School Science Lessons
2024-10-28
(UNBiology6a)
Photosynthesis
Please send comments to: J.Elfick@uq.edu.au.
Contents
6.5.0 Photosynthesis
6.6.0 Chlorophyll, chloroplasts
6.5.0 Photosynthesis
6.5.1 Photosynthesis, 3 types of photosynthesus, RuBisCo
RuBisCo C5H12O11P2
Experiments
6.5.2 Air is necessary for photosynthesis, "Vaseline"
6.5.3 Carbon dioxide is necessary for photosynthesis, Nasturtium
6.5.4 Growth rate of Isochrysis galbana, on nitrate culture medium
6.5.5 Light is necessary for photosynthesis, Experiments
6.5.7 Microalgae
6.5.8 Oxygen gas is formed during photosynthesis, Elodea
6.5.9 Photosynthesis and light intensity, Na2CO3 solution, Selaginella
6.5.10 Photosynthesis and light intensity, NaHCO3 solution, leaf discs
6.5.11 Photosynthesis in shaded waterweed leaves
6.5.12 Photosynthesis in waterweeds, Elodea
6.5.13 Plants growing in the dark, phototropism
6.5.14 Waterweeds in the light and in the dark
6.5.15 Waterweeds use carbon dioxide for photosynthesis
6.6.0 Chlorophyll, chloroplasts
E140 Chlorophylls and chlorophyllins
Carbon dioxide from photosynthesis
9.0.2 Green algae, Phylum Chlorophyta, Class Chlorophyceae
6.6.1 Photosynthetic pigments, chlorophyll a and chlorophyll b
Experiments
6.6.2 Chlorophyll is necessary for photosynthesis, variegated leaf
6.6.3 Chlorophyll fluorescence
6.6.4 Chlorophyll from green leaves
6.6.5 Chlorophyll pigments separated with paper chromatography
6.6.6 Chloroplasts in cells of waterweeds
6.6.7 Chloroplasts, Spirogyra, Zygnema, Closterium
6.5.1 Photosynthesis, 3 types of photosynthesus, RuBisCo
1. Photosynthesis is the process by which carbon dioxide is converted into organic compounds using the energy of light absorbed by chlorophyll to form oxygen gas from water and to synthesize chemical compounds.
Photosynthesis is an anabolic set of reactions, i.e. plant cells can synthesize complex molecules from simple ingredients.
The reactions for photosynthesis take place in chloroplasts in the leaf cells.
Chloroplasts contain pigments that can capture energy from sunlight and then use the energy to produce compounds containing carbon.
Summary equation for photosynthesis:
6CO2 + 12H2O + solar energy -->; C6H12O6 + 6H2O + 6O2
The rate of the photosynthesis reactions is affected by light intensity and light wavelength.
2. The three types of photosynthesis
Type 1. C3 photosynthesis
It is called C3, because the carbon dioxide is first incorporated into a 3-carbon compound.
The stomata are open during the day.
Photosynthesis occurs throughout the leaf.
It is the most efficient type of photosynthesis for plants under cool and moist conditions and under normal light, because requires fewer enzymes and no specialized anatomy.
Most plants are C3. RuBisCo, (Ribulose-1,5-bisphosphate carboxylase oxygenase), the enzyme involved in photosynthesis, is also the enzyme involved in the uptake of carbon dioxide.
Type 2. C4 photosynthesis
It is called C4, because the carbon dioxide is first incorporated into a 4-carbon compound.
It uses PEP carboxylase for the enzyme involved in the uptake of carbon dioxide.
This enzyme allows carbon dioxide to be taken into the plant very quickly, and then it "delivers" the carbon dioxide directly to RuBisCo for photosynthesis.
Photosynthesis takes place in certain inner cells, called "Kranz anatomy", which photosynthesizes faster than C3 plants under high light intensity and high temperatures, because the carbon dioxide is delivered directly to RuBisCo, not allowing it to grab oxygen and undergo photorespiration.
It has better water use efficiency, because PEP Carboxylase brings in carbon dioxide faster and so does not need to keep stomata open as much, less water lost by transpiration, for the same amount of carbon dioxide gain for photosynthesis.
C4 plants include saltbush, maize, many summer annual plants.
Type 3. CAM photosynthesis
In CAM photosynthesis, (crassulacean acid metabolism), the carbon dioxide is stored as an acid, before use in photosynthesis.
The stomates usually open at night and are closed during the day.
The carbon dioxide is converted to an acid and stored during the night.
During the day, the acid is broken down and the carbon dioxide is released to RuBisCo for photosynthesis.
Better water use efficiency than C3 plants under arid conditions.
When conditions are extremely arid, CAM plants can leave their stomates closed.
The oxygen given off in photosynthesis is used for respiration and carbon dioxide given off in respiration is used for photosynthesis.
This is a little like a perpetual energy machine, but there are costs associated with running the machinery for respiration and photosynthesis, so the plant cannot CAM-idle forever.
However, CAM-idling does allow the plant to survive dry spells, and it allows the plant to recover very quickly when water is available again, unlike plants that drop their leaves and twigs and go dormant during dry spells.
The CAM plants include cactuses, agaves, orchids and bromeliads.
6.5.2 Air is necessary for photosynthesis, "Vaseline"
See diagram 9.147: Leaf smeared with petroleum jelly.
Smear a band of petroleum jelly ("Vaseline"), over both sides of leaf on a growing plant.
Do this on both sides of the leaf making the areas coincide with each other.
After two days, remove the leaf from the plant and scrape off the petroleum jelly.
Do the iodine test for starch on the whole leaf.
See the pattern of the band where the petroleum jelly was on the leaf.
6.5.3 Carbon dioxide is necessary for photosynthesis, Nasturtium
See diagram 9.151: Split rubber stopper.
See diagram 9.146: Carbon dioxide is necessary for photosynthesis, Nasturtium.
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 a leaf outside the vessel.
The potassium hydroxide has absorbed the carbon dioxide in the enclosed air.
2. Put a flowerpot containing a soft leaf plant, e.g. Nasturtium, in the dark.
The next day put the flowerpot next to a 20% potassium hydroxide solution in a container with a slotted cover.
Bend a stem and insert one leaf into the upper section of the container.
Seal the slot around the stem with petroleum jelly or adhesive tape.
Potassium hydroxide absorbs carbon dioxide so the leaf sealed inside the container is in an atmosphere without carbon dioxide.
Put the plant and container in sunlight.
After three hours, detach the leaf inside the container and detach other leaves that had equal exposure to light.
Drop the leaves in boiling water to kill the cells.
Put the leaves in methylated spirit then remove them when they are almost colourless.
Rinse the leaves in water then do the iodine test for starch.
Compare the blue-black colour of the leaves.
6.5.4 Growth rate of Isochrysis galbana, on nitrate culture medium.
Use: 100 ml conical flasks, 4 X 1ml pipettes, aluminium foil, haemocytometer, microscope Isochrysis galbana stock culture, Walne’s medium artificial seawater or 1 micron filtered, autoclaved sea water, lux meter, thermometer, measuring cylinder.
1 Add to flask 50 ml of sterile artificial seawater or filtered sea water from an aqua culture facility.
2. Add Walne’s medium.
3 Add 25 ml of Isochrysis galbana, (concentration: 1 x106 cells per ml).
4. Swirl the conical flask for equal distribution of the culture solution.
5. Do initial cell counts with the haemocytometer at 400X magnification.
6. Close flask with aluminium foil.
7. Place flask in 1000 lux light for 12 to 14 hours below 25oC.
8. Record the cell count on every 2nd day for 14 days.
9. Repeat above steps for different concentration of nitrate in Walne’s medium.
6.5.5 Light is needed for photosynthesis
1. All food comes originally from green leaves in the sunlight.
A process is a change in something, or a way in which something is made.
So you can say that photosynthesis is the process by which green plants make food.
Show the students the prepared demonstrations.
Where does all your food come from? [Green plants in the sunlight.]
In sunligh, green plants use carbon dioxide gas from the air, water, and plant nutrients from the soil to make food.
This process is called photosynthesis.
("Photo" means light, "synthesis" means putting together).
Plant nutrients are chemicals in the soil that plants need, e.g. plant nutrients are needed to make the green colour that absorbs the sunlight energy.
This energy remains stored in the food.
The first food made in green plants is the simple sugar, glucose.
2. Photosynthesis equation
6CO2 + 12H2O + solar energy --> C6H12O6 + 6H2O + 6O2
Carbon dioxide gas + water + sunlight energy --> simple sugar (food) + oxygen gas.
The oxygen gas produced by photosynthesis goes out into the air.
All your food comes originally from photosynthesis in a green plant, because your food is either plants, or animals that have eaten plants.
Animals breathe in oxygen gas and breathe out carbon dioxide gas that is a waste.
Plants use the carbon dioxide gas during photosynthesis and give out oxygen gas.
3. To make food crops grow well they need:
1. Sunlight, but baby plants need some shade,
2. Water, but soil must not be waterlogged (filled with water),
3. Plant nutrients, but they must be the kinds of chemicals the plants need,
4. Green leaves.
The leaves must not be damaged by insects or disease, and they must not turn brown or yellow.
Experiments
See diagram 9.145.1: Band of foil on leaf.
1. The maximum absorption of light by chlorophyll a, C55H72O5N4Mg, is at wavelengths 430 nm and 662 nm.
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.
Put a silver paper band around a leaf on a tree and leave it there for three days.
Pick the leaf and test it for starch.
The part of the leaf shaded by the silver paper shows no starch present.
The part of the leaf not shaded by the silver paper band shows starch.
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.
See diagram 9.150: Cork discs on leaves.
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.
4. Compare the growth of the bean plants growing in sunlight and growing kept in the dark.
The plants kept in the dark become etiolated, i.e. they 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.
Such plants die if kept too long in the dark.
5. Punch out small discs from leaves of sun-adapted and shade-adapted plants, e.g. pelargonium and aspidistra leaves.
Float the discs in a syringe of sodium hydrogen carbonate solution.
After gas is evolved by photosynthesis, the leaf discs rise and fall.
Compare the rise and fall movement of the discs of different plants.
Repeat the experiment at different light intensities, using thin, smooth, not-hairy leaves, e.g. Brassica cotyledons and youngh spinach.
6.5.7 Microalgae
Microalgae (microphytes), occur in freshwater and marine environments.
They are unicellular, but may exist individually or in chains or groups.
Experiments may include the effect of the following on the growth of microalgae:
1. Light intensity,
2. Different coloured light,
3. Carbon dioxide,
4. Nutrients (change nitrate, phosphate)
5. Micro-nutrients, e.g. vitamins.
Culture the microalgae in test tubes and record the daily growth.
You can use a haemocytometer for counting the microalgae.
If you culture under ideal conditions for 10 days, you will get a graph similar to bacterial growth (exponential graph).
CSIRO supplies microalgae "starter" cultures and technical advice to industry, research and educational institutions in more than 65 countries since 1986.
In the aquaculture industry, microalgae are the essential first foods for larval and juvenile animals.
All our cultures are grown under controlled-environment conditions in a purpose-built, Australian Quarantine Inspection Service AQIS) accredited algal culture facility.
CSIRO supplies cultures in 20 millilitre and 250 millilitre quantities, algal plate cultures for selected aquaculture strains, a selection of discounted strains available for educational purposes.
Available microalgae strains include those from marine, estuarine and freshwater species.
Also available are aquaculture strains and toxin-producing species.
Selected strains are axenic (bacteria-free).
These algae can be maintained for a long time in the laboratory by sub culturing in agar plates.
Some micro-algae used in aquaculture for penaeid shrimp larvae, bivalve mollusc larvae, freshwater prawn larvae, postlarvae, abalone larvae, marine rotifers (Brachionus), brine shrimp (Artemia), saltwater copepods, freshwater zooplankton.
6.5.8 Oxygen gas is formed during photosynthesis, Elodea
See diagram 9.152: Waterweeds in the light, Waterweeds in the dark.
6CO2 + 12H2O + solar energy --> C6H12O6 + 6H2O + 6O2
1. Oxygen gas is given off during photosynthesis.
Try out the demonstration a few days before the lesson.
The demonstration works best if you can start it the day before the lesson.
Or,
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.
2. Tests for oxygen gas
Light a thin piece of wood then blow out the flame leaving the wood glowing red.
If you put this into oxygen, the glowing wood will burst into flame.
3. Put green waterweed in a test-tube and invert it under water.
The test-tube contains no air or bubbles in it.
This is left in the sunlight.
Put another piece of waterweed under a similar test-tube.
This is left in the dark.
After some hours look at the waterweeds again.
The waterweed in the light has bubbles of oxygen gas coming from it.
No bubbles of oxygen gas come from the waterweed in the dark.
The gas was oxygen, because it made a glowing piece of wood burst into flames.
Conclusion: During photosynthesis oxygen gas is given off.
6.5.9 Photosynthesis and light intensity, Na2CO3 solution, Selaginella
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 light source, placed at 1. 00 in, 0.90 m, 0.75 in, 0.60 and 0.50 m from the plant.
Use a 5% sodium hydrogen carbonate solution 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.
6.5.10 Photosynthesis and light intensity, NaHCO3 solution, plant leaf discs
Indirect measurement of the effect of light intensity on photosynthesis using plant disc assays.
1. Use a hole punch or plastic straw to cut out 10 discs from a spinach leaf or other hairless leaf.
Do not punch out areas that contain major leaf veins.
2. Remove the plunger from a 60 mL plastic syringe.
3. Put the 10 discs in the syringe barrel.
4. Replace the plunger in the barrel and gently push down until only a small amount of air remains with the discs.
Do not crush the discs.
5. Pour 30 mL of 0.2% sodium bicarbonate solution into the beaker.
Add 2 drops of liquid soap to wet the hydrophobic surface of the leaf and allow the solution to be drawn into the leaf.
If leaf discs are difficult to sink, add more liquid soap.
6. Draw up 10 mL of the solution into the syringe and tap the syringe to ensure that the discs are suspended in the solution.
7. Place a finger over the tip of the syringe and gently draw back to form a vacuum.
Hold the vacuum for 10 seconds while gently swirling the discs to keep them in solution.
8. Release the vacuum by removing the finger.
Check to see if the discs are still floating.
9. Repeat the above action up to 3 times if any discs are still floating.
Continue the experiment only if all the discs have sunk to the bottom.
10. Pour the solution and discs into a plastic cup with a reference line.
Add bicarbonate solution into the cup until it is just below first line.
Make sure to use the same depth for each run.
11. For a control, infiltrate leaf discs with a solution of only water with a drop of soap, no bicarbonate.
Set the microscope light control to provide with varying light intensities or use other methods, e.g. darkened rooms.
12. Set a timer to zero, place the cup under the light source, record the number of discs not floating.
13. Each minute record the number of discs now floating, and swirl the discs to dislodge any stuck together or on the side of the cup.
Continue recording until all discs are floating or until 12 minutes.
The floating discs may be removed as they come to the surface.
14. Record the data for that light intensity.
Repeat the experiment for other light intensities.
15. Calculate the mean for each intensity and graph your results.
16. The oxygen produced by photosynthesis is released into the air spaces between the cells within the leaf and later reaches the atmosphere by passing through the stomates on the surface of the leaf.
Leaf discs usually float, but the air spaces are filled with the sodium bicarbonate solution, so as the density of the leaf discs increases they tend to sink.
Then the bicarbonate ion becomes the carbon source for photosynthesis, which releases oxygen into the interior of the leaf to change the leaf disc buoyancy, causing them to rise and float.
However, respiration that consumes oxygen occurs at the same time, so the rate the discs rise is only an indirect measurement of the net rate of photosynthesis.
2NaHCO3 -->; Na2CO3 + H2O + CO2.
6.5.11 Photosynthesis in shaded waterweed leaves
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.
6.5.12 Photosynthesis in waterweeds, Elodea
Waterweeds lose bubbles of oxygen during photosynthesis
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 canadensis American waterweed, pondweed), Hydrocharitaceae
See diagram 9.3.41: Elodea producing oxygen.
1. The photosynthesis activity of leaves can be shown by putting waterweeds 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 that 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 eight 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.
6.5.13 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 9.125: 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.
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.
4. Plant ten bean seedlings, each having roots 1cm 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.
6.5.14 Waterweeds in the light and in the dark
See diagram 9.150: Cork discs on leaves.
See diagram 9.152: Waterweed in the light and in the dark.
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 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.
6.6.1 Photosynthetic pigments, chlorophyll a and chlorophyll b
See diagram: 16.3.5.2.3: Chlorophyll a and chlorophyll b.
The plant pigment chlorophyll a (C55H72O5N4Mg), bright blue-green colour, occurs in all green plants and in all oxygenic photosynthetic organisms.
The plant pigment chlorophyll b (C55H70O6N4Mg), olive colour, occurs in green plants especially in shade-adapted and green algae.
Chlorophyll pigments contain a porphyrin ring with electrons that move freely and be added to the ring or lost from the ring.
The ring of carbon and nitrogen atoms with magnesium at the centre is soluble in water and it absorbs light.
The fat-soluble tail of 16 carbon atoms attaches the molecule to the chloroplast membrane.
During photosynthesis, electrons become more energized by sunlight and can be passed on enable the process of sugar formation.
The site of photosynthesis is the thylakoid membrane, a flattened sac or vesicle lined with a pigmented membrane in interconnected stacks constituting a granum of the chloroplast.
. The pigment chlorophyll C occurs in photosynthetic Chromista and dinoflagellates.
E140 Chlorophylls and chlorophyllins (colour: olive to dark green), (used to dye oils and wax in medicines and cosmetics)
E141 Copper complexes of chlorophylls and chlorophyllins (colour: bright green)
Photosynthesis or Solar Cell Experiments - Mains Powered
Photosynthesis or Solar Cell Experiments - Battery or Mains
6.6.2 Chlorophyll is necessary for photosynthesis, variegated leaf, Abutilon
See diagram 9.149: Iodine test on 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.
6.6.3 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.
6.6.4 Chlorophyll from green leaves
6.5.7 Chlorophyll from green leaves, potato, onion, nasturtium, fuchsia, hyacinth, lilac
Use leaves of 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.
6.6.5 Chlorophyll pigments separated with paper chromatography, spinach
1. Cut dark green spinach leaves into small pieces then crush them with a spoon or mortar and pestle or an electric mixing machine.
Put the crushed juice in a beaker.
Cover with acetone nail polish remover.
Let the leaves settle down to the bottom of the liquid.
Hang a rectangle paper towel coffee filter napkin over a pencil so that one end is dipped in the liquid.
Leave to stand for several hours.
Note colours called pigments have moved up the paper.
Study the pigments in order of separation from top to base:
First: Carotenoid pigment (yellow) and some decomposition products move with the solvent front.
Second: Carotenoid pigment (yellow)
Third: Chlorophyll a (blue-green), C55H72O5N4Mg
Fourth: 1.4 Chlorophyll b (yellow-green), C55H70O6N4Mg
2. Use 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 twice 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:
First: Carotenoid pigment (yellow) and some decomposition products move with the solvent front
Second: Carotenoid pigment (yellow)
Third: Chlorophyll a (blue-green)
Fouth: Chlorophyll b (yellow green)
6.6.6 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.
6.6.7 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, e.g. 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 describe the shape of their chloroplasts.