OP  +  TP
Both DPD of plasmolysed cell is greater than O.P.    
The demand for water:-
Plasmolysed cell > Flaccid cell > Partially turgid cell > Fully turgid cell.
1.5.    Water Potential (\Psiw)
It is the difference in the free energy of water molecules in the solution and that of pure water at the same temperature and pressure.
\Psiw  =  – DPD
1.6.    Solute Potential (\Psis) 
It is defined as the amount by which the water potential is reduced as a result of the presence of the solute. The solute potential is also called as the osmotic potential which is always expressed in negative values. Osmotic pressure and osmotic potential are numerically equal but osmotic potential has a negative sign.
1.7.    Pressure Potential (\Psip) 
The pressure potential operates in plant cells as wall pressure and turgor pressure and usually has a positive value.
\Psiw  =  \Psis  +  \Psip  +  \Psig   +  \Psim
\Psig  →  gravitational potential
\Psim  →  matric potential
\Psig  +  \Psim  are negligible
\therefore    \Psiw  =  \Psis  +  \Psip
Where \Psiw  and  \Psis  are negative values.
Movement of water molecules:-
Higher DP →  Lower DP (Diffusion pressure)
Lower OP  →  Higher OP
Lower DPD  → Higher DPD
Higher (less negative) → Lower (more negative)
\Psiw                →    \Psi
Higher TP      →    lower TP
Hypotonic      →    Hypertonic
Lower concentration of solution → Higher concentration of the solution

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