The following points highlight the top five experiments on imbibition. The experiments are: 1. Demonstration of Imbibition Process 2. Demonstration of the Effect of Osmotic Concentration (Or Osmotic Pressure) On Imbibition 3. Demonstration of Pressure Released Due to Imbibition 4. Demonstration of Release of Heat Energy during Imbibition 5. Demonstration of Imbibition of Water by Different Types of Seeds.
Experiment # 1
Demonstration of Imbibition Process:
Experiment:
A cubical piece of a potato tuber and that of a wood are taken, their volumes (by displacement of water) arid weights are noted. These are immersed in a bottle of water (if the piece of wood floats, a sinker may be used). After 2 hours the pieces are taken out, blotted well and the final volumes and weights of each are recorded.
Results:
The difference between the final and initial volumes and that between weights for each type of material gives the amount of water imbibed by them.
Discussion:
Imbibition is a physical process of diffusion and is independent of the presence of a differentially permeable membrane which is the characteristic of all living cells.
The amount of liquid imbibed depends upon:
(a) The diffusion pressure gradient between the imbibant and the liquid imbibed, and
(b) The affinity between the components of the imbibant and the liquid.
Hence, there may be a difference in the initial rate of imbibition between a piece of wood and a potato tuber when imbibing medium is pure water because in the former case only imbibition is operative while in the latter case both imbibition and absorption by osmosis arc in action.
N.B. Imbibition is influenced by some physical conditions of the external medium such as pH, temperature, etc. The osmotic concentration of the imbibing medium also affects the rate of imbibition because this rate depends upon the diffusion pressure gradient between the imbibant and the imbibing liquid.
Imbibition differs from osmosis in that, when the colloid absorbs liquid and swells, considerable solution may be absorbed, and not just water as in an osmotic system having a semipermeable membrane. (For example, blocks of gelatine or agar may swell nearly as much in a half-molar solution of sucrose as in pure water).
Experiments may be suitably designed to study the effects of these external factors on the rate of imbibition by different types of plant materials.
Experiment # 2
Demonstration of the Effect of Osmotic Concentration (Or Osmotic Pressure) On Imbibition:
Experiment:
Graded molar solutions of NaCl within the range of 0.1 to 1.0 M are prepared. Several cubical pieces of potato tuber and wood are prepared. Each of such pieces is initially weighed or volume measured and immersed individually in petridishes containing different graded solutions.
Water control for each type of material is also kept. Final weights or volumes of the pieces for each concentration are taken after 2-3 hours and the difference in final and initial weights or volumes in each case is calculated.
A graph is plotted taking concentration as abscissa and percentage increase (or decrease) in weight or volume as ordinate.
Results:
From the curve it is observed that there is increase in percentage weight or volume in case of both potato tuber and wood but the amount, of water imbibed decreases with increasing molarity of NaCl.
In case of potato tuber (a living tissue), a point (isotonic conc.) is reached where no change in weight or volume takes place and beyond this point (hypertonic conc.) there is a decrease in percentage weight or volume but in case of a piece of wood there is no such effect.
Discussion:
In case of a piece of wood (non-living) water enters by imbibition only which is dependent upon the diffusion pressure of the imbibing medium. The addition of a solute to pure water lowers its diffusion pressure.
The decrease in diffusion pressure gradient decreases the rate at which water is imbibed and the amount of water taken up. But in case of potato tuber consisting of living cells, having semipermeable membrane, diffusion of water is mainly controlled by osmosis.
Hence diffusion of water takes place due to endosmosis so long the external solution remains hypotonic but as soon as an isotonic concentration is reached no change in weight or volume takes place. At a hypertonic concentration, weight or volume decreases due to exosmosis.
Experiment # 3
Demonstration of Pressure Released Due to Imbibition:
A. Imbibition pressure:
Experiment:
A suitable can with a lid (cigarette tin) may be suitably employed for this experiment. A few small holes are made at its bottom and are completely filled with dry gram seeds. The lid is then fixed tightly and the bottom of the can is then placed in a petridish containing water.
Results:
The lid comes off forcefully by itself after some time and the time required for this is noted.
Discussion:
The dry seeds swell on coming in contact with water below. The swelling results in development of considerable imbibition pressure which pushes the lid to come off.
N.B. Some known weight may be placed on the top of the lid and imbibition pressure required to just overcome this may be taken as a measure of imbibition pressure.
B. Measurement of imbibition pressure:
Experiment:
Some dry seeds are taken in a glass jar and a few ml of water are poured into them. The seeds are then covered with a flat disc which is centrally fitted with a graduated piston. The piston passes through a cork fitted at the mouth of the jar.
Care should be taken that the graduated piston moves easily through the cork. The top of the piston is fitted with a small disc so that different weights can be placed on it. The initial mark of the piston just above the cork is noted (Figure 5).
Results:
As the seeds swell due to imbibition after some time, the graduated piston moves upward and gives the indication of imbibition pressure.
Now weights are placed on the top of the disc to bring the piston to its initial mark. This amount of weight gives a measure of the imbibition pressure. The pressure is determined at different time intervals using different weights required to bring the piston down to the initial mark.
Discussion:
The swollen seeds exert imbibition pressure on the disc which pushes the piston upward indicating imbibition force. The rate of imbibition pressure gradually increases with time ultimately reaching a constant value.
N.B. The rate of imbibition pressure for different types of seeds may be compared with the help of this experiment.
Experiment # 4
Demonstration of Release of Heat Energy during Imbibition:
Experiment:
200gm of starch is weighed out quickly which was previously dried in an oven at 100°C., transferred to a desiccator and cooled to room temperature which is recorded. The starch is placed in a clean dry thermos-flask and 250 ml of water is immediately added.
It is then thoroughly mixed with the help of a glass rod and the mouth of the flask is closed tightly with a cork through which passes a thermometer and its initial reading is recorded. The rise in temperature by the starch water mixture is recorded after a few hours of imbibition.
Discussion:
The imbibed water is actually compressed occupying less space than when present as free water. Part of the water is adsorbed with great force and thus concentrated at the colloidal interfaces. This adsorption releases the potential energy of the system which was present before the water was placed in contact with the colloids.
The large amount of heat evolved when water is added to such colloidal materials which have been first thoroughly dried offers a demonstration of this change from potential to kinetic energy in the form of heat. Thus the surface energy within the colloid resulting from withdrawal of water and drying is released when water is again added.
Experiment # 5
Demonstration of Imbibition of Water by Different Types of Seeds:
Experiment:
This experiment may be performed with three types of seeds, viz., proteinaceous pea, starchy rice and fatty groundnut. Seed coats are removed and 4 ml of each type of seeds is taken (by displacement of water in a measuring cylinder the volume may be, measured). These are then blotted well and weighed.
Each type of seed is then taken in a beaker and equal volume of water is poured in each so that the seeds are completely immersed. The seeds are then taken out at an interval of 15 minutes, blotted properly and increase in weight is noted.
Results:
The percentage increase in weight by each type of seed is determined and plotted graphically.
Discussion:
When dry seeds are put in water after removing their coats, which render a physical barrier to absorption, water is absorbed by the seeds mainly by imbibition owing to the colloidal nature of the protoplasm of seeds.
As a result seeds become turgid and osmotically active and absorption of water is then regulated by osmotic process. The rate and amount of absorption by different types of seeds vary because of their different protoplasmic constituents which control their imbibing capacity.
Results of the present experiment show that the percentage of absorption of water is maximum in case of proteinacous seeds and minimum in case of fatty seeds. Proteins and polypeptides are hydrophilic colloids which have strong affinity for water. Cellulose and starch also absorb water strongly to the surfaces of these hydrophilic colloids.
N.B. (a) Imbibition by intact seeds leads to considerable error due to formation of water layer in the air space between the endosperm and the seed coat. In order to determine the imbibition of water by intact seeds amount of imbibition without seed coat is to be added to the amount of imbibition by the seed coat only.
(b) Effect of seed coat on imbibition can be studied by setting experiment with seeds with and without seed-coats.
(c) 0.10 of absorption of water can be studied by allowing the seeds to imbibe water at room temperature, 10°C below and above it.