Imbibition,Diffusion, Osmosis



It is the absorption of water by the solid particles without forming a solution. Solid substances are called imbibants. The liquid (usually  water) is known as imbibate. The imbibate is held in between   and over the surface of particles of imbibant through the processes  of capillary  and adsorption. Plant imbibants are hydrophilic colloids. Imbibition increases the volume of the imbibant but the increase is less than of heat (heat of wetting). The swelling imbibant also develops a pressure called imbibition pressure (matric potential).It is the maximum  pressure which can develop in an imbibant when it is brought in contact with water without  allowing  the former to swell up. It is usually  very high.  Imbibition  capacity  is maximum  in phycocolloids followed by proteins,  starch and cellulose. Lignin is hydrophobic. It, therefore, does not show imbibition  of water.  Imbibition  is influenced by a number of factors like texture ( looseness  more and compactness less imbibition) of the imbibant, temperature ( rises with rise of temp), pressure  ( decreases if pressure  is against imbibant), electrolytes ( decreases) and pH  ( decreases or increases depending  upon charge of imbibant). 


(i) Absorption of water by young  cells and germinating seeds is mostly through imbibition.  If it were not for the pressure  due to imbibition, seedlings would not have been able to emerge out of the soil into the open. 

(ii) Breaking of the seed coat in germinating seeds is due to greater imbibitional  swelling  of the seed kernal ( starch and protein) as compared to seed coverings  ( cellulose). 

(iii) Imbibition pressure developed  during germination  of seeds and spores can break pavements. 

(iv) Jamming  of wooden frames during rains is caused by swelling of wood due to imbibition.  

(v) Previously, it was used in breaking  the rocks. Wooden pegs were inserted in rocks. They were  then made wet. The wooden pegs developed huge imbibition  pressures that could break the rocks. 

(vi) The hydrophilic colloids of young cells serve as the chief defence against dessication in dry environment. They absorb sufficient water and reduce the  portion of free water in the cell sap. 

(vii) The water economy of resurrection plants of Selaginella and lichens is dependent  upon the presence of hydrophilic colloids. They can absorb water from slight downpour and remain air dry for considerable periods. 


It is the tendency on the part of molecules, atoms, ions, etc., of gases, liquids, and solids to  get evenly distributed  throughout  the available  space on account of their random kinetic motion. It is the movement  of the particles  of different  substances from the region of their higher  concentration, free energy or diffusion  pressure  to region of their low concentration,  free energy or diffusion pressure. 

        Diffusion of particles  of one substance  is independent  of the diffusion of particles  of another substance, provided the  two do not react. It is  known as independent diffusion. The diffusion pressure  of the individual  substance is then known as partial pressure.  In case of non_ electrolytes, it is the concentration gradient which determines their diffusion. In  case of ions, diffusion is dependent  upon electrical gradient across, the membrane  as well as chemical gradient. Electrical and chemical gradients together  constitute electrochemical gradient. 

Diffusion Pressure ( D.P.): 

The pressure exerted by the tendency  of the particles to diffusion from the area of its higher concentration  to the region of its lower concentration  is called diffusion pressure. It is proportional  to the concentration  of diffusing  particles.

Factors Influencing  Diffusion: 

(i) Density: Rate  of diffusion of a substance is inversely  proportional  to square  root  of its relative density ( Graham's  Law). 

(ii) Permeability   of Medium : Rate of diffusion decreases with density  of the medium. It,  however,  does not influence  the final  equilibrium. 

(iii)  Temperature: A  rise  in temperature increases the rate of diffusion with Q10 = 1.2 _ 1.3. Because of it sugar crystals do not  dissolve  easily  in ice_ cold water while they do so easily in warm water. 

(iv) Diffuion Pressure  Gradient : Rate of diffuion is directly  proportional  to the difference  of diffuion pressure  at the two  ends of a system and inversely proportional  to the distance between  the two. 


(i) It keeps the cell walls of the internal plant tissue moist. 

(ii) It is a means  of spreading  of ions and other substances  throughout  the protoplast. 

(iii)  Transpiration  is a diffusion  process. 

(iv) Exchange of gases ( CO2  and O2) between  the plant interior and outside air occurs through  diffusion. 

(v) Osmosis is a special type of  diffusion  in which  water  diffuses through  a semi_ permeable  membrane. 

(vi) Aroma of flowers is due  to diffusion  of volatile  compounds. 

(vii) Uptake  of minerals  is partly controlled  by diffusion.

(viii) The exchange  of gases during  respiration  takes place through  diffuion. 


Depending  upon their permeability, membranes are of four types __ impermeable, permeable, semipermeable and selectively  ( or differentially) permeable. Impermeable membranes do not allow the  passage of solute or solvent, e.g.,cutinised and suberised cell walls. Permeable  membranes allow the passage  of both solute and solvent, e.g., cellulose cell walls. Semipermeable membranes allow the passage of solvent molecules but prevent the movement of solute particles, e.g., animal bladder. Selectively  permeable membrane is normally semi permeable but allows selective passage of solutes through it. Plasma membrane  and tonoplast are selectively permeable membranes.

Differences between Semi Permeable and Differentially Permeable Membranes 

(A) Semi Permeable Membrane

1) It allows passage of only solvent molecules. 

(2)  Inward and outward permeability  is the same. 

(3) No biomembrane is perfectly  semi permeable. 

(4) It does not have any specific  sites  for passage of materials.

(5) Its maintenance does not require constant  expenditure  of energy. 

(B) Differentially Permeable Membrane

(1) It allows passage of solvent molecules  as well as some selective solute particles. 

(2) There is a difference  in the inward and outward permeability. 

(3) All living  membranes are differentially permeable. 

(4)  It possesses specific  sites for passage of specific  particles. 

(5) Its  maintenance  requires constant  expenditure  of energy. 

Definition of Osmosis 

(i)  Diffusion of water from  its  pure  state   to solution  or dilute  solution  into  a stronger solution  when the two are separated  by a semi permeable  membrane; 

(ii) The movement  of water from  its  higher chemical potential  to its  lower chemical potential  through a semi permeable  membrane and 

(iii) It is movement  of solvent or water molecules from the region of their higher diffusion  pressure  or free energy  to the region of their lower diffusion pressure  or  free energy  across a semi permeable  membrane. 

 A solution which can cause an osmotic entry of water into it is said to be osmetically active  solution. 

Explanation : 

Solute particles  decrease  the chemical potential  of water. The number of free and active water molecules decreases  according  to the strength of solute particles. A semi permeable membrane the semi permeable membrane on both the sides  and pass through  the same. 

 Since more free water molecules  are present  on the side of pure water, A, more of them pass through  the membrane to enter the solution B as compared  to the reverse flow. 

Types of Osmosis 

Osmosis  is of two types  _ 

(i)  endosmosis _ the osmotic entry of water into a cell,  organ or system 

(ii) Exosmosis __ The osmotic withdrawal  of water from a cell, organ or system.

Demonstration  of Osmosis 

Thistle Funnel Experiment 


Take a thistle funnel   having a long stem. Close the mouth of the funnel with membrane  of animal bladder or parchment paper.

Pour 10% sugar solution in the thistle funnel till it stands at about  1/3  of the height of the stem. Now dip the converd end of the thistle funnel in a beaker containing  water. Mark the level of sugar solution  as A. 

Note that after a few minutes  the level of the sugar  solution has risen in the  stem  of thistle funnel to a point B while the level of water  falls  down in the beake. Taste the water  of the beaker. It is  not  sweet. 

Results : 

The rise of sugar solution  in thistle funnel  can only be due to the  entry  of water into it through  the  animal bladder. But no sugar has   gone out  into  the water of the beaker as it  does not taste sweet. The experiment, therefore, proves that 

(i) Animal bladder or parchment paper is a semipermeable membrane.

(ii) Sugar solution is an osmetically  active solution. 

(iii)Water diffusion into solution when the two are separated by a  semipermeable membrane. 

Osmotic Concentrations 

A solution having a low osmotic concentration (  and  low  osmotic pressure  but less negative solute  potential) as compared to  another solution is known as hypotonic solution. A solution having high osmotic concentration ( and high  osmotic pressure but more negatively solute potential) as compared  to another solution  is termed as hypertonic solution. The two solution with the same concentration  are termed as isotonic solutions. 

Importance  of Osmosis  

(1) Entry of soil water  into root 

(2) It performs  cell _to_cell movement of water. 

(3) Living cells remain  turgid by the osmotic  entry of water.

(4) Various  cell organelles  will collapse if they fail to maintain a proper osmotic concentration. 

(5) 70% of cell  water  is held  in vacuoles. It is kept  in its place due to osmotic concentration of solutes dissolved in it. 

(6) The soft organs like leaves, flowers, fruits and young  stems are kept stretched and swollen  due to turbidity  of their cells brought  by osmosis. 

(7) It plays a key role  in the growth of radicle and plumule during the germination of seeds. 

(8) Many  plant movements like the folding and drooping  of leaves  in Mimosa are   brought  about  by osmosis.

(9) The stomata open and close due to the process of osmosis.

(10) A high osmotic pressure  protects the plants against  drought and frost injury.  Seeds and spores are able to pass through  the  unfavourable  periods due to high osmotic pressure. 

Osmotic Pressure 

It is  the maximum  pressure  which  can develop  in an osmetically  active solution  when  it is separated  from its pure solvent  by a semipermeable membrane. The apparatus  used to measure  it is called osmometer (e.g., Pfeffer's  Osmomter). In some  osmometers,  osmotic pressure  is measured  as minimum  force  required to stop osmotic entry of water into a solution  e.g., Berkeley and Hartley's  Osmometer.It  is  influenced  by temperature  ( increases  with rise in temperature), solutes ( directly  proportional to  the number  of particles) and pressure  ( decreases  osmotic pressure  if applied against  the same).

  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 under  drought conditions. Halophytes have the maximum  osmotic pressure.



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