Dialysis tumbling



The diffusion of the small dissolved molecules from one solution through the semipermeable membrane into another is known as dialysis.


Membranes of the cells and subcellular organelles are semipermeable. Dialysis, therefore, plays a role in the passage of dissolved materials into the cytosol from the surrounding fluid and into the organelles from the cytosol and vice versa. The membranes around the food vacuoles and the contractile vacuoles  are also semipermeable, and the  materials pass from or into these vacuoles by dialysis. Dialysis is, however, a slow process and is insufficient for the requirements of the cells. Moreover, it permits the movement of only small molecules and lipid soluble molecules through the membranes. 



A different type of diffusion is seen if a membrane is permeable to the small water molecules,  but not to the large molecules of the dissolved substances,  saybsugar.The beaker is fitted with  a semipermeable membrane, such as fish bladder or egg membrane. Its part A contains 15% sugar solution and part B pure water. Sugar solution and water stand at equal levels in the two parts. Sugar molecules, being large and unable to diffuse through the membrane, remain within the part A. The water molecules, being small and diffusible , pass through the membrane, in both directions. The rate of movement of water molecules from one side of the membrane to the other will depend upon the number of water molecules which strike the membrane in a given time period.In our experiment, more water molecules pass from the part B into the part A, because of their higher concentration ¹( 100%) in the former than in the latter ( 85%). This results in the rise in the level of sugar solution and corresponding fall in the level of water. Now the water molecules in sugar solution are more numerous and exposed to a larger area of the membrane than before. This increases the number of water molecules which cross back into part B.Moreover, pressure on the high side due to gravity, squeezes proportionately more water molecules back into the pure water side.This establishes an equilibrium between the two sides. Diffusion then occurs with equal rate in both directions through the membrane and there is no further rise in the level of sugar solution.In other words, the hydrostatic pressure of the column in part A checks the further net gain of water molecules. This happens in spite of the concentration difference between the two sides of the membrane. 

Osmosis is the diffusion of water or solvent through a semipermeable membrane from a solution of lower concentration of solutes to a solution of higher concentration of solutes to which the membrane is relatively impermeable. The solvent in all biological system is water.

1. Some water molecules are bound to solute molecules in hydration shells and lose their movement. Therefore, the concentration of unbound water molecules,  that are free to cross membrane,  causes osmosis. 

Osmotically Active Solution A solution which drawn water into it by osmosis is said to be  osmotically active solution, e.g., sugar solution in the fore_ going experiment. 

Demonstration in Cells.

Cells are osmotic systems.The cell membrane is semipermeable and water enters and leaves the cells by osmosis. This is very nicely shown by the red blood cells.They remain normal outside the blood stream if kept in an isotonic solution, on where the concentration of water and solutes is the same as in the cytoplasm ( G  isos = equal, same; tonos= tension) .The reasons for this is that water leaves and enters the red blood cells at the same rate.The solutions that are isotonic with the body fluids are used for injecting drugs into the blood. The 0.9 percent NaCI and 5% glucose solutions are isotonic for the mammalian red blood cells.

      ● If the red blood cells are placed in a hypotonic solution, in which the concentration of solutes is less and concentration of water is greater than inside the cells ( G. hypo= below; tonos= tension), say distilled water, they will take up water to equalize the water concentration, swell up and may burst. This shows the danger of intravenous injection of nonsalinated water. The burst cells leave  behind cell ghosts.

         ● If the red blood cells are placed in a hypertonic solution,one in which the concentration of solutes is greater and concentration of water is less than in the cytoplasm ( G. hyper= above; tonos= tension), say 10% salt solution, they will quickly lose water, stink and appear wrinkled. Shrinking of animal cells is known as crenation.
         It is clear from the above experiments that the cells, red blood corpuscles, may be destroyed when surrounded by  hypotonic or hypertonic solution. Survival of cells depends upon balancing water uptake and loss. 
       Tonicity: The osmotic environment around a cell is described in terms of its tonicity.Tonicity is the osmotic pressure or tension of a solution development due to the presence of solute particles in it. A Cell survived in isotonic environment, and is destroyed in hypotonic and hypertonic environments. The animal cells burst when placed in a hypotonic solution because their plasma membrane cannot withstand the high hydrostatic pressure exerted by the cell contents when swollen. For thus reason, solution that are isotonic with the body fluids are used for injecting drugs into the blood and for washing contact lenses.

For the red blood corpuscles, (i) 0.9% solution is isotonic, 0.66% is hypotonic and 1.25% is hypertonic; and (ii) 5% glucose solution is isotonic, 10% is hypertonic and 0.2% is hypotonic. 

      Red  Corpuscles in Blood .

The red blood cells neither shrink nor swell up excessively in the blood. This is because the concentration of water in the plasma is almost the same as that within the  cells. 
No doubt, water continuoues to pass through the membrane in both directions, there is no net flow because the flow rates in the two directions are equal. Indeed, this is the case with all the cells of an animal body since they are bathed in a body fluid which is in equilibrium with the cell contents.

            Passive Transport: Diffusion of a substance across a biological membrane is known as passive transport because the cell neither shows special activity nor uses energy to make it go. The concentration gradient itself represents potential energy and drives diffusion. 

 Adaptation For Countering Osmosis: Animal living in the hypotonic freshwater have certain adaptations to deal with the problem of endosmosis. Most of their body has an impermeable coating that resists water takeup. The coating consists of mucus in worms and fishes, and of chitin in insects and spiders. In addition,  these animals pass out large volumes of very dilute urine to get rid of excess water. Fresh water protozoans have contractile vacuoles that collect excess water and throw it out periodically. Many plant and animal cells are able to regulate their contents of dissolved materials and thereby control water loss or gain.They do so by creating a difference in osmotic potential across their membrane by moving solutes from one side of the membrane to the other .The control of water balance is called osmoregulation. 

Difference Between Diffusion and Osmosis  


1. It is the movement of all materials ( solid, liquid or gas) from a region of higher concentration to a region of lower concentration. 

2. It can occur both in air and water.

3. It can take place without or through a semipermeable membrane. 

4. It equalizes the concentration of diffusible molecules throughout the medium of media.


1. It is the movement of solvent ( water) molecules from the area of higher concentration to the area of lower concentration. 

2. It can occur in a liquid medium only.

3. It always takes place through a semipermeable membrane. 

4. It does not equalize the concentration of solvent molecules in the media involved. 


A plant cell placed in a hypotonic solution receives water by osmosis but does not burst  because it is surrounded by a rigid cell wall, which can withstand the hydrostatic or turgor pressure of the turgid ( distended) cell contents.The cell wall countries the turgid pressure by exerting wall pressure. This pressure stops the gain of water by a plant cell beyond a certain limit. After this, the cell wall squeezes water back out as fast as it enters.

In a plant cell placed in a hypertonic solution, the cytoplasm along with plasma membrane shrinks and separates from the cell wall as water flows out from the vacuole The vacuole shrinks but remains enclosed by the cytoplasm. This process of shrinkage of protoplast from the cell wall due to osmosis caused by a hypertonic solution is called plasmolysis.

         Stages of Plasmolysis: Gradual loss of water causes the protoplast stop exerting pressure on the cell wall. This is the beginning of Plasmolysis.It is called limiting Plasmoysis. Plasmolysis starts at the corners of the cell when it is called incipient Plasmolysis. When progressed to maximum, it is called evident Pladmolysis.Plasmolysis reduced the pressure on the cell wall so that the elastic cell wall shrinks slightly ,making the cell a bit smaller. 


If a Plasmolysed plant cell is immediately pla ced in a hypotonic solution, water enters the cell and plasmolysed cytoplasm swells up and comes in contact with the cell wall. This process is called deplasmolysis.The cell also regains it's normal size. Prolonged Plasmolysis kills the protoplast.Hence deplasmolysis can occur immediately after Plasmolysis. 

     Plant cells placed in isotonic solution do not gain water and become flaccid( limp),  and this causes the plant to wilt.

Passage of water through lipid Bilayer

Water does not dissolve readily in lipids, still it crosses lipid bilayer quickly. The is partly due to the small size of the water molecule and partly to the fact that the water molecules unique polar structure somehow permits it to pass the bilayer's hydrophilic outer sides easily.

Practical Applications of Plasmolysis.

Plasmolysis has practical applications 
(i) Addition of salt to pickles and of sugar to jams kills fungi spores and bacteria by Plasmolysis. 
(ii) Salting can check the growth of plants in wall cracks.

(iii) Salting of tennis lawns can kill the weeds.
( iv) A high concentration of chemical fertilizer should be avoided as it can damage the crop by exosmosis of water from the cells of the roots.

(v) Plasmolysis is shown by the living cells only. Therefore, its occurrence indicates that a cell is alive. 

 Forms of Osmosis.

Entry of water into the cells when surrounded by a hypotonic solution is called endosomosis.Exit of water from the cells when surrounded by hypertonic solution is termed exosmosis.
  Raisins swell up on soaking in water due to endosmosis.Surface of a sliced cucumber, when salted, exudes water due to exosmosis. In the foregoing experiments, red blood corpuscles swell up in hypotonic solution due to endosmosis and shrink in hypertonic solution due to exosmosis. Similarly, a plant cell shows Plasmolysis in hypotonic solution due to exosmosis and deplasmolysis in hypotonic solution due to endosmosis. 

    Osmotic Pressure (Osmotic Potential).

The osmotic pressure is the pressure that must be applied to a solution to prevent the passage into it of solvent when solution and pure solvent are separated by a  membrane permeable only to the solvent. Sometimes less correctly defined as the force with which the  solution draws the solvent to itself through the semipermeable membrane. The osmotic pressure is directly proportional to the concentration of the solution, i.e., the more molecules a solution has dissolved in it, the greater is its osmotic pressure. A has sugar molecules dissolved in water. Hence, it had osmotic pressure. Part B of the same figure contains pure water and no dissolved molecules. Therefore, it has zero osmotic pressure. Thus, water tends to flow from a place of lower osmotic pressure to a place of higher osmotic pressure. 

     The cell membrane maintains a balance between the osmotic pressure of the intracellular and intercellular fluids.

        Role of Osmosis I.

Cosmos is of a great importance for the plants___

(i) Roots absorb soil water by osmosis. 

(ii) Water moves from cell to cell by osmosis.

(iii) Endosmosis of water keeps the living cells fully turgid.

( iv) Leaves, flowers and young shoots are kept stretched by cell turbidity provided by osmosis.

( v)  Seeds swell up and soften due to osmotic entry of water to facilitate germination. 

( vi) Certain plant movements, such as folding and dropping of the leaves of the sensitive plant and opening and closing of stomata, result from osmosis. 

(vii) High osmotic pressure by the cells from the digestive tract by Osmosis in animals. 

II . Water is absorbed by the cells from the digestive tract by Osmosis in animals. 

Relation between Diffusion, Dialysis and Osmosis. 

The above discussion points out that dialysis and osmosis are simply two forms of diffusion. Diffusion is the general term for the movement of molecules and ions form a region of higher concentration to a region of lower concentration brought about by their kinetic energy. Dialysis is  the diffusion of the molecules of dissolved substances ( solutes) rather than of solvents through the semipermeable membranes, and osmosis is the diffusion of the water or solvent molecules rather than solute molecules through the semipermeable membranes.

Role of Diffusion. 

Diffusion plays an important role in biological systems: 
( i) It enables the cells to take up and turn out some materials. 

( ii) It brings about respiratory exchange of gases between cells and their surrounding medium. 

( iii) It spread solutes and ions throughout the cytoplasm in cells.

(iv) It keeps the cell surface moist in the internal plant tissues. 

(v) It contributes to transpiration, a necessary evil for plants.

( vi) It spreads fragrance from the flowers into the air to attract the pollinating creatures. 

( vii) It spread pheromones from certain glands in animals into the air to guide the distant members of the species. 

4. Filtration.

 Filtration is the movement of molecules through semipermeable membranes with pressure developed by extraneous factors. 

Examples It occurs in the uriniferous tubules of the kidneys.  There is a  high blood pressure  in the glomerular capillaries present in the Bowman's capsule.  The high blood pressure in the glomerular capillaries causes  a correspondingly high filtration pressure, which is normally about twice as high as that af an arterial end of an ordinary capillary bed in the tissues. The high filtration pressure exceeds the osmotic Bowman's capsule through the fine pores between the cells that form the walls of the capillaries and Bowman's capsule. The filtered out fluid is called the   glomerular or capsule filtrate.  contained sodium and Potassium ions, glucose, amino acids, urea and a large amount of water. The blood is left with only corpuscles and plasma proteins. Filtration pressure also forces water out of the blood capillaries into the tissue spaces.

Stomach               Pancreas

RNA                    Nucleic Acids


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