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DIFFUSION
Diffusion is the net movement of anything (for example, atoms, ions, molecules, energy) generally from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in Gibbs free energy or chemical potential. It is possible to diffuse "uphill" from a region of lower concentration to a region of higher concentration, like in spinodal decomposition. Definition By Wikipedia
All substances are made up of minute particles called molecules, e.g. the smallest particle of carbon dioxide is a molecule consisting of one atomu of carbon joined to two atoms of oxygen. In a solid the molecules are packed relatively closely together with little or no freedom to move; in liquids the molecules are spaced further apart and are free to move; the molecules of a gas are very much further apart and are moving about at random colliding with each other and the walls of whatever contains the gas. Because of this constant random movement the molecules of a gas tend to distribute themselves evenly tnroughout any space in which they are confined. The same pmnciple. holds true for substances which dissolve in a liquid, e.g. if a crystal of copper sulphate is placed at the bottom of a beaker of water the blue colour of the solid will eventually spread throughout the water as the copper sulphate dissolves. This molecular movement in gases or liquids which tends to result in their uniform distribution is called diffusion.
The following experiments illustrate the process of diffusion in air and water.
Experiment 1 Diffusion of a gas
Squares of wetted red litmus paper are pushed with a glass rod or wire into a wide glass tube, corked at one end, so that they stick to the side and are evenly spaced out. The open end of the tube is closed with a cork carrying a plug of cotton wool saturated with a strong solution of ammonia.
The alkaline ammonia vapour diffuses along inside the tube at a rate which can be determined by observing the time when each square of litmus paper turns completely blue. If the ex- periment is repeated using a more dilute solution of ammonia the rate of diffusion is seen to be slower.
Experiment 2. Diffusion in a liquid
Diffusion in a liquid is very slow and liable to be affected by convection currents or other physical disturbances in the liquid. In this experiment the water is kept still" so to speak, by dissolving gelatin in it. 10 g gelatin is dissolved in 100 g hot water and the solution is poured into test-tubes to half fill them.
Some of the liquid gelatin remaining is coloured with methylene blue and when the first layer of gelatin in the test-tube has set firmly, a narrow layer of blue gelatin is poured into it. When the blue layer of gelatin is cold and firm, the test-tube is filled with cool but liquid gelatin and cooled quickly so that the blue gelatin is sandwiched between two layers of clear gelatin. After a week, the blue dye is seen to have diffused into the clear gelatin, upwards and downwards to equal extents.
The rate of diffusion of a substance depends to a large extent on the size of the molecule, the temperature of the substance and its concentration. The larger the molecule, the more slowly it diffuses and the warmer the substance, the more rapidly it diffuses. Experiment 1 shows that the more concentrated the source of a substance, the more rapidly it diffuses and that the direction of diffusion is from the region of high concentration to the region of low concentration. The difference in concentration which results in diffusion is called a diffusion gradient and the "steeper" the diffusion gradient the more rapid is the resulting diffusion. For example, when rapid photosynthesis is going on in a palisade cell of a leaf carbon dioxide is being removed from the cytoplasm and incorporated into carbohydrate. The carbon dioxide concentration in the cell falls and so sets up a diffusion gradient between the air in the intercellular spaces and the cell. As a result, carbon dioxide diffuses into the cell from the intercellular space. The same cell will be producing oxygen in the course of photosynthesis, creating an oxygen gradient in the opposite direction to that for carbon dioxide. Oxygen consequently diffuses out of the cell into the intercellular space.
Diffusion in living organisms. Diffusion plays a part in most cases of uptake or expulsion of substances within an organism or between the organism and the environment. Sometimes, as in microscopic single-celled organisms, diffusion may be rapid enough to account entirely for the uptake of oxygen and the removal of carbon dioxide and other excretory products . n many cases, however, diffusion is to slow to meet the demands of the living tissues and is superseded by, on a large scale, such processes as the circulation of blood and, on a cellular scale, *active transport" whereby substances are moved by some form of chemical activity into, across or out of cells.
Specific instances of diffusion in plants and animals are mentioned in the appropriate chapters or they can be sought out by referring to the index.
OSMOSIS
Osmosis can be regarded as a special case of diffusion; the diffusion of water from a weaker to a stronger solution. A weak solution of salt, for example, will contain relatively less salt and more water than a strong solution of salt. Thus the diffusion gradient for salt is from the strong to the weak solution, but for water the diffusion gradient is from the weak to the strong solution. If two such solutions were in contact, the water molecules would move one way and the salt molecules the other until both were uniformly distributed. If, however, the two solutions are separated by a membrane which allows water but not salt to pass through, only water can diffuse. Such a membrane is said to be selectively permeable or *semi- permeable" and the water movement is called osmosis.
Osmosis, then, is the passage of water* across a selectively permeable membrane from a weak to a strong solution.
Experiment. Demonstration of osmosis
A length of dialysis tubing (cellophane) is filled with a strong solution of syrup or sugar and fitted over the end of a capillary tube with the aid of an elastic band. The dialysis tube is lowered into a beaker of water and the tube clamped vertically.
In a few minutes, the level of liquid is seen to rise up the capil- lary tube and may continue to do so for a metre or more according to the length of the tube.
Interpretation. The most plausible interpretation is that water molecules have passed through the cellophane tubing into the sugar solution, increasing its volume and forcing it up the capillary tube. This movement should theoretically con- tinue until the hydrostatic pressure of the column of syrup in the capillary is equal to the diffusion pressure of water entering the dialysis tube. In practice, the dialysis tubing is not fully selective. It does allow sugar molecules to pass through into the water but more slowly than it allows water molecules in. Eventually the concentrations of sugar in the beaker and the dialysis tubing would become the same.
The selectively permeable membrane. It is not very clear what the properties are that make a membrane selectively permeable.
One theory supposes that the membrane acts as a molecular sieve, having tiny pores in it that are too small to allow large molecules like (C,H120,) to pass through, bat large enough to let the small water molecules (H,O) go through.
Osmotic potential. The results of Experiment show that pressure builds up in the syrup solution and forces liquid up the capillary. If the capillary were blocked, the pressure would burst the dialysis tubing. The syrup solution is thus said to exert an osmotic pressure. A solution that not in a situation where osmosis is occurring is not exerting a pressure, but is said to have an osmotic potential.
A weak solution has relatively more water molecules than a strong solution and so is said to have a higher osmotic potential than a strong solution; i.e. if the solutions are separated by a selectively permeable membrane, water will flow from the weaker to the stronger. Pure water has the highest possible osmotic potential.
OSMOSIS IN PLANTS
The surfaces of plants and animals and the membranes in their cells frequently have semi-permeable properties. When these organisms or their individual cells are surrounded by fluids weaker or stronger than their own, osmotic forces are set up.
Turgor
The cellulose wall of plant cells is freely permeable to water and dissolved substances. The cytoplasm, however, behaves as a selectively permeable membrane, while the cell sap in the vacuole, since it contains salts and sugars, has an osmotic potential less than pure water. If an isolated plant cell is surrounded by water, osmosis would cause water to enter the cell sap. The vacuole would expand, pushing the cytoplasm against the cell wall eventually the outward pres- sure of the vacuole would be equalled by the resistance of the inelastic cell wall and the cell could take in no more water. Such a cell is turgid and the vacuole is exerting turgor pressure.
The normal source of water for producing turgidity in cells is the xylem vessels or neighbouring cells.
A plant structure made of turgid cells is resilient and strong; the plant stem stands upright and the leaves are held out firmly. Many young plants depend entirely upon this turgidity for their support but in older plants, woody and fibrous tissues take over this function.
Growth. In the growing regions of plants, the cell walls are still fairly plastic. After new cells have been produced by division at the growing point, vacuoles begin to form in their cytoplasm water enters the cell by osmosis and the vacuoles join up and increase in volume. Since the cell wall is plastic, the cell is extended as the vacuole pushes outwards on the walls. Hundreds of cells extending in this way produce expansion growth.
Wilting. When plants are exposed to conditions in which they lose water to the atmosphere faster than it can be obtained from the soil, water is lost from the vacuoles. The turgor pressure of the vacuoles decreases and they no longer push out against the cell wall. The cell becomes limp or flaccid (like a deflated football). A plant structure made of such cells is weak and flabby, the stem droops and the leaves are limp: in other words, the plant is wilting.
Plasmolysis. If a plant cell is surrounded by a solution more concentrated than the cell sap, water passes out of the vacuole to the outside solution. Loss of water causes the vacuole to shrink and pull the cytoplasmic lining away from the cell wall (Fig. 10.5b). There is now no pressure outwards on the cell wall and the cell is flaccid. This.condition, called plasmolysis, can be induced experimentally in living cells without necessarily harming them, but it is an extreme condition and rarely occurs in nature.
Stomata. Although the details of the stomatal mechanism are not fully worked out, it seems that when a leaf is illuminated and photosynthesis is rapid, the consequent fall in the carbon dioxide concentration in the leaf causes the guard cells to convert stored starch to sugar with a corresponding fall of osmotic potential in their vacuoles. The higher osmotic potential of the neighbouring epidermal cells forces water into the guard cells, increasing their turgor. Since the inner walls bounding the stoma, are thicker than the others the 1ncrease in turgor pressure causes the guard cells to curve and open the stoma between them. Thus the stomata are usually open in daylight when carbon dioxide is needed for photosynthesis and closed at night when photosynthesis ceases.
Movement of water in a plant
Cells. Osmosis is considered to be largely responsible for the passage of water from one cell to another in a plant.
Imagine two adjacent cells A and B. A has a lower concentration of sugar in its cell sap and hence a higher Osmotic potential than B. The more dilute cell sap in A will thus force water by osmosis into cell B. The water entering cell B will dilute its cell sap and raise both its osmotic potential and its turgor pressure so tending to force water out into the next cell in line. Thus water passes from cell to cell down an osmotic gradient.
However, if cell B is fully turgid it is unable to expand and so can take in no more water even though it has a lower osmotic potential than its neighbour A. In fact, if cell A is not fully turgid and still capable of expansion, the high wall pressure of cell B will force water out into cell A against the osmotic gradient., The movement of water between plant cells depends, therefore, not only on the osmotic potential of their vacuoles but also on how turgid they are.
Roots. In this way, water may be absorbed from the soil by the roots, though this is not established fact, but rather one of the more widely held theories. shows the general direction of water-flow, and represents a few cells along the radius of the root.
The root hairs are thin-walled extensions from the cells of the outer layer of a root. They grow out, pushing between the soil particles to which they adhere closely. T'he film of water which surrounds the soil particles also surrounds the root hairs. Although soil water has mineral salts dissolved in it, they make only a very weak solution, and the cell sap of the root hair is more concentrated than this. WWater pases from the soil through the cell wall and its thin eytoplasmic lining into the vacuole of the root hair. This extra water dilutes the cell sap so that it is now weaker than that in R2. 1t also inereases the volume of the vacuole and hence its turgor pressure. The rise of Osmotic potential and turgor pressure in Rl forces water into R2. In the same way water passes to R3. By such a chain of local osmotic effects, water ie passed from cell to cell into the centre of the root until it enters the water vessels of the xylem and is carried up the root and stem into the trans- piration stream.
In an actively transpiring plant there is often a tension rather than a pressure in the xylem vessels so that water enters the xylem from the root cortex.
In some plants at certain times of the year, it is possible to demonstrate a positive pressure, root pressure, in the xylem. For example, sap oozes from the stump of a tree if it is cut down in the wet season. If a glass tube containing a little water is fitted to the cut stem of a well-watered potted plant, as shown in the water can be seen to rise several centimetres in the tube. There is some evidence that this root pressure derives from the sort of osmotic activity described above but its significance is not certain. In some plants, root pressure 1s thought to be responsible for the exudation of drops of water, guttation, from the tips of the leaves.
The explanation of water movement between cells as given above, describes the passage of water by osmosis from the vacuole of one cell to the vacuole of another. However, there are alternative pathways for water and dissolved substances, namely (1) through the intercellular spaces between the cortical cells in roots, (11) through the cell walls from one cell to another
Without passing through the cytoplasm and (11)) from the cytoplasm of one cell to the cytoplasm of another without passing through the vaeuoles. The relative importance of the differend routes is not known but it must be borne in mind that, by its very definition, osmosis cannot be invoked to explain the movement of dissolved substances into the roots and through the plant. This is discussed more fully in the next chapter.
The selectively permeable properties of cytoplasm depend on its being alive. Anything which kills the cytoplasm also destroys its selective permeability. At the same time, of course, it will destroy all transport systems which depend on living processes such as active transport or movements of solutes in the phloem.
Leaves. shows a few adjacent cells in a leaf. The palisade cell (a) i losing water by evaporation from its surface into the intercellular spaces. This loss of water results in a reduction of turgor pressure and an increase in the con- centration of the sugars and salts in its vacuole. The cell sap in the vacuole becomes more concentrated than that of its neighbouring cell, its osmotic potential falls and water will flow into it by osmosis from the neighbouring cell. Such a loss of water from cell (b) will increase the concentration of its cell sap and reduce the volume of its vacuole and hence its outward
pressure on the cell wall. These two changes will give cell (b) a lower osmotic potential and turgor pressure and it will absorb water from its neighbour (c). Cell (d) is next to a water vessel running in one of the veins of the leaf, and it will receive water from it, again by osmosis. In this way water can flow from the vein to the cells of a leaf.
Experiment. To demonstrate root pressure
A piece of glass tubing is connected by rubber tubing to the freshly cut stem of a potted plant. A little coloured water is placed in the tube and its level marked. If the roots are kept well watered, the coloured water will rise a few cm in the tube. This demonstrates root pressure.
Experiment . To demonstrate osmosis in living tissue
Cut three 8 cm cubes from a yam and make a large cavity in each cube as shown in. Immerse one of them in boiling water for a few minutes t0 kill the protoplasm. Place each in a Petri dish of water and place some salt or sugar in the cavity cut in the boiled yam and in one of the raw ones. After 2-3 hours the cavity of the raw yam with salt in it will be full oof fiuid, since first of all the salt dissolves in the sap exuding from the damaged cells, and then the salt solution so formed withdraws further water by Osmosis. In the boiled yam there will be no such accumulation of fluid, showing that semi- permeability is a property of living protoplasm. The raw yamn
Without salt is a control to eliminate the possibility of wateer forming in such a cavity irrespective of the salt.
OSMOSIS IN ANIMALS
The skins of many aquatic animals are more or less selectively permeable and this results in osmotic effects between them and their surroundings.
Fresh-water animals. The blood of fishes and amphibians and the body fiuids of invertebrates that live in fresh water are more concentrated than the pond or stream water that surrounds them. Water tends to enter their bodies by osmosis, through their skins or such patches of skin as are exposed. If this water were not removed continually, the animal's blood would be diluted and the whole creature would swell and become water- logged. Vital processes in the body would be interfered with and the animal would die. In a great many fresh-water animals it has been possible to show that certain organs are able to eliminate the excess water, or osmo-regulate and keep the concentration of the body fluids constant. In frogs and fish the kidneys extract excess water from the blood as it passes through them, and it is passed out of the body as a dilute urine.
Impermeable coverings, like the cuticle on the exoskeleton of beetles and other aquatic insects, greatly reduce the surface Over which water might be absorbed. In some fresh-water animals it Is not clear how osmosis affects them or how their osmo regulation is carried out.
Salt-water animals. Sea water is a more concentrated solution than the blood of many marine fish, in consequence there is a tendency for water to pass out of their bodies and into the sea, by osmosis. Sea-water fish swallow water and in some way absorb it through their alimentary canals. There is also evidence that they can eliminate the excess salts taken in.
Animals in estuaries have to withstand extreme osmotic changes, being alternately covered with fresh and salt water.
Land animals. Osmotic effects occur within the bodies of all animals, but in general, land animals lose water from their body surfaces by evaporation and gain it from their food and drink. The concentration of the blood is nevertheless kept very constant by the regulatory action of the kidneys and other organs . The impermeable cuticle of insects, the fur of mammals and the feathers of birds reduce water-loss by evaporation and contribute to the success of these groups on land.
ADDITIONAL EXPERIMENTS
1. Plasmolysis in cells. Some of the purple epidermis from a Rheo leafis stripped and mounted, outer surface uppermost, on a slide with some water. The slide iS arranged so that a group of cells with purple vacuoles can be seen under the lower power of the microscope. A few drops of salt or sugar solution are placed on the tissue. If the cells are watched, the purple vacuoles will be seen to shrink away from the cell walls as water passes into the solution by osmosis. If the solution is now blotted up with filter paper and replaced with water, the cells will take up water once again and the vacuoles will swell to their normal Size.
2 To show osmotic pressure in cells. A S mm piece of young onion leaf is cut along its length into strips about 2 mm wide. The outer epidermis is inelastic while the thin-walled inner cells, freed from the restraint of the epidermis, tend to expand. Thus the strips curl outwards, with the epidermis on the inner circumierence, One strip 15 placed in a watch glass containing water, another in sugar solution, and a third left as a control. The first strip will absorb water by osmosis. The inner cells will expand while the inelastic epidermal cells will not, and the strip will curl up more. In the sugar solution, water will escape from the cells, which will lose their turgor; the unequal stresses in the strip will disappear and it will straighten,
3. The effect of salt solutions on red blood cells. Prepare four solutions of sodium chloride in distilled water, 0-2, 0-6, 0:8 and 1-6 per cent strengths. Label each of four microscope slides With one of the above figures and place a drop of salt solution of the appropriate strength on each slide. Obtain a blood sample as follows: thoroughly clean the ball of the little finger with cotton w0ol soaked in ethanol; squeeze the little finger between the thumb and first finger of the same hand and stab the cleaned area with a sterile lancet"; touch a clean glass capillary on to the drop of blood which oozes out; the blood will run up into the capillary. By blowing gently down the capillary, place some blood on each of the drops of salt solution, cover each drop with a cover slip and examine under the high power of the microscope
Result. The red cells in the weak salt solution (0:2 per cent) will have absorbed water by osmosis and burst, so no intact cells will be seen. In the strong (1:6 per cent) solution, the red cells will have lost water by osmosis and shrunk so that their outlines have a crinkled (orenated) appearance. In one or other of the remaining solutions, the red cells will appear normal and thus the concentration of this solution must approximate to that of blood plasma since no water is taken in or lost by the red cells.
Interpretation. The cell membrane of the red cells is selectively permeable. The cytoplasm inside has a low osmotic potential as a result of the proteins and other dissolved substances it contains. Thus, water will be lost to or taken up from solutions stronger or weaker than the cytoplasm. The cell membrane is very thin and easily ruptured or distorted by changes in the volume of the cell.
The results illustrate the importance of maintaining the osmotic concentration of blood plasma within narrow limits