Osmosis,  Definition of Osmosis,Explanation,Types of Osmosis, Working and Result, importance of Osmosis, Osmotic Concentrations and pressure,


Osmosis is the most important physical process that brings about the movement of water in living systems as well as in their environments. 

Osmosis may  be defined as : 

Osmosis  is the most diffusion  of water molecules from a dilute solution  to the concentrated  solution  when the two are separated  by  means of a semipermeable membrane. If the solution  is separated  from it's pure solvent, the molecules of solvent move from pure solvent to the solution. 

When the two  solution  having different  osmotic concentration are separated  by means of a semipermeable  membrane, the molecules of water ( or solvent) move from the region  of its higher chemical potential  to the region of its chemical  potential . This movement of water ( or solvent) is called osmosis. 


When the two solution having different osmotic concentrations are separated  by means of a semipermeable membrane, the molecules of solvent or water move from the region of their higher diffusion pressure ( or free energy) to the region of their lower diffusion  pressure ( or free energy). This movement  of water or solvent is called osmosis.


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.

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.

Osmosis NCRT Biology: summary 

The plant cell is surrounded  by a cell membrane  and a cell wall. The cell wall is freely permeable  to water and substances in solution hence is not a barrier to movement. In plants the cells usually  contain a large central vacuole, whose contents, the vacuolar sap, contribute to the solute potential  of the cell. In plant cells, the cell membrane and the membrane of the vacuole, the tonoplast together are important determinants of movement of molecules in or out of the cell. 

Osmosis  is the term used to refer specifically  to the diffusion   of water across  a differentially_ or selectively  permeable  membrane. Osmosis  occurs spontaneously in response to a driving force. The net direction and rate of osmosis depends on both the pressure gradient  and concentration gradient. Water will move from its region of higher  chemical potential ( or concentration) to its  region of lower chemical  potential  until equilibrium  is reached. At equilibrium  the two chambers should have nearly  the same water potential. 

  You may  made a potato  osmometer in your earlier  classes stage in school. If the tuber is placed in water, the water enters the cavity in the potato tuber containing  a concentrated solution  of sugar due to osmosis.  

The  two  chambers, A and B, containing  solutions  are separated  by a semi_ permeable membrane.  

(a) Solution of which chamber has a lower water potential? 

(b) Solution of which chamber  has a lower solute potential? 

(c) In which direction  will osmosis occur?

(d) Which solution has a higher solute potential? 

(e) At equilibrium  which chamber will  have lower water  potential? 

(f) If one chamber has a Ψ  of  _  2000 kPa, and  the other _ 1000 kPa, which is the chamber that has the higher Ψ ? 

(g) What will be the direction of the movement  of water when two solutions with Ψw = 0.2 MPa and Ψw = 0.1 MPa are separated  by a selectively permeable membrane? 

Let us discuss  another experiment where a solution  of sucrose  in water  taken in a funnel is separated  from pure water in a beaker by a selectively  permeable membrane. 

You can get this kind of a membrane in an egg .Remove the  Yolk and albumin  through a  small hole at one end of the egg, and place the shell in dilute solution  of hydrochloric acid for a few hours. The egg shell dissolves leaving  the membrane intact. Water will move into the funnel, resulting  in rise in the level of the solution  in the funnel. This will continue  till the equilibrium  is reached. In case sucrose does diffuse out through the membrane, will this equilibrium be ever reached? 

 External pressure  can be applied  from the upper part of the funnel such that no water diffuses into the funnel through  the membrane. This pressure required to prevent water   from diffusing in fact, the osmotic pressure and this is the function of the  solute concentration; more the solute concentration, greater will be the pressure  required to prevent water from diffusing in. Numerically  osmotic pressure  is equivalent  to the osmotic potential, but  the sign is opposite. Osmotic pressure  is the positive  pressure  applied, while osmotic potential  is negative. 



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