IMBIBITION

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IMBIBITION

(1) Imbibition is a special type of diffusion where water is adsorbed by solids and

colloids causing an enormous increase in volume. Examples include the imbibition of water

by dry seeds and dry wood. The pressure that is produced by the imbibition or swelling of

wood had been used by the prehistoric man to split rocks and boulders. If it were not for the

pressure due to imbibition, seedlings would not been able to emerge out of soil. Thus

imbibition is a phenomenon of adsorption of water.

(2) Imbibition is also a phenomenon of diffusion since water movement is along a

concentration gradient. The seeds and other such dried materials have almost no water

hence they absorb water easily. Water potential gradient between the absorbent (imbibant)

and the imbibed solvent is essential for imbibition. For any substance to imbibe any solvent

or liquid, affinity between the absorbent and the solvent is also a pre-requisite.

Imbibition is due to presence of large number of hydrophilic colloids both in living and

dead cells in the form of proteins, carbohydrates such as starch, cellulose, pectic substances

etc. which have strong attraction towards water.

(3) In imbibition there is also absorption of water by hydrophilic colloids. Matrix

potential contributes significantly to water in such substances. Examples of plant material

which exhibits imbibition is the dry seeds which imbibe sufficient amount of water before

germination. Imbibition can also entrain the genetic clock that controls circadian rhythms in

Arabidopsis thaliana and some other plants. Another example is the imbibition in the Amott

test (Amott test is an empirical wettability measurement for reservoir cores in petroleum

engineering which combines two spontaneous imbibition measurements).

(4) Different types of organic substances have different imbibing capacities. Proteins

have a very high imbibing capacity than starch and cellulose. That is why proteinaceous pea

Seeds swell more on imbibition than starchy wheat seeds.

(5) Imbibition of water increases the volume of the imbibant, which results in

imbibitional pressure. This pressure can be of tremendous magnitude. This fact can be

demonstrated by the splitting of rocks by inserting dry wooden stalks in the erevices of the

rocks and soaking them in water, a technique used by early Egyptians to cleave stone blocks.

(6) For ascent of sap, a theory known as imbibition theory was proposed by Sach

(1878), who believed that water is imbibed through the cell wall materials and translocated

upwards. However this theory was not accepted by the plant physiologists.

(7) An important role of imbibition in plants is that before the absorption of water by the

roots of higher plants, imbibition of water occurs by the cell walls of the root hairs.

(8) Using thermodynamic terminology a solvent say water, moves by imbibition into a

substance (imbibant) only when its (solvent) water potential exceeds that of the imbibant. The

old term imbibition pressure is replaced by the term matric or matrix potential (},m). The

matric potential in an imbibant results primarily from absorptive forces which bind water

molecules to micelles or molecules of imbibant and is analogous to the osmotic potential of a

solution. In an imbibant, the greater the amount of water present in proportion to a given

amount of imbibant, the less negative (or higher) is the matric potential. With reference to

pure H2O, the values of matric potential are always negative like that of osmotic potential.

The sum total of matric potential and pressure potential (/p) 1s equal to water potential

(1w) of an imbibant i.e. Vw =Vm+ 4p

However if the imbibant is not confined within a definite space, no pressure (turgor or

other pressure) is involved so that the aboOve equation simplifies to

w =Pm» and here water potential is equal to matric potential.