School Science Lessons
Plant physiology, photosynthesis, chlorophyll, chloroplasts
2012-05-13 SPwp
Table of contents
9.8.0 Photosynthesis
6.5.2 Chlorophyll, chloroplasts
9.8.0 Photosynthesis
6.5.01 Air is necessary for photosynthesis
6.5.02 Bean plants growing in the light and in the dark
6.5.1 Carbon dioxide in the air is necessary for photosynthesis, nasturtium
6.5.2 Chlorophyll, chloroplasts
6.5.5 Chlorophyll is necessary for photosynthesis, variegated leaf
6.5.0 Light is necessary for photosynthesis
6.5.16 Photosynthesis and light intensity, sodium hydrogen carbonate solution
3.36 Photosynthesis equation, carbon dioxide and photosynthesis
6.5.15.3 Photosynthesis in a shaded waterweed leaf
6.5.15.0 Photosynthesis in waterweeds
5.03 Photosynthesis, sunlight is necessary for photosynthesis
5.04 Photosynthesis, oxygen gas is formed during photosynthesis
6.4.6 Phototropic responses of seedlings
6.5.17 Plants growing in the dark, phototropism
1.30 Plants need sunlight (Primary)
9.9.5 Seedlings growing in the light and in the dark, e.g. pea
9.142 Tests for starch, Fehling's tests for starch
9.132 Tests for starch, iodine tests for starch
9.140 Tests for sugars, simple sugars, reducing sugars, Fehling's test
6.5.15.1 Waterweeds lose bubbles of oxygen during photosynthesis
6.5.15.2 Waterweeds in the light and in the dark
6.5.15.4 Waterweeds use carbon dioxide for photosynthesis
6.5.2 Chlorophyll, chloroplasts
3.36 Carbon dioxide and photosynthesis
16.3.5.2.3 Chlorophyll a and chlorophyll b
6.5.5 Chlorophyll is necessary for photosynthesis, variegated leaf
6.5.9 Chlorophyll fluorescence
6.5.7 Chlorophyll from green leaves, potato, onion, nasturtium, fuchsia, hyacinth, lilac
6.5.8 Chlorophyll pigments separated with paper chromatography
6.5.15.5 Chloroplasts in cells of waterweeds
6.5.4 Chloroplasts, Spirogyra, Zygnema, Closterium
3.24 Separate pigments from green leaves with paper chromatography
9.132 Tests for starch, iodine tests for starch
6.5.0.1 Bean plants growing in the light and in the dark
Compare the growth of the bean plants growing in sunlight and growing kept in the dark. The plants kept in the dark have a pale yellow colour due to absence of chlorophyll, small leaves and long thin stems because of abnormal lengthening of the internodes and small leaves. These plants are describes as being etiolated. Plants ultimately die if kept too long in the dark.
6.5.0 Light is necessary for photosynthesis
See diagram 9.145.1: Band of foil on leaf
1. Fix a band over a leaf. Fold "silver paper" from a chocolate packet, or aluminium cooking foil, in a band around a large leaf on a tree growing in the sunlight. After three days, remove the band and drop the leaf in boiling water to kill the cells. Put the leaf in methylated spirits to remove the chlorophyll. Do the iodine tests for starch on the whole leaf. The part of the leaf not covered by the band paper turns a blue-black colour. The part of the leaf covered by the band does not turn blue-black because of lack of sunlight for photosynthesis to make starch.
2. Three days before the lesson fold a piece of silver paper from a chocolate packet to form a band around a leaf on a tree growing in the sunlight. After three days, pick the leaf off the tree and remove the silver paper. Drop the leaf in boiling water to kill it. Put the leaf in methylated spirits to remove the green substance called chlorophyll. Place the leaf in an iodine solution. The part of the leaf not covered by the silver paper turns blue black colour. The part of the leaf covered by the silver paper does not turn blue black because there was no sunlight for photosynthesis to make starch. Students could form their own initials out of silver paper and "write" them on a leaf. The simple sugars produced by photosynthesis are soon changed to starch in the leaf. When iodine solution is added to starch it changes from a white colour to a blue black colour. A leaf with a silver paper band around it is left on a tree for three days. The leaf is then picked and tested for starch. The part of the leaf shaded by the silver paper shows no starch present. The part of the leaf not shaded shows starch.
See diagram 9.150: Cork discs on leaves
3. Use pins to fix cork discs in pairs on leaves of a potted nasturtium plant so that each leaf has two discs exactly opposite each another on the upper and lower surface. In this way, you exclude light from part of the leaves. Do this in the evening. The next morning, place the plant in strong sunlight. After six hours, cut these leaves from the plant and kill the cells by putting the leaves in boiling water for one minute. Then put them in 96% ethanol. After one hour, the leaves will have become almost colourless. Rinse off the ethanol with water and do the starch test with iodine solution. Draw the leaves and record where you can detect starch.
6.5.01 Air is necessary for photosynthesis
See diagram 9.147: Leaf smeared with petroleum jelly
1. Choose a green leaf but do not remove it from the plant. Smear petroleum jelly ("Vaseline") over some of its area. Do this on both sides of the leaf making the areas coincide with each other. Allow the leaf to remain in such a condition on the plant for two days. Remove the leaf from the plant and scrape off the petroleum jelly. Test the whole leaf for starch using the iodine test. Note where the leaf was smeared with petroleum jelly.
2. Smear a band of petroleum jelly over both sides of leaf on a growing plant. After two days, remove the leaf from the plant and scrape off the petroleum jelly. Do the iodine tests for starch on the whole leaf. See the pattern of the band where the petroleum jelly was on the leaf.
6.5.02 Bean plants growing in the light and in the dark
Compare the growth of the bean plants growing in sunlight and growing kept in the dark. The plants kept in the dark have a pale yellow colour due to absence of chlorophyll, small leaves and long thin stems because of abnormal lengthening of the internodes and small leaves. These plants are describes as being etiolated. Plants ultimately die if kept too long in the dark.
6.5.1 Carbon dioxide in the air is necessary for photosynthesis
See diagram 9.151: Split rubber stopper
1. Insert a split rubber stopper firmly. Smear petroleum jelly over the surface of the stopper to prevent any air entering the vessel either between the stopper and the neck or between the stopper and the plant stem. Place the apparatus in sunlight. Leave for two days, then do the iodine tests for starch on a leaf from that part of the twig inside the vessel and also a leaf outside the vessel. The potassium hydroxide has absorbed the carbon dioxide in the enclosed air.
See diagram 9.146: Nasturtium
2. Put a mature nasturtium plant in a plant pot in a dark place in the evening.. On the following morning, fill a glass container one third full with 20% potassium hydroxide solution. Place the nasturtium plant beside the glass container. Bend a leaf stem without breaking it and insert one leaf into the upper air filled section of the glass container. Cover the jar with a slotted cover to let the leaf stem to pass through the slot. Seal the slot around the stem with adhesive tape. Completely seal the glass container with adhesive tape Potassium hydroxide absorbs carbon dioxide so the leaf sealed inside the glass container is in an atmosphere without carbon dioxide. Put the plant and culture jar in direct sunlight so that the leaf inside the glass container is exposed to the light source. After three hours, detach the leaf inside the glass container from the plant. Also detach some other leaves which have had equal exposure to light. Kill the cells in the leaves by dropping them into boiling water. After one minute, remove them with tweezers and place them in 96% ethanol. After 2 hours the leaves become almost colourless. Rinse the leaves in water and pour potassium iodide solution over them. Note which leaf is coloured blue violet by the iodine potassium iodide solution.
6.5.4 Chloroplasts, Spirogyra, Zygnema, Closterium
See diagram 9.39.1: Filamentous algae, Spirogyra, Zygnema, Closterium
The green leaf pigment chlorophyll is not usually present in plant cells in solution, but in chloroplasts. To study the shape of the chloroplasts in various plant species, use forceps to detach a few leaves from a stem of moss that has large leaves. Put them in a drop of water on a microscope slide. Mount a coverslip. Examine the slide under low power. Use forceps to place a few cell filaments of Spirogyra in a drop of water on a slide. Mount a coverslip. Examine the slide under high power. Study cell filaments of a stellate algae, Zygnema. Use a pipette to put cells of a desmid, Closterium, on a microscope slide. Mount a coverslip. Examine the preparation under high power. Note the shapes of the chloroplasts and their position inside the cell. Study other species of plants including seed bearing plants, ferns, mosses, and algae to determine the shape of their chloroplasts.
6.5.5 Chlorophyll is necessary for photosynthesis, variegated leaf
See diagram 9.149: Variegated leaf
Use a thin variegated leaf, that has green or other colour parts and white parts, e.g. Chinese lantern (Abutilon). The white parts contain no pigment necessary for photosynthesis. Leave the plant for days in strong sunlight, then pick a leaf and drop it in boiling water to kill the cells. Put the leaf in methylated spirits to remove the chlorophyll. Do the iodine tests for starch on the whole leaf. The green part, or other coloured part, turns a blue-black colour. The white area part of the leaf does not turn blue-black.
6.5.7 Chlorophyll from green leaves, potato, onion, nasturtium, fuchsia, hyacinth, lilac
Heat a beaker of water with an electric heater. Do NOT use a Bunsen burner. Use thin leaves and kill them by putting in boiling water for a few minutes. Put half a large test-tube of ethyl alcohol in a beaker containing recently boiled water. Immerse the killed leaves in the alcohol. Note how the leaves gradually lose their colour because the alcohol dissolves the chlorophyll. To show that only green leaves make starch by photosynthesis, choose a leaf and treat it as above until it becomes whitish through loss of colour. Then use the iodine tests for starch.
6.5.8 Chlorophyll pigments separated with paper chromatography
1. Cut dark green spinach leaves into small pieces then crush them with a spoon or a mortar and pestle or an electric mixing machine. Put the crushed leaves and juice in a beaker. Cover the crushed leaves and juice with acetone, nail polish remover. Let the pieces of leaves settle down to the bottom of the green liquid. Cut out a rectangle of paper towel or paper coffee filter or paper napkin. Put a pencil across the beaker. Hang the rectangle of paper over the pencil so that one end of the paper is in the green liquid. Leave the beaker to stand for several hours. Note the colours called pigments that have moved up the paper.
Study the pigments in order of separation from top to base:
1.1 Carotenoid pigment (yellow) and some decomposition products move with the solvent front
1.2 Carotenoid pigment (yellow)
1.3 Chlorophyll a (blue-green)
1.4 Chlorophyll b (yellow-green)
2. Dry dark green leaves, silver beet, in an oven at 70oC. Grind the leaves then add 80% acetone to extract the chlorophyll. Add the extract to a separating funnel containing petroleum ether. On the addition of water to dilute the acetone the pigments become less soluble in the dilute acetone and dissolve in the petroleum ether. A complete transfer of pigment occurs from acetone to petroleum ether. Keep the solution in the dark or it will decompose to a brown colour. Dip a strip of filter paper in the chlorophyll solution and let it run up 2 cm evenly. Pull out the filter paper and dry by waving in the air. Repeat two times with the same piece of filter paper. Open a specimen tube containing two organic solvents, petroleum ether and benzol. Fix the filter paper in the specimen tube so that it does not touch the sides then close the tube. Watch the chromatogram develop leaving the specimen tube closed. The solvent moves ahead of most of the pigments. The pigments move at different rates depending on their relative solubility in the solvent and on their relative absorption by the paper.
3. Study the pigments in order of separation from top to base:
3.1 Carotenoid pigment (yellow) and some decomposition products move with the solvent front
3.2 Carotenoid pigment (yellow)
3.3 Chlorophyll a (blue-green)
3.4 Chlorophyll b (yellow green)
6.5.9 Chlorophyll fluorescence
1. Grind green leaves in acetone with a mortar and pestle then filter through a coarse filter then absorbent paper. The filtrate is an acetone solution of almost pure chlorophyll. Note the red fluorescence as seen by looking through the solution held against a dark background. Direct a bright beam of light at the solution and note the deep red glow. The fluorescent light emitted by chlorophyll is red light at a longer wavelength, lower energy, than the absorbed light. The chlorophyll electrons become excited by the light energy, but the acetone has dissolved the chloroplast membranes so the absorbed energy cannot be used for photosynthesis but instead the chlorophyll electrons lose their excited energy state as a reddish glow. In the normal situation sunlight energy is converted to a chemical form and used in photosynthesis, not emitted as fluorescent light. Chlorophyll strongly absorbs blue and red light. Leaves appear green because wavelengths of light from the red and blue regions of the visible spectrum are necessary to excite the chloroplast electrons, so the unused green light is reflected.
2. Make an extract of chlorophyll in 85% acetone from dried nettle leaf powder. Place some of the powder in a funnel fitted with a filter paper, slowly add the acetone and collect the filtrate in a beaker or test-tube. The filtrate is an acetone solution of almost pure chlorophyll. Note the red fluorescence as seen by looking through the solution held against a dark background. Chlorophyll strongly absorbs blue and red light. The fluorescent light emitted by chlorophyll is red light at a longer wavelength, lower energy, than the absorbed light. The energy converted to chemical form and used in photosynthesis is not emitted as fluorescent light.
6.5.15.0 Photosynthesis in waterweeds
Waterweeds are fast growing oxygenating plants that compete with algae for plant nutrients to keep the water clean in ponds and aquariums. In some countries these plants are listed as weeds because they are invasive and may congest waterways. They include: Canadian waterweed (elodea), American eelgrass, fanworts, hornworts (coon's tail).
6.5.15.1 Waterweeds lose bubbles of oxygen during photosynthesis
See diagram 9.3.41: Elodea producing oxygen 1 | See diagram 9.146.1: Elodea producing oxygen 2
1. The photosynthesis activity of leaves can be shown by putting waterweeds, e.g. elodea, in a funnel, inverting the funnel in a large beaker of water and putting a test-tube over the small end of the funnel. A fine piece of tubing or plastic electrical insulation is used in the manner of a drinking straw to remove the air from the test-tube, thus filling it with water. Several dabs of putty put between the funnel and the beaker will permit free circulation of the water from a beaker into the funnel. The water plants should not be in contact with a zinc container before putting in the apparatus. Test the gas that bubbles from the plant by collecting in the test-tube and putting in a glowing wood splint and watching for it to flame. Elodea has a hollow stem, so if you punch at the end with a pin it will release oxygen bubbles faster and in a stream that you can count for quantitative results.
2. Put waterweed into the barrel of a 50 cm3 syringe. Fill the syringe with a 5% sodium hydrogen carbonate solution. Insert plunger into the barrel of the syringe. Avoid trapping any air inside the barrel. Connect the syringe to a graduated 1 cm3 pipette with a short length of rubber tubing. Expel any air trapped inside the syringe or pipette turning the apparatus with the open end of the pipette pointing upwards and slowly pushing the plunger into the barrel. Fix the apparatus in a vertical position with clamp and stand. Adjust the position of the plunger until the liquid level lies in the upper region of the 1 cm3 pipette. Oxygen produced by the plant collects above the sodium hydrogen carbonate solution, increasing gas pressure to push the meniscus reading level down the pipette. Record the volume of oxygen released every 10 minutes. When the meniscus reaches the lower end of the pipette, it can be moved up again by adjusting the position of the plunger.
3. Drop some pieces of glass tubing in a beaker of water and invert a glass funnel in the beaker so that its edge rests on the glass tubing . Hold a test-tube under water to remove any air. Seal the opening with your thumb and invert the test-tube over the spout of the funnel. Pour water containing waterweed, e.g. elodea, into the beaker. Use forceps to shake the waterweed free of bubbles then place it under the inverted funnel. The waterweed should not be in contact with a zinc container prior to placement in the beaker. Place the apparatus in bright sunlight or use an electric lamp. Test the gas which bubbles from the plant in a small test-tube with a glowing wood splint. Elodea has a hollow stem so you can pinch it at the end with a pin to release oxygen bubbles faster and in a stream so they can be counted to give quantitative results.
4. To detect the release of oxygen during photosynthesis, fill a glass container with water to 2 cm below the top, drop in 8 well developed shoots of a waterweed, e.g. elodea, and add a little mineral water to enrich the carbon dioxide content. Put an inverted bell jar with its tap open over the waterweed and push it down into the vessel until water reaches up to the tap. Then close the tap, place the slotted lid on the glass container and suspend the inverted bell jar from the lid by its tap, cushioned on a piece of cotton wool. Put the vessel in as bright a place as possible so that it is exposed, at least for some time, to direct sunlight. If this is not possible, expose the vessel to electric light (from a microscope lamp) for several hours. What can you see in the bell jar on the following day? Hold a glowing wood splint immediately over the outlet of the inverted bell jar and open the tap. What happens? What gas has been detected by this method?
6.5.15.2 Waterweeds in the light and in the dark
See diagram 9.145.1: Waterweed in the light and in the dark | See diagram 9.150: Cork barrier
1. Tap water usually contains enough carbon dioxide to support photosynthesis for submerged plants. Put fresh green waterweed, e.g. Elodea, in a test-tube full of water. Place the apparatus in bright sunlight or under a 100 watt lamp. Note the small bubbles rising from the cut ends of the waterweed. Collect the gas in the test-tube by simple downward displacement of water. When the test-tube is full of gas, remove it and test the gas with a glowing splint. The gas is oxygen. Repeat the experiment by adding a 1% solution of sodium hydrogen carbonate (sodium bicarbonate) or potassium bicarbonate. The rate of bubbling increases. Put the waterweed in the dark for a few days. Bubbles of oxygen no longer rise from the surface of the leaves.
2. Start the demonstration at the beginning of the lesson and allow students to see it again in the next lesson. A suitable waterweed is Elodea. The demonstration works better if you add some sodium bicarbonate (baking soda) to the water.
Put green waterweed is placed inside a test-tube then invert it under water. The test-tube should not contain any air bubbles. Leave the test-tube in the sunlight. Repeat the experiment by leave the test-tube in the dark. After some hours the waterweed in the light has bubbles of oxygen gas coming from it but no bubbles of oxygen gas come from the waterweed in the dark. To test for oxygen gas, light a thin piece of wood then blow out the flame leaving the wood glowing red. If you put the glowing wood into oxygen, the glowing wood will burst into flame. The bubbles of gas is oxygen if the gas in the test-tube makes a glowing piece of wood burst into flames.
6.5.15.3 Photosynthesis in a shaded waterweed leaf
See diagram 9.150: Cork discs on leaves
Choose two small corks equal in size and use two pins to fix them opposite each other on the surfaces of a leaf of the waterweed, Elodea, without removing the leaf from the plant. Do not crush the tissues of the leaf. After 24 hours and towards the end of the day, test the leaf for starch with the iodine test.
6.5.15.4 Waterweeds use carbon dioxide for photosynthesis
See diagram 9.3.45: Measure leaf activity of elodea
Fill four test-tubes three-quarters full of water. Add 25 drops bromothymol blue solution to each test-tube. Put a length of waterweed, e.g. elodea, in test-tube 1. and test tube 2. Use a drinking straw to blow bubbles into test-tube 3. and test-tube 1. Note the colour change of the bromothymol blue solution that shows the presence of carbon dioxide. Attach stoppers to the four test-tubes and observe the changes every 15 minutes for an hour. Repeat the experiment, in a dark place.
6.5.15.5 Chloroplasts in cells of waterweeds
See diagram 1.1: Filamentous algae, Spirogyra
Use a glass rod to transfer a drop of water from a beaker to a microscope slide. Using tweezers pluck a leaf from a shoot of waterweed, e.g. Elodea. Put the leaf in the drop of water. Mount a coverslip. Examine the specimen with 50 × magnification. The leaf is built up from individual cells as was the onion epidermis examined in 3.2.1. However, besides the parts seen in the onion cells the cells of the waterweed contain many green grains. Examine the slide under low power. Draw the shape of the green grains. Examine the slide under high power. Draw the shape of the green grains, called chloroplasts. They contain the green leaf pigment chlorophyll. Chlorophyll is usually found only in the chloroplasts. Note the large spiral chloroplast in the green algae Spirogyra.
6.5.16 Photosynthesis and light intensity, sodium hydrogen carbonate solution
1. Vary the light intensity by changing the distance of the light source from the apparatus, projector light
2. Vary the light quality by covering the syringe with cellophane of different colours. Include a control identical to the experiment but without a plant.
3. Vary the light quality on pond Selaginella, Use an overhead projector to provide a strong and uniform light source, placed at 1. 00 in, 0.90 m, 0.75 in, 0.60 and 0.50 m from the plant. A 5% sodium hydrogen carbonate solution was used to provide an abundant supply of carbon dioxide. The volume of oxygen released was measured at 10 minute intervals, from which the rate of photosynthesis was calculated. For each light intensity, three measurements were taken and the mean value was used for plotting the graph.
4. Vary the concentration of sodium hydrogen carbonate solution from 0 to 5% w / v.
6.5.17 Plants growing in the dark, phototropism
Compare the growth of the bean plants growing in sunlight and growing in the dark. The plants in the dark have a pale yellow colour due to absence of chlorophyll, long thin stems because of abnormal lengthening of the internodes, and small leaves. These plants are describes as being etiolated. Plants ultimately die if kept too long in the dark.
See diagram 9.3.58: Phototropism
1. Cover seedlings growing in pots or a sprouting potato with a black box with a hole in one side. Observe the change in direction of growth as the plants turn towards the light from one side. Repeat the experiment by removing the growing tip from some plants. These plants do not turn towards the light. Repeat the experiment with the hole in the box covered with red, then yellow then blue cellophane. Phototropism is caused by increased concentration of the growth hormone auxin on the dark side of the growing tip of the shoot. So the plant cells grow more on the shaded side. Plants growing in the dark grow faster than plants growing in the light, but they become etiolated, pale yellow, due to lack of sunlight. Phototropism is more responsive to blue light than any other colour.
2. Find a plant that grows in the shade and put it in a flowerpot. Turn the flowerpot on its side and observe the resulting direction of growth of the shoot and the leaves. The shoot turns towards the light and the leaves turn to be at right angles to the source of light. Leaves are diaphototropic, orientated at right angles to the vertical in response to light.
3. Observe plants that turn towards the sun, e.g. sunflower.
4. Tropism as a reaction to light stimuli, phototropism, is shown by the shoots and roots of higher plants. Put a few erect bean seedlings with roots 5 cm long on a cork disc with seven holes. Insert the roots through the holes. Put the disc on the liquid surface of a glass container filled with nutrient solution. Put the apparatus in a window box and check the growth of the seedlings daily. The shoots bend towards the light but the roots turn away from the light. The shoots and roots react in positive and negative phototropic manner respectively. In this way shoots can adapt to the light conditions necessary for the plant and roots grow into the soil to obtain nutrients.
5. Plant ten bean seedlings, each having roots 1 cm long in two flowerpots containing garden soil. Put the pots, each sitting in one half of a dish, in a window box. Put one of them on a clinostat and set it in motion. Regularly water and check the growth of the plants. The plants in the static pot still bend towards the light. The plants rotated on the clinostat grow straight up. The plants on the rotating clinostat all receive an equal amount of light so do not bend to one side.