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Chapter: Biology: Physiological Activities in Plants Nutrition


If one side of a piece of blotting paper is touched water surface, the blotting paper will gradually absorb water and becomes wet.



If one side of a piece of blotting paper is touched water surface, the blotting paper will gradually absorb water and becomes wet. We can do this experiment with a piece of newspaper too. Imbibition is also a process of water absorption. Absorption of water by dry, semi-dry colloidal substances called Imbibition.


More easily it can be said that the process of absorption of water by plant cell wall, protoplasm etc. is called imbibition. The amount of water absorbed by dry seeds during germination is done by imbibition.


The substances, which absorb water, are called Hydrophilic (water loving) substances. Hydrophilic substances contracted for shortages of water, again when it gets water become swollen by absorbing it.


It is seen that in winter the wooden doors and windows make gaps for contraction and in the rainy season they swells up by absorbing water and it become harder to open the doors and windows. It is a result of imbibition.




If a spoon of sugar dissolves in a glass of water, the sugar molecules spread throughout the whole glass of water; as a result the whole water becomes equally a sweet. If a spoon of salt is dissolved in a glass of water, it is seen that whole water becomes equally salted. Let us see what happens if some crystals of Potassium permanganate (KMn04) is dropped into a glass of water. After some times it will be seen that the whole water in the glass becomes violet. Dropping a drop of ink in the glass may do this experiment.


Actually in a crystal of sugar, salt or potassium permanganate there are any molecules compactly combined together. When the crystal is dropped in water the water molecules attract its molecules very strongly. As a result the crystal is completely dissolved in water and its molecules are spread throughout the water evenly. The molecules of sugar, salt or potassium permanganate distributed throughout the water by the process diffusion. At a constant temperature and atmospheric pressure movements of the molecule of a substance from a region of high concentration to a region of low conecentration is known as diffusion. The term diffusion means to distribute throughout.


Diffusion Pressure :


The potential ability of a substance to move from a region of high concentration to a region of low concentration at a constant, temperature end atmospheric pressure is called diffusion pressure. The diffusion pressure of gas is a gas balloon is higher than the diffusion pressure of its surrounding air. So when the balloon bursts, the gas inside the balloon spread throughout the surrounding air due to its higher diffusion pressure.


Factors Affecting Diffusion:


Diffusion of a substance depends on temperature, atmospheric pressure. Concentration of molecules of that substance and concentration of the medium.


Temperature: with the increase of temperature diffusion usually increases.


Concentration of the substrate: The rate of diffusion will be higher if theconcentration of molecules of the substrate is higher; if the concentration is less, rate of diffusion will be less.


Concentration of the medium: If the concentration of the medium, like wateror air, is more, the rate of diffusion will be less; and if the concentration is less the rate of diffusion will be high.


Atmospheric pressure: Increase of atmospheric pressure will reduce the rateof diffusion and decrease of atmospheric pressure will increase the rate. Usually temperature and pressure at a certain place remain same at a certain time, in that case the concentration of the substance and the concentration of the medium become the factors to control diffusion. If the medium and the diffusion substance are same (as in the balloon and the surrounding air) then diffusion will continue till the concentration of the two become same.

Diffusion through membrane :


If two solutions (mixture) of different concentrations is separated by a membrane then the rate of diffusion between two solutions will depend on the nature of the membrane. Usually we find three types of membrane in plants: -

Permeable Membrane: Molecules of both solute and solvent can be diffused through this membrane. Cell wall, made of cellulose, is of this type.


Selectively permeable (Semi permeable) membrane: Only the solvent molecules can diffuse through this membrane, not the solute molecules. Cell membrane is a semi permeable membrane.


Impermeable membrane: molecules of both solute and solvent are impermeable (can not pass through= can not diffuse) through this membrane. Cell wall coated with cutine or suberine is impermeable membrane.


Importance of diffusion :


Diffusion plays a great role in plant life. CO2 - necessary compound for photosynthesis is absorbed from air (atmosphere) by diffusion process. The process happens in the spongy parenchyma of leaves. Oxygen - necessary for respiration, is also intakes from air by diffusion. Movement of water and mineral salts in the plant body takes place by diffusion. Diffusion also responsible for loss of water from the aerial shoot of the plant.

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