The following points highlight the top five experiments on permeability. The experiments are: 1. Effect of Temperature on Permeability (Thermal Death Point) 2. Permeability of Different Dyes through Cell Membrane 3. Experiments on Differential Permeability of a True Solution and a Colloidal Solution 4. Impermeability of Seed Coat 5. Effect of Lons on Permeability and Their Antagonism.
Experiment # 1
Effect of Temperature on Permeability (Thermal Death Point):
Experiment:
6 equal slices of beet root are cut with the help of a cork borer. These uniform slices are thoroughly washed in water to remove adhering anthocyanin pigment from the wounded cells.
The slices are then transferred to a small beaker containing distilled water. The beaker is then maintained at ice-cold temperature and then at room temperature. The temperature of the beaker is then raised by 1°C using hot water and any excretion of colour in water is carefully noted.
The temperature (recorded by a thermometer) at which red anthocyanin pigment has just started diffusing out into the outer medium is cautiously recorded.
Observation:
No diffusion of red anthocyanin pigment takes place until the temperature of the medium is raised to ‘X’ °C. (X is to be found out experimentally). On further increase of temperature the intensity of colour deepens.
Inference:
The red colour of water at higher temperature is obviously due to the diffusion of red anthocyanin pigment from the cell sap into the external medium. This diffusion of pigment takes place only when the cell has been partially or completely killed at higher temperatures and its semipermeable property is lost.
The cell membrane now becomes permeable and the temperature at which the cell loses its semipermeable nature is called ‘the thermal death point’.
N.B. The thermal death point varies from species to species and depends on the physical conditions of the materials.
Chemicals like alcohol, copper sulphate, etc., also kill the cell destroying its semipermeable nature. At a particular conc. of these substances the excretion of red colour takes, place indicating the destruction of semipermeable nature of the membrane.
Experiment # 2
Permeability of Different Dyes through Cell Membrane:
Experiment:
A solution of 0.05 M of CaCl2 (5-55gm/litre) is prepared. Equal quantities of methylene blue and neutral red dye solutions (100mg dye in 100 ml of 50% alcohol) are then poured into two beakers containing equal quantity of CaCl2 solution (the ratio of dye: CaCl2 is 1:1000). Two clean slides are taken and to each slide one drop of neutral red-calcium chloride solution is taken.
To one slide onion scales and to the other Azolla root hairs are placed. The time of penetration of the dye as indicated by the coloured cytoplasm is noted under the microscope.
Similar experiment is done with methylene blue-calcium chloride solution and the time of penetration of the dye is also noted.
Observation:
It is observed that the time of penetration in case of methylene blue is much longer than that of neutral red. (Compare the rate of penetration of dyes in the two materials).
Inference:
The rate of entrance of different non-toxic dyes is different. This differential rate is due to the molecular arrangement of dyes, their adsorptive capacity and their solubility in water and lipid substance which is one of the important components of the cell membrane. The purpose of using CaCl2 solution is to keep the physical characteristic of the membrane unaltered.
Experiment # 3
Experiments on Differential Permeability of a True Solution and a Colloidal Solution:
Experiment:
1 % iodine solution is taken in a test tube and its mouth is covered with a semipermeable membrane (parchment or egg membrane may be used). The test tube is then inverted in a beaker containing 1 % starch solution.
In another test tube 1 % starch solution is taken, covered with a semipermeable membrane and is inverted in another beaker containing 1% iodine solution (1gm I2+ 2gmKI + 300 ml distilled water).
Observation:
After about an hour the characteristic blue colour of starch-iodide complex appears in the beaker containing starch solution. But there is no change in colour in the beaker containing iodine solution. The test tube containing starch solution is also coloured blue.
Inference:
The experiment clearly indicates that the molecules of iodine pass easily through the membrane in either direction while starch, which is a colloidal solution, does not.
N.B. To prepare 1 % starch solution, 1gm soluble starch is mixed with 100 ml water, warmed until the solution is clear. To avoid contamination, a pinch of salicylic acid should be added for preservation purpose.
Experiment # 4
Impermeability of Seed Coat:
Experiment:
3gm each of castor and gram seeds is taken and weighed. The seeds are then placed in two beakers containing equal quantity of water. After 1 hour seeds are taken out, blotted well and weighed again.
Observation:
It is observed that no marked increase in weight takes place in case of castor seeds whereas considerable increase in weight takes place in case of gram seeds.
Inference:
The castor seeds having hard and waxy seed coat are relatively impermeable to water. The gram seeds having no such physical barrier are quite permeable to water.’ Hence marked increase in weight in case of gram seeds occurs.
Experiment # 5
Effect of Lons on Permeability and Their Antagonism:
Experiment:
Two stock solutions of NaCl and CaCl2 (anhydrous) of conc. 1.88% (which is more or less isotonic to cell sap) are prepared.
Six petridishes are taken and the solutions are added in the following proportions:
Equal slices of beet root of uniform thickness are cut with the help of a cork borer and these are thoroughly washed in distilled water to remove the adhering anthocyanin pigment from the wounded cells.
Four such slices are then transferred to each petridish containing the above solutions in the aforementioned proportions. Observations arc made at 5 minutes interval for half an hour to note whether any red colour diffuses into the external solution.
Observation:
Diffusion of red colour appears first in petridish (b), then in (c) and lastly in (a). No colour diffusion takes place in case of petridishes (d), (e) and (f).
Inference:
NaCl increases the permeability of the plasma-membrane to a point which is injurious to it but CaCl2 acts in a reverse way, i.e., it decreases the permeability to a point which is injurious. When these two salt solutions are mixed in a proper ratio, their combined effect on the plasma-membrane neutralizes the individual effect and the permeability remains unaltered.
Since Ca is a divalent cation (having two positive charges) and Na being a monovalent cation (having one positive charge), the effect of Na ions (increasing permeability) is counterbalanced by the effect of Ca ions (decreasing permeability) at 2 : 1 ratio of Na : Ca. Thus one Ca ion antagonises two Na ions.
N.B. The above experiment may also be performed with potato discs by weighing method. Here at the null point there will be no change in weight.