SOME PHYSICAL PROCESSES OCCURRING IN LIVING ORGANISMS: MOLECULAR MOVEMENT

Experiment 4—To show Gaseous Diffusion

A FEW drops of bromine are poured into a gas jar, a plate is put over the mouth and the jar is shaken until it is full of the brown vapour. On partly removing the plate and tilting the jar, the vapour will stream out and can be poured into a vessel below, showing it to be denser than air.

The jar, closed by the plate, is now stood mouth upwards, and an inverted jar similar in size should be stood upon it. The plate is slid away and the two jars left for about ten minutes, when it will be seen that brown vapour now occupies the lower part of the upper inverted jar. In time it will fill the jar completely, showing that the vapour, although denser than air, is in a state of motion and moves in all directions. This movement is termed gaseous diffusion. The higher the temperature, the more rapidly will the vapour move or diffuse of its own accord from place to place. Moreover, it will tend to diffuse to places where its concentration is less, until it occupies all the available space, and the concentration throughout this space is uniform.

Experiment 5—To show Liquid Diffusion

Two hundred cubic centimetres of distilled water are poured into a tall narrow gas jar. Distilled water must be used as copper sulphate produces a precipitate with tap-water unless it is very soft. One hundred cubic centimetres of a saturated solution of copper sulphate are allowed to run from a pipette, slowly, below the water. The surface dividing the solution from the water should be clearly seen. The jar should now be left undisturbed for a long period. When examined later it will show a gradual ’shading off ’ of colour from the deepest at the bottom to colourless at the top. Much later it will be seen to exhibit uniform colour, illustrating the fact that the coloured copper sulphate particles 2l have diffused of their oion accord until their concentration is uniform throughout the liquid. This experiment shows clearly that dissolved particles are in a state of motion and diffuse from those parts of the solution where there is a high concentration to other parts where the concentration is lower.

OSMOSIS

Experiment 6—To show Osmosis A circular piece of cellophane is tied tightly around the top of a thistle funnel and the joint waxed with melted Faraday wax to make it watertight. This is inverted in a beaker of distilled water, being held by a clamp, and is filled with a saturated solution of copper sulphate, a special filler being used. The funnel is adjusted until the levels of the liquid inside and out are the same.

After a time the level inside will be seen to have gone up. Therefore the volume of liquid inside has increased. This can only be due to the addition of water, which can get into the funnel only by passing through the membrane of the pig ’s bladder. This movement of water is called osmosis, and may be defined as ’the movement of water through a membrane into a solution, or from one solution into another of higher concentration. ’ Upon repeating the above experiment, using a saturated sugar solution instead of copper sulphate, the level will be found to rise more slowly, but sugar will not appear in the beaker. Such a substance as sugar which, when in solution, causes osmosis to occur but does not itself diffuse through the membrane, is called an osmotic substance. The membrane is said to be semi-permeable, since it will allow water to pass through it but not the sugar. The height to which the solution rises in the funnel is a measure of the osmotic pressure of the solution.

In living cells the outer layer of the protoplasm, e.g. the ectoplasm in Amoeba or that which lines the cell wall in plant cells, acts as a semi-permeable membrane. Osmotic substances occur in the endo-plasm or in the vacuoles of plant cells. The living semi-permeable membrane of protoplasm, however, can control or vary the degree of permeability which enables both osmosis to occur and at the same time diffusion of solutes in and out of the cell by restricting the passage of some solutes and allowing that of others.

EXPERIMENTS ON OSMOSIS IN LIVING CELLS

Experiment 7—To show the Absorptio?1 of Water by Osmosis in Living Cells A few filaments of Spirogyra are placed in a drop of calcium chloride solution or in a drop of artificial sea-water on a microscopic slide and observed under the microscope. The strong solution outside the cells withdraws water from them, and the protoplasm lining each cell wall and sur rounding the central vacuole will be seen to shrink away from the walls towards the centre of the cell. The cells are now said to be plasmolysed. The filaments are then placed in a drop of tap-water and again watched. Water will now pass into the cell sap of the vacuole by reason of the solutes in the cell sap producing osmosis into the cell. The protoplasmic contents of the cell will be seen to expand until the vacuole regains its former size and the protoplasmic lining once again presses up against the cell wall. or cells from holly berry skin, can be used instead of Spirogyra. A drop of freshly drawn blood is mixed with a little salt on a slide and observed under the microscope. The red corpuscles will be seen to shrink and crinkle as water is withdrawn from them by the strong salt solution outside.

If, on the other hand, a drop of blood is mixed with a little distilled water and observed, the red corpuscles will be seen to swell up and burst owing to water passing into them by osmosis.

In the foregoing experiments the cells of Spirogyra do not burst owing to the restraining cell wall of cellulose. This is relatively inelastic, and when stretched by the entry of water into the cell by osmosis produces a state of rigidity in the cell called turgor.

EXPERIMENTS TO DEMONSTRATE TURGOR IN PLANT CELLS

Experiment

Some thin slices of potato are placed in water and left overnight. Some similar slices are placed in strong calcium chloride solution and also left overnight. Next day the first slices will be found to be stiff and not readily bent, while the second ones will be found to be limp. In the first case the cells are turgid, I.e. in a state of turgor, being ’blown up ’with water entering by osmosis ; in the second the cells are ’deflated ’by water passing out of them by reversed osmosis.

Experiment

A flower stalk of a dandelion is split lengthwise into thin strips. These will curl outwardly. If placed in distilled water the curling will become more pronounced owing to the absorption of more water by osmosis with increase in the turgor of the cells. The strips are then placed in strong calcium chloride solution, when they will be seen to curl rapidly in the opposite direction as water is withdrawn from the cells, causing shrinkage of the cell walls.

Experiment 10—To show that the Living Protoplasmic Lining beneath the Cell Wall is a Semi-permeable Membrane

Three thin slices of raw beetroot are washed to remove any red sap exuding from cells damaged by the cutting.

One is placed in a basin of tap-water, the second in a basin of water which is boiled for two or three minutes, while the third is placed in a basin of water with a few drops of chloroform. The basins are then set aside for half an hour or so. It will then be found that the slice in the first basin remains turgid and that little or no red sap has escaped into the water. The other two slices become limp and the water becomes coloured red by the sap which has oozed from them. In both these latter cases the protoplasm has been killed and its semipermeable nature destroyed.

The erect position of herbaceous plant stems depends largely on the turgor of its cells. This turgor can be lost in each of the following ways :—

By loss of water by evaporation. Cf. wilting of herbaceous stems on a hot summer ’s afternoon when the roots cannot supply water fast enough.

By placing the cut end of a stem in strong salt solution. Cf. the damage done to plants by sea-water in the fields of Holland owing to the breaching of the dykes during the war.

By immersion in boiling water. Cf. the limpness of the stalks and leaves of boiled vegetables due to the death of the protoplasm.

RATES OF DIFFUSION

Experiment 11—To show Rates of Diffusion of Solutes

To show that substances in solution diffuse at different rates, a dialyser is set up. This includes a frame of two gutta-percha rings, one fitting into the other, so that a sheet of parchment paper may be stretched tightly to make a

Supplied by Messrs Philip Harris. base. The frame is stood on a petrie dish or other suitable support, standing in an outer vessel of distilled water. The petrie dish raises the under side of the parchment away from the outer vessel. A solution containing 1 per cent. sodium chloride, 5 per cent, glucose and 1 per cent, soluble starch, is poured into the dialyscr. At intervals of two minutes the frame is slid sideways and a few drops of the liquid underneath are removed with a dropping tube. These are tested for chloride, using silver nitrate solution ; glucose, using Fehling ’s solution ; starch, using iodine solution. After having discovered how long it takes for detectable quantities of salt and sugar to get through, the third test need not be done until the end of the school period. The results should show that the rate of diffusion decreases as the size of the molecules increases.

PROPERTIES OF DISSOLVED SUBSTANCES

The solution formed when a substance is dissolved is one of two kinds, crystalloid and colloid. A crystalloid solution is formed by a substance which readily dissolves, is transparent, and when not too concentrated, does not greatly affect the physical properties of the solvent. On boiling away the solvent, the substance, if solid, can be obtained once more in a crystalline form. Such a solution is formed when an acid, an alkali or a soluble salt is dissolved in water. Sugar solutions are also included. A colloid solution, on the other hand, is formed from a substance which usually dissolves with difficulty, is cloudy in appearance, and greatly affects the properties of the solvent, making it viscous or jelly-like and sticky to the touch. On boiling away the solvent, the substance tends to char. White of egg, gelatine, soluble starch and glue all form colloid solutions.

To the above we may add that crystalloids have small molecules and diffuse relatively rapidly, whereas colloids with enormous molecules diffuse extremely slowly, if at all, and will not pass through a membrane. An example of an intermediate type is soap, the tiny sodium ions of which will pass through a membrane, whereas the other part of the soap molecule forms very large ions which will not pass through.

Soluble Substances of Biological Importance include : Mineral salts, carbon dioxide, simple sugars, amino-acids ; all of which form crystalloid solutions and readily diffuse ; complex sugars, which also form crystalloid solutions but are osmotic substances ; polysaccharides such as starch and glycogen and proteins and peptones, which form colloid solutions. The nature of much biological activity depends upon the type of solution present, since protoplasm is a complex substance containing many of the above-mentioned substances.

SURFACE TENSION

The surface of a liquid acts as if it were a thin membrane tending to keep the liquid particles as closely packed as possible. This is commonly shown by the fact that the drop which hangs from a tap has the same shape as that of a rubber sheet tied across the end of a wide vertical tube which has been filled with liquid ; here the sheet is strong enough to withstand the weight due to the liquid. The explanation of this phenomenon lies in the fact that all liquid particles have an attraction for each other. A particle at the surface will be attracted by those in the surface surrounding it and by those in the body of the liquid, so that the surface is in a state of strain. As a result of this, the particles in the surface will move or tend to move inwards. This property of the surface of a liquid is called surface tension. Since the particles at the surface are affected differently from those in the body of the liquid, we say that the liquid has a surface film. Where a liquid wets a surface, the area involved may be quite extensive. The effect of surface tension is best seen in small drops, e.g. mercury, water on a dusty surface, drops of oil floating in water. They take up a spherical shape, which is the most compact shape possible, e.g. vacuoles and oil droplets in many cells. In the case of oil in water, when the oil drops rise to the surface they spread out, showing that the surface tension effect of water is greater than that of oil.

COHESION

Experiment 12—To show Cohesion of Water Molecules

The attraction of similar particles for each other is termed cohesion. In the case of water it can be shown quite easily. A flat tin lid is suspended from the arm of a balance and counterpoised. The balance is then put to rest. A beaker is placed under the tin lid and water added until the bottom of the lid just rests on the water. A large beaker must be used, otherwise the pan will drift to one side. The beam of the balance is raised, when the bottom of the lid will be found to adhere to the water. More water is put into the beaker from a pipette until the beam is nearly horizontal. To make the beam horizontal it will now be found necessary to add quite heavy weights to the right-hand pan, and in so doing a column of water is raised on the

CAPILLARY ACTION Experiment 13—To show Capillarity

Many liquids wet solid surfaces. We explain this by the fact that the force of attraction between the solid and the liquid is greater than that between adjacent liquid particles. This causes the meniscus which one so commonly sees. In narrow ’capillary ’tubes the effect is very marked, a liquid rising in such tubes until the weight of the little column of liquid counterbalances the surface tension. The rise of the liquid we call capillary action. Blotting – paper ’soaks up ’liquid by capillary action. A strip of it hung over the side of a beaker, with one end dipping into liquid, makes an effective siphon.

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