School Science Lessons
Plant Physiology
2012-05-13 SPwp
Please send comments to: J.Elfick@uq.edu.au
Table of contents
9.3.8.0 Cellulose
9.12.0 Colloids, diffusion
9.14.0 Osmosis and osmotic pressure
9.15.0 Plasmolysis
16.4.1.0 Vitamins
9.12.0 Colloids, diffusion
9.12.0 Colloids, diffusion, semipermeable membrane, dialysis
9.165 Cellophane as a semipermeable membrane
9.167 Copper ferrocyanide as a semipermeable membrane
9.172 Colloidal nature of egg white
9.180 Colloidal solution of starch
9.162 Diffusion through a colloidal gel
9.163 Diffusion with copper (II) sulfate solution
9.168 Prussian blue as a semipermeable membrane
9.166 Sausage skin as a semipermeable membrane
9.181 Separate a colloid from a crystalloid by dialysis
9.14.0 Osmosis and osmotic pressure
9.164 Osmosis and osmotic pressure, reverse osmosis
9.179 Imbibition in seeds and dried fruit, broad bean, pea, bean
9.170 Osmosis with a carrot
9.171 Osmosis with an egg
9.174 Osmosis with dialysis tubing
9.171.1 Osmosis with honey on bread
9.216 Osmotic behaviour of red blood cells
9.173 Osmotic pressure and suction potential in dandelion
9.113 Swelling of seeds, imbibition, during germination
9.175 Turgor pressure in a potato
9.15.0 Plasmolysis
9.176 Plasmolysis in beetroot
6.1.16 Plasmolysis in Elodea
9.178 Plasmolysis in hairs on the stamens of Tradescantia
9.177 Plasmolysis in onion epidermis
6.1.17 Plasmolysis in Spirogyra
9.169 Suction potential and tissue tension in celery
9.175 Turgor pressure in a potato
16.4.1.0 Vitamins
16.4.1.1a Vitamin A
16.4.1.2 Vitamin B1, thiamine, Vitamin B2, riboflavin
16.4.1.3 Vitamin C (ascorbic acid)
Vitamin C, ascorbic acid
16.4.1.4 Vitamin D
16.4.1.5 Vitamin E
19.2.1.6 Vitamin E, Antioxidant phenols, antioxidants, vitamin E, beta-carotene
6.1.16 Plasmolysis in Elodea
See diagram 9.3.68: Plasmolysis in Elodea 1. Mount a complete leaf in water on a slide and examine cells under the high power. Note the small green granules, chloroplasts, which contain the chlorophyll. Then irrigate the leaf with a strong solution of sugar or salt. The green chloroplasts help one to see plasmolysis taking place more easily. Now place a small sprig of Canadian pond weed (Elodea) in boiling water for a few minutes. This kills the cells. Then mount one leaf and treat it as before. Note that plasmolysis does not occur now. This shows that only living cells possess semipermeable membranes and are therefore able to absorb water by osmosis.
2. Note absorption of methylene blue by Elodea. Place a shoot of Elodea in a very dilute solution of methylene blue. Leave for a few hours and note that the Elodea plant becomes deeply coloured by adsorption of the dye.
6.1.17 Plasmolysis in Spirogyra
See diagram 1.1: Spirogyra
1. Put the algae, Spirogyra, in water on a microscope slide. While looking at the cells under the microscope add drops of sodium chloride solution near the algae. The cytoplasm shrinks away from the cell walls and forms a clump. Absorb the salt water with absorbent paper. While looking at the algae cells under the microscope, add drops of water near the algae and absorb excess water with absorbent paper. The cytoplasm swells to occupy most of the space in the cell. When you put salt water near the algae, water diffused out of the cytoplasm causing it to shrink. When you mop up the salt water and put pure water near the algae, water diffused back into the cytoplasm in the cells making it swell.
9.3.8.0 Cellulose, (C6H10O5)n, cellulose ashless, cotton wool, hemicellulose, lignin, tests for wood: 16.3.1.6
Cellophane
Cellulose, iodine test: 9.135
Cellulose nitrate (nitrocellulose, guncotton)
Digestion of cellulose (by cellulolytic bacteria and fungi): 9.214
Tests for cellulose, iodine test: 9.135
Tests for cellulose, solubility test: 9.136
9.3.8.1 Cellulose compounds
Cellophane
Celluloid film (cellulose nitrate + camphor)
Cellulose acetate, (cellulose ethanoate), Burning test, easy to ignite, yellow flame, burns after removing flame, acidic fumes, acetic acid smell
Cellulose acetate, CA, acetate plastic, cellulose fibre, artificial fibre, Cellulose acetate butyrate, CAB: 3.5.8
Cellulose acetate sheet: 31.1.17
Cellulose nitrate, nitrocellulose, (celluloid), flash paper: 3.5.9
Cellulose nitrate, cellulose acetate, thermoset plastics: 3.5.7
Cellulose propionate, cellulose butryate, ethyl cellulose
Cellulose triacetate: 3.5.8
Chemical sources of polymer materials: 3.4.02
Collodion
Electrostatic series, triboelectric series, ranking of insulators: 31.1.02 (See 17. Cellulose acetate)
Ethyl cellulose, Burning test, when ignited forms drips on ignition, blue-yellow flame with a green base, burns after removing flame, burning wood smell
Identification tests for plastics: 3.102
Identification tests for polymers: 3.103
Prepare rayon: 3.4.8
9.12.0 Colloids, diffusion, semipermeable membrane, dialysis
See diagram 9.171: Measure osmosis
Diffusion occurs when two substances flow into each other until both substances are completely mixed. Osmosis occurs when a semipermeable membrane separating a water and sugar solution allows only water molecules to diffuse through it and so decrease the concentration of the sugar solution. One mole of a non-electrolyte dissolved in water and made up to 22.4 L of solution causes an osmotic pressure, at 0oC of 760 mm. of mercury (1 atmosphere). The cell membrane inside the cell wall of plant cells allows water to diffuse in when the cell is surrounded by a lower concentration solution. The cell swells with the absorbed water and develops an extra pressure called turgor pressure. If the cell is surrounded by a higher concentration solution, water diffuses out of the cell, and the protoplasm shrinks away from cell wall. This process is called plasmolysis.
9.113 Swelling of seeds, imbibition, during germination
See diagram 3: Soaked bean seed
1. After soaking dry bean seeds in water the seed coats swell and wrinkle. Water also enters a tiny hole in the seed coats, the micropyle. The cotyledons absorb water and swell pushing out the wrinkles in the seed coat. So the bean seed becomes larger and the seed coats become smooth again.
2. Fill a small cheap glass tube with dry peas. Fill the tube with water, plug with wet cotton wool, leaving no air in the tube and attach a cork stopper. Secure the stopper in position with wire. Put the tube in a closed plastic container. By the next day the tube breaks because the peas enclosed in the test-tube have swollen with water and exerted a strong pressure inside the tube. The seeds become larger by imbibition.
3. Measure the increase in size of pea seeds by imbibition. Half fill a measuring cylinder with water, add fifty dry pea seeds and shake the measuring cylinder to remove air bubbles. Record the level of the water in the graduated cylinder. Pour the seeds into a flat dish of water. After two days, pour out the water from the flat dish, take the pea seeds out and dry their surfaces between the absorbent paper. Half fill a measuring cylinder with water, add the fifty swollen pea seeds and shake the measuring cylinder to remove any air bubbles. Calculate the percentage increase in volume of the seeds. Some people do this experiment by just leaving the peas in the measuring cylinder but they can swell so much as to jam and even break the measuring cylinder!
4. Swelling seeds exert pressure on their surroundings. Explain why pushing peas or beans into their nose or ear is dangerous for children. Fill a test-tube with dry peas. Cover the mouth with a triple layer of muslin tied tightly below the flanged rim. Fill a Petri dish with tap water to 1 cm below the rim. Hold the test-tube containing the peas obliquely in the Petri dish. Its mouth must be below the surface of the water so the water can run into the test-tube. Move the tube to and fro until all the air between the peas is displaced by water, and leave lying in the water in the Petri. The next day the test-tube in the Petri dish breaks. The peas enclosed in the test-tube have swollen in the water. They have expanded and exerted so strong a pressure on the test-tube that it has shattered. Seeds absorb water before germinating. This process, by which the seeds become appreciably larger is called swelling or imbibition. When the seeds imbibe water, they swell.
5. Fill a wine glass with dry peas. Shake the wine glass and keep adding peas until no more can fit in without forcing them in. Add water to the wine glass until no more can be added without it spilling over. Put a light lid over the wine glass and peas and place the wine glass over a baking tray. During the next hours, as the peas swell some peas will be pushed over the rim and fall with a sound on the baking tray.
6. Determine the increase in size of pea seeds through swelling. Fill a graduated cylinder of 100 mL capacity with water up to the 50 mL mark. Add 50 dry pea seeds and shake the graduated cylinder several times to remove any air bubbles that may have been locked in between the seeds. Read the level of the water in the graduated cylinder. By how many millilitres has it risen through the addition of the so dry seeds? What can be deduced from this rise in the water level? Shake the pea seeds with the water into the beaker of 100 mL capacity and cover it with an open flat glass dish of 100 mm diameter. After three days pour out the water from the beaker, take the pea seeds out and dry their surfaces between the filter papers. The pea seeds have swollen. Again fill the graduated cylinder up to the 50 mL mark with water. Put the fifty swollen pea seeds in the graduated cylinder and shake it several times to remove any air bubbles. that may have been locked in between the seeds. Read the level of the water in the graduated cylinder again. By how many millilitres has it risen this time, as a result of adding the 50 swollen pea seeds? What can be deduced from this rise? What was the volume of the 50 dry pea seeds what is their volume after swelling? By how much has the volume increased?
9.162 Diffusion through a colloidal gel
1. Make a 10% solution of gelatine by warming in water and add a few drops of phenolphthalein solution. Be careful! Phenolphthalein may cause eye, skin and respiratory irritation. Use safety glasses and insulated heat-proof gloves and work in a fume cupboard, fume hood. Fill a wide test-tube, with a stopper at one end, with the warm solution and leave to cool. When it has set, invert the test-tube over a 10% solution of caustic soda. A reddening of the gelatine will show the upward diffusion of the alkali through the colloidal gel.
2. Note how gelatine absorbs dye. Cut three rectangles of exactly equal dimensions from sheet gelatine. Let one rectangle remain dry as a control. Place a second rectangle in water. Note that its dimensions increase because of the imbibition of water. Measure its final area and compare it with the control. Place the third rectangle in a very dilute aqueous solution of methylene blue. A progressive staining of the gelatine accompanies the swelling. Eventually the gelatine adsorbs practically all the dye from the solution. Imbibition is the absorption of fluid by a solid.
9.163 Diffusion with copper (II) sulfate solution
1. Put 75 mL of water in a 100 mL beaker. Using a graduated pipette, put 10 mL of copper (II) sulfate solution under the water layer. Mark the boundary between the two liquids with a grease pencil. Leave the beaker to stand in a place free from vibrations. During the following days the boundary between the demineralized water and the saturated copper (II) sulfate solution becomes more indistinct and moves upwards. After three weeks, the liquid in the beaker appears uniformly coloured. Compared with the original saturated solution the colour is less intense. At first, the lower layer of saturated copper (II) sulfate solution is sharply separated from the demineralized water above it because of its greater density. However, because of the individual movement of the molecules of both layers, the initial marked difference in concentration at the boundary becomes diffuse because of mixing of a sequence of layers of lower concentration. The result is equalization of concentration.
9.164 Osmosis and osmotic pressure, reverse osmosis
See 24.1.06 Morse equation, osmotic pressure equation
See diagram 9.164: Osmosis
Osmosis is the process by which a solvent passes through a semipermeable membrane from a region of lesser solute concentration into a region of greater solute concentration until the concentrations are equal. Osmotic pressure is the pressure that must be applied to a solution to keep it in equilibrium with the pure solvent separated only by a semipermeable membrane. If a container is separated by two parts A and B by a semipermeable membrane, e.g. a bladder or a film of copper ferrocyanide, A containing water and, B containing a substance dissolved in water. Water molecules pass through the semipermeable membrane from A into B to dilute the solution in it. The osmotic pressure of the solution is the pressure that must be applied to it to prevent water entering it by osmosis. If the solutions A and B originally had the same level in the container and osmosis ceased when the level of the solution in B had risen to h cm the original osmotic pressure of B was [h × density of the solution × g (gravity acceleration)]. The osmotic pressure obeys the gas laws so osmotic pressure increases proportionally to the absolute temperature. Osmotic pressure depends on the number of particles so when a solute dissociates in solution the osmotic pressure increases. The approximate value of the osmotic pressure, Π (capital pi) of a dilute solution (atmosphere or bar) can calculated from the Morse equation, (osmotic pressure equation). Isotonic solutions have the same osmotic pressure so at the same temperature they must contain the same number of particles of the solute per litre if they are neither dissociated or nor coagulated. If a solution is more concentrated than another solution, it is the hypertonic solution and if less concentrated than another solution, it is the hypotonic solution.
Reverse osmosis uses pressure to drive solutes out of a solution through a semipermeable membrane by applying pressure to the solution , e.g. desalination of seawater, concentration of milk.
9.165 Cellophane as a semipermeable membrane
See: Cellophane
1. Mix cornstarch with water then test a sample of it with drops of iodine solution. The cornstarch turns dark blue. Put the sample aside and put the rest of the cornstarch in a cellophane bag. Wash outside the bag under the tap. Half fill a beaker with water and add drops of iodine solution or tincture of iodine. Suspend the cellophane bag containing cornstarch solution in the iodine solution. The cornstarch in the cellophane bag starts to turn blue showing that iodine solution is moving through the wall of the cellophane bag. However, the iodine solution in the beaker does not change colour because the cornstarch cannot pass through the walls of the cellophane bag. The cellophane is semipermeable. It is permeable to the iodine solution but not permeable to the cornstarch solution.
2. Tie a piece of cellophane over the mouth of a funnel. Invert the funnel and partly fill it with a concentrated sugar solution coloured with red ink. Tie tightly the string around the funnel and cellophane so that no leaks occur. Place the funnel with its mouth in a beaker of water and clamp it to a stand. Set the apparatus aside and note the rising level of the sugar solution in the stem of a funnel. Note the decrease in the red colour as the level of sugar solution in the stem of the funnel rises. Note the height when the level of the sugar solution stops rising. This height gives a measure of the difference in concentration of the two solutions.
Repeat the experiment with sugar solution in the beaker and water in the funnel. The water level now falls in the funnel. In osmosis, the net movement of water is from the weaker solution to the stronger solution until both solutions are isotonic. The result is to dilute the more concentrated solution.
3. Pour molasses or "golden syrup" on the centre of a square piece of cellophane. Lift the corners of the cellophane then tie a string around the cellophane above the molasses to make a "pudding". Hang the pudding in water, keeping where it is tied above the surface of the water. The "pudding" swells as water enters by osmosis.
9.166 Sausage skin as a semipermeable membrane
1. Immerse a specimen tube in a concentrated solution of cane sugar or common salt. Work under the solution to avoid air bubbles. Fix sausage skin across the mouth of the specimen tube and secure with wire. Wash outside the specimen tube and sausage skin membrane under the tap to remove any concentrated solution. Immerse the filled specimen tube in tap water. The sausage skin membrane stretches outwards as water moves through it into the concentrated solution.
9.167 Copper ferrocyanide as a semipermeable membrane
1. Drop a concentrated solution of potassium ferrocyanide into dilute copper (II) sulfate solution. A layer of copper ferrocyanide will surround the drop formed. The drop sinks but later rises because of osmosis causing alteration in density.
2CuSO4 + K4Fe(CN)6 --> 2K2SO4 + CU2Fe(CN)6
2Cu2+ + Fe(CN)64- --> Cu2Fe(CN)6 (s)
9.168 Prussian blue as a semipermeable membrane
1. Put a solution of 0.5 g in 1 litre of potassium ferrocyanide in an evaporating basin and add a lump of solid iron (III) chloride. A semipermeable membrane layer of Prussian blue forms at the surface of the concentrated solution of iron (III) chloride. Then water passes from the dilute potassium ferrocyanide solution through the semipermeable membrane and the layer of Prussian blue swells. Prussian blue, iron (III) ferrocyanide, Fe7(CN)18(H2O)x, engineers blue, blueprint blue, was used in washing blue to make yellowing cotton sheets appear white and in blue rinse to dye the hair of old ladies.
9.169 Suction potential and tissue tension in celery
1. Use a segment of celery "stalk" stored in a dry place for a few days, i.e. you can bend it and it is not crisp. Celery "stalks" are enlarged petioles or leaf stalks. Cut a thin transverse section, put it in water, and examine it with a magnifying glass. On the outer convex side is a dark green epidermis in folds. It is a single layer of cells with a thick cuticle on the outer surface. Associated with each of the outer folds is a vascular bundle that appears as a white circle. If you rub your finger across a dry transversely cut surface, you can feel them. Each vascular bundle has phloem on the outside, then cambium then xylem. Phloem cells have cytoplasm and carry organic food materials. Cambium cells are closely packed and produce new cells by repeated cell division. Xylem cells have no cytoplasm and conduct water. A cap of woody sclerenchyma cells strengthens the vascular bundles. Inside the epidermis and around the vascular bundles are collenchyma cells with cellulose thickening in the corners. Most of the cross-section consists of the light green parenchyma cells, a packing tissue with thin cellulose cell walls. On the inside concave surface is a second layer of epidermis. In contrast to the outer convex layer, this epidermis consists of thin cells with no cuticle.
2. Cut across the celery stalk to make a segment 8 cm long. Measure the length of the segment. Then put it in water to allow the cells to become turgid and measure the length again. Cut longitudinally between the folds to produce strips. Put the strips in water and note how the strips curl with the whiter parenchyma tissue on the outside of the curl and the greener epidermis on the inside of the curl. The parenchyma cells take up water and expand because the longitudinal cuts reduce tissue tension. Wall pressure in this tissue is low because the cells have thin extensible walls. Suction potential increases in the parenchyma cells, water moves in and they increase in size. Curling occurs because the inner cut surface increases in length, while the outer cut surface remains the same because of the epidermis with its thick cuticle on the outside and the collenchyma confined by its own cell walls. Wall pressure remains high, so no change in suction potential, no water uptake and no increase in length of this surface. Salad cooks know how to cut celery, radishes and other uncooked vegetable to make them curl attractively.
9.170 Osmosis with a carrot
See diagram 9.3.47: Osmosis with a carrot
1. Select a carrot that has a large top and is free of breaks in its surface. In the centre of the carrot, cut a 5 cm deep round hole with a cork borer so that the diameter is the same as a one-hole stopper fitted with a length of glass tubing. Add ink to a concentrated sugar solution in a beaker and note the resulting colour of the solution. Fill the hole with the concentrated sugar solution coloured with red ink. Fit the one-hole stopper into the entrance of the hole. Put the carrot in a tall beaker of water. Seal around the stopper with wax from a burning candle. Be careful! Melted wax can cause skin burns so wear safety glasses and insulated heat-proof gloves. Move the glass tube through the stopper to adjust the height of the coloured sugar solution in the tube. Observe the coloured sugar solution rising inside the glass tubing. Record the height and note any change in the colour of the coloured sugar solution.
2. Repeat the experiment using a potato.
3. Use a carrot or potato that has been stored for a long time. Squeeze the carrot and notice the limp feel. Put the carrot in water and squeeze again later. The carrot now feels firm because water has entered its cells because of osmosis.
9.171 Osmosis with an egg
1. Remove the shell of a fresh egg by dissolving it in dilute (10%) hydrochloric acid or vinegar. The shell is mostly calcium carbonate. A thin outer skin, shell membranes, now encloses the egg. Put the egg in pure water. It will swell because water passes into it by osmosis. The liquid in contact with the inner surface of the membrane is an aqueous solution. Place a similar egg with no shell in a concentrated salt solution. The egg shrinks. Water passes out of the egg solution into the salt solution because the salt solution is more concentrated. Even after one day in the hypertonic or hypotonic solutions the change in volume is not easy to observe. However, the change is more obvious with a very small egg, e.g. a quail's egg.
2. Put an egg in a beaker, cover it with vinegar and leave the beaker and egg in the refrigerator for one day. Replace the vinegar and leave the beaker and egg in the refrigerator again for one day. Use a wooden spoon to remove the egg from the beaker and observe the loss of the shell. Feel the rubber-like surface of the egg. Replace the egg in the beaker, cover it with corn syrup and leave for one day. Use a wooden spoon to remove the egg from the beaker and observe its flabby softness by gently picking it up with your fingers. Place the egg in a new beaker and cover it with water and leave for one day. Use a wooden spoon to remove the egg from the beaker and observe its rubber-like toughness by gently picking it up with your fingers.
9.171.1 Osmosis with honey on bread
1. Spread honey on a flat slice of bread but do not use any butter. Pick up the slice of bread by holding the opposite crusts and hold the slice horizontally. The middle of the slice dips down. The concentrated sugar solutions in honey have attracted moisture out of the side of the bread next to the honey bread by osmosis which shrinks causing the concave bowing of the slice of bread.
9.172 Colloidal nature of egg white
1. Note the irreversible coagulation of egg white by warming some egg white in a test-tube placed in boiling water. Examine the reversible precipitation of a colloid, with egg white. The colloidal nature of egg white is due chiefly to the presence of the protein albumen. Mix the white of an egg with 100 mL of water. Filter with a filter pump. To about half the clear solution add powdered ammonium sulfate while continuously shaking. As the sulfate approaches saturation, the albumen will precipitate as a white curd. Then shake this liquid with about the same volume of water and note that the precipitated albumen re-enters the colloidal solution.
9.173 Osmotic pressure and suction potential in dandelion
See diagram 9.36.11: Circular strips of dandelion in sucrose solutions
1. Cut the stalk into circular segments. Then make a vertical cut through one side of each segment to form circular strips. After you make the vertical cut, each strip curves outwards because of the expansion of the stalk cells. Put the strips in demineralized water. The stalk cells can still absorb water so the strip curves more outwards. If the stalk cells had no suction potential they could not absorb water and no change in curvature would occur. If the cells lost water the curvature of the strip would be reduced and eventually the strip would become straight. Put a circular strip in each of the following concentrations of sucrose: 0.3M, 0.4 M, 0.5 M, 0.6 M, 0.7 M. The concentration that causes no change in the curvature of the strip is equal to the suction potential of the cells of the strip. A lower concentration causes more curvature and the circle becomes a spiral, at 0.3M and 0.4 M. A higher concentration causes an opening out of the circle, at 0.6 M, 0.7 M. So the suction potential is equivalent to a 0.5 M solution of sucrose where no change of curvature occurs.
9.174 Osmosis with dialysis tubing
See diagram 9.36.12: Osmosis with dialysis tubing
1. An osmometer measures osmotic pressure. Tie a length of dialysis tubing filled with a sugar solution to a capillary tube and note the rise in the level of the sugar solution. However, in this osmometer, achieving a watertight junction between the dialysis tubing and the capillary tube is difficult because the thread is not elastic and the wall of the glass capillary tube is slippery. Note any small initial rise in the level of sugar solution in the capillary tube followed by a steady drop. Leakage at the junction usually causes this drop when enough hydrostatic pressure has built up in the liquid column.
2. Make a watertight junction between the dialysis tubing and the glass tubing using a polypropylene connector with a wide end of bore diameter 10 mm and a narrow tapered end of bore diameter 5 mm (or fix the dialysis tubing to the T-shape connector with a rubber band, taking care not to trap air bubbles in the dialysis tubing when filling with the sugar solution). Tie a knot tightly at one end of a length of soaked dialysis tubing about 16 cm long. Fix the other end of the dialysis tubing to the wide end of a polypropylene connector by winding a rubber band tightly around it to form a watertight junction. Fill the dialysis tubing with a sugar solution. Join the tapered end of the polypropylene connector to a T-shape connector with rubber tubing. Join the other two ends of the T-shape connector to a 10 mL syringe filled with the sugar solution and to a calibrated 1 cm3 pipette. Rinse the outer wall of the dialysis tubing with water to remove any trace of sugar solution. Immerse the dialysis tubing into a beaker of water. Move the plunger of the syringe to adjust the position of the meniscus of the sugar solution in the pipette to a suitable position. Start taking measurements. When the meniscus reaches the top of the pipette, move it to the starting position by adjusting the plunger to make more measurements. Add a few drops of Congo red (blue in acid and red in alkali) to the sugar solution to see the liquid column in the pipette. Congo red may be harmful if swallowed or inhaled so use amyloid red dye as a safer substitute.
3. Investigate the following:
3.1 The effect of temperature or solute concentration on the initial rate of osmosis of a sugar solution.
3.2 Compare the initial rates of osmosis of different solutions, starch, sucrose and glucose solutions.
9.175 Turgor pressure in a potato
1. Make a potato thimble by peeling the potato then scooping out the inside. Half fill the potato thimble with 10% sugar solution. Suspend the filled potato thimble in a beaker of pure water. Note the rise in level of the sugar solution in the potato thimble.
2. Cut 3 mm thick slices from a potato tuber. Bend the slices between the fingers to test their comparative firmness. The cells are full of water and thus turgid. Put the slices in a 3% solution of common salt and leave for about half an hour. Again bend the slices between the fingers to test their comparative firmness. The slices become flabby. Put the slices in demineralized water and leave for half an hour. Again bend the slices between the fingers to test their comparative firmness. The cells regain their firm texture. If a slice of potato is put in a solution that is hypertonic to the cytoplasm, the size of the cells will decrease slightly and the slice of potato will feel soft.
3. Prepare 30 identical thickness and area slices of potato or cut discs from a 3 cm thick slice with a large cork borer. Dry the slices or discs with absorbent paper and weigh them. Half fill test-tubes with 20%, 15%, 10%, 7.5%, 5%, and 0% sucrose solutions. Put a potato slice or disc into each test-tube with just enough sucrose solution to cover the strip and take them out after 40 minutes. Dry the slices or discs with absorbent paper and weigh them. Plot a graph of final weight divided by initial weight against percentage of sucrose solution. From the graph note which sucrose concentration is isotonic with the potato cell contents. If sucrose is the only osmotically active material in the cell, determine the sucrose concentration of the cytoplasm. A 1 molar sucrose solution contains 342 g of sucrose in 1000 mL of solution, equivalent to 25.69 atmospheres. A 0.1 molar solution is 34.2%, so if the isotonic solution is 5%, then the turgor pressure will be 5 divided by 34.2 of the osmotic pressure of the 1 molar solution.
9.176 Plasmolysis in beetroot
1. Cut thin sections of beetroot and mount them in water on a microscope slide. Cut the section as a very fine wedge then find an area of the section where you can see cells clearly. Put drops of 30% sugar solution next to the coverslip. Use absorbent paper to draw the sugar solution across the section of beetroot tissue. Note the cells that plasmolyse. Put drops of tap water next to the coverslip. Use absorbent paper to draw the water across the section of beetroot tissue. Note the cells that deplasmolyse.
9.177 Plasmolysis in onion epidermis
See diagram 9.36.15: Microscope technique
1. Remove the outer layer of skin from an onion with a razor blade to expose the leaf scales. Use forceps to detach a small piece of the epidermis from the external convex side of a leaf scale. Put this piece immediately in a drop of demineralized water on a microscope slide. Cover the drop with a coverslip and examine the epidermis under low power. Put drops of 6% sodium chloride solution to one side of the cover slip and draw it across under the coverslip with absorbent paper at the other side. Note any change in the cells. Place drops of demineralized water to one side of the coverslip and draw them across under the coverslip with absorbent paper at the other side. Note any change in the cells. Repeat the experiment with red rhubarb stalks.
9.178 Plasmolysis in hairs on the stamens of spiderwort, Tradescantia
See diagram 9.36.16: Plasmolysis in Tradescantia
1. Mount a complete leaf in water on a slide and examine cells under the high power. Note the green chloroplasts that help to define the extent of the cytoplasm. Use absorbent paper to draw concentrated sugar or salt solution across the cell while still looking down the microscope. The cytoplasm shrinks into the centre of the cell away from the cell wall, taking the chloroplasts with it. Plasmolysis has occurred.
2. Repeat the experiment by drawing pure water across the same waterweed cells. The cytoplasm and chloroplasts spread out through the cell. Plasmolysis has been reversed.
3. Repeat the experiment with waterweed, Elodea, dipped in boiling water for two minutes. Use absorbent paper to draw concentrated sugar or salt solution across the cell while still looking down the microscope. Plasmolysis does not occur because only living cells possess semipermeable membranes that control plasmolysis.
9.179 Imbibition in seeds and dried fruit, broad bean, pea, bean
1. Measure the displacement of dry broad bean or pea seeds with a measuring cylinder partly filled with water. Put the seeds in moist sawdust for two days and then again measure their displacement. Estimate the percentage increase in volume because of the imbibition of water.
2. Put sultanas, grains, dried apricots in pure water and leave them for some time. Then place them into a concentrated solution of sugar or salt. Each gains water and swells when placed in pure water, then shrinks in the concentrated solution.
9.180 Colloidal solution of starch
1. Shake 2 g of starch with a little cold water in a test-tube until it forms a paste. Boil 100 mL of water in a beaker, and while the water is boiling, slowly pour the paste from the test-tube, drop by drop, into it. After cooling, note that this colloidal solution is opaque. Test the solution for starch by adding a few drops of iodine solution.
9.181 Separate a colloid from a crystalloid by dialysis
Dialysis is the use of a semipermeable membrane to separate a colloidal solution from a true solution, i.e. separating small molecules from large molecules. A Soxhlet thimble is a filter made usually of cellulose or thick paper. It looks like a white test-tube. Collodion, cellulose tetranitrate, is made by dissolving nitrocellulose (gun cotton, nitrate movie film) in acetone. It was used in medicine to cover wounds and with salicylic acid as a wart remover. Also, it was used in wet plate photography.
1. Dip a finger in collodion, wave it in the air until the collodion dries, then pull off your dialyser thimble. Wash the dialyser with tap water. Add sodium chloride solution to a 2% colloidal solution of gelatine to produce a mixture of a colloidal solution and true solution. Put the mixture in the dialyser. Put the dialyser in a beaker of water so that the level of water in the beaker and in the dialyser are the same. After one day, test the liquid in the dialyser with tannic acid that precipitates gelatine out of solution.
2. Test the liquid in the beaker with silver nitrate solution that reacts with sodium chloride to produce a white precipitate of silver chloride. The crystalloid sodium chloride passes through the membrane but the colloid collodion does not. An artificial kidney works in the same way. Be careful! Silver nitrate is poisonous if swallowed or inhaled and leaves silver to black stains on the skin that cannot be removed with soap and water. Students should not do the silver nitrate test. Repeat the experiment with a colloidal solution of starch. Test the liquid in the dialyser with the iodine tests for starch.
3. Make a dialyser by soaking a Soxhlet thimble in a 5% solution of collodion in glacial acetic acid. Alternatively, you can try making a dialyser by dipping your finger in collodion then waving you finger in the air. Wash the dialyser with tap water. Add sodium chloride solution to a 2% colloidal solution of gelatine to produce a mixture of a colloidal solution and true solution. Put the mixture in the dialyser. Put the dialyser in a beaker of water with the water level with the mixture in the dialyser. After one day test the liquids. Test the liquid in the dialyser with tannic acid that precipitates gelatine out of solution. Test the liquid in the beaker with silver nitrate solution that reacts with sodium chloride to produce a white precipitate of silver chloride. The crystalloid sodium chloride passes through the membrane but the colloid collodion does not. An artificial kidney works in the same way. Repeat the experiment with a colloidal solution of starch. Test the liquid in the dialyser with the iodine tests for starch.
10.2.0 Osmosis, osmotic pressure, reverse osmosis
Osmosis is a modification of diffusion, namely the penetration of liquids and solutions through a porous membrane. Some membranes, which are permeable to one type of liquids and solutions, are partially or completely impermeable to other liquids and solutions. Such membranes are called semipermeable membranes.
Reverse osmosis membrane elements are used in a variety of applications, desalination of seawater and brackish water to produce pure water, treatment and recycling of effluent for te recovery of valuable process materials and the concentration of foodstuffs, e.g. milk.
10.2.1 Osmometer, carrot or potato osmometer
See diagram 9.3.47: Osmosis with a carrot
Prepare a one-hole stopper with a long glass tube inserted through the hole, 2 cm below the stopper and 20 cm or more above the stopper. Be careful when inserting the stopper! Cut a hole in the side of a large carrot, or potato, the same diameter as the middle of the one-hole stopper. Insert a spoon or knife into the hole to make the carrot hollow. Fill the carrot with concentrated sucrose solution, containing drops of ink, by pouring through the hole. Put some petroleum jelly on the rim of the hole. Insert the stopper with the glass tube into the hole of the carrot. It must fit tightly. By moving the glass tube through the stopper you can adjust the height of the coloured sugar solution in the tube. Fill any gap between the carrot and the stopper with hot candle wax. Hold the carrot in a tall beaker. Pour water into the beaker to submerge the carrot and a short length of the glass tube. Clamp the carrot upright without squeezing it. Record the height of the coloured sugar solution in the glass tube and note the depth of the ink colour. Also, record the height of the water in the beaker. Later, record the heights again and note depth of colour in the glass tube. Water penetrates the wall of the carrot to dilute the colour of the sugar solution that rises in the tube. The level of water in the beaker falls.
16.4.1.1a Vitamin A
Vitamin A is a group of substances including retinol and retinal. Deficiency of vitamin A leads to anaemia and night blindness When light strikes the retinal / opsin complex in the retina, a double bond in retinal is converted from (cis-) to (trans-) to send a signal to the optic nerve. Vitamin A is used to maintain epithelial tissue. Normal blood contains 15 to 60 mg retinol per 100 mL of serum. The vitamin A precursor is β-carotene.
16.4.1.2 Vitamin B1, thiamine, vitamin B2, riboflavin
See diagram 16.4.1.2: Thiamine
Vitamin B1, thiamine, thiamin, C12H17ClN4OS, is a water soluble factor that is a cofactor for many enzymes, e.g. enzymes that release carbon dioxide from beta-keto acids. Thiamine occurs in the brown coating of unpolished rice and other cereal grains. Thiamin deficiency causes the disease beriberi.
Vitamin B2 complex vitamins occur in cereals, liver and yeast. For example, riboflavin (lactoflavin, vitamin B2) (Latin: flavus, yellow), orange-yellow crystalline compound, C17H20N4O6, is a growth promoting factor, occurs in milk, leafy vegetables, fresh meat, and egg yolks.
In fresh milk, droplets and suspended particles reflect light in all directions so the milk appears white and opaque. As milk freezes and transparent ice crystals form, the remaining liquid has a higher concentration of riboflavin so the milk appears yellow.
16.4.1.3 Vitamin C (ascorbic acid)
See diagram 16.4.1.3: L-Ascorbic acid (vitamin C)
Vitamin C, ascorbic acid, is the water soluble antioxidant and used for formation of collagen, bone, teeth, and tendons and for amino acid metabolism. Lack of ascorbic acid results in scurvy that can be prevented by a Recommended Daily Allowance, RDA of 60 m for young adult males.
16.4.1.4 Vitamin D
Vitamin D is a group of compounds including pyridoxine, pyridoxal and pyridoxamine. The latter two are cofactors for some metabolic enzymes for catalysis, biosynthesis and degradation of amino acids. Sunscreen inhibits the production of Vitamin D. People need 15 minutes per day of direct exposure to the sun outside peak UV times, i.e. 10 am to 2 pm. Office workers who always wear long-sleeved clothing and women from Middle East countries living in the Northern hemisphere may have insufficient levels of vitamin D. However, people who eat oily fish may have sufficient levels of vitamin D at the end of winter.
16.4.1.5 Vitamin E
Vitamin E, α-tocopherol, an oil soluble anti-oxidant found in polyunsaturated oils in amounts necessary to protect the them against oxidation.
See 19.2.1.6: Antioxidant phenols, antioxidants, vitamin E, beta-carotene