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EXPERIMENTS ON PHOTOSYNTHESIS
The occurrence of photosynthesis in plants can be shown by experiments. The easiest proof that photosynthesis has taken place is to test for the presence of starch in the green leaves. Experiments should be carried out to show the importance of carbon (IV) oxide, sunlight energy and chlorophyll in photosynthesis. There is need to prove that oxygen is given off during photosynthesis.
Testing a leaf for starch
The usual laboratory test for starch is to bring the testing substance in contact with iodine solution. A blue-black colour confirms the presence of starch. With the green leaf, the test is not so straight forward. Green leaves contain chlorophyll which blocks up the reaction of iodine with starch. Steps are therefore taken to remove the chlorophyll from the leaf before bringing it in contact with iodine.
The following steps must be carried out:
1) The leaf to be tested is detached from the plant after 4-6 hours of exposure to light and put into boiling water for about 10 -15 minutes. This kills the protoplasm of leaf and stops all enzyme activities in the cells. Boiling will also make iodine more easily permeable to starch granules at the time of test.
2) The boiled leaf is put into 70% boiling ethyl alcohol or methylated spirit. This is done to extract the chlorophyll from the leaf. The alcohol is heated to a boiling point in a water bath to avoid inflammation.
When the chlorophyll is completely removed, the leaf becomes white and brittle. It is then dipped into hot water to soften the tissues.
3) The bleached leaf is spread out on a flat surface such as a white tile and few drops of iodine solution are added. It is then allowed to stand for a few minutes. The iodine solution is washed off in cold water. The colour of the leaf an now be observed against a light source.
- If the tested leaf appears bluish-black in colour, it contains starch.
- If the colour is yellowish-brown, it contains no starch.
Destarching the leaves of a plant
It has earlier been stated that the presence of starch in a leaf is an evidence that photosynthesis has occurred in that leaf. It is very important that in any experiment on photosynthesis, the leaves to be used must be destarched. Green leaves can be destarched by keeping a potted plant in a dark cupboard for 48 – 72 hours. During this period photosynthesis stops. The starch already formed in the leaves is hydrolysed to sugar and translocated to other parts of the plant to be used for respiration. At the end of 48 hours it is expected that all the starch in the leaves must have been completely hydrolysed and translocated. The leaves are therefore without starch. This can be confirmed by testing one of the leaves for starch as previously described. If the leaves have been destarched, the starch test result is negative, that is a yellowish-brown colour with iodine solution. Potted plants are more suitable for experiments on photosynthesis as the pots can easily be carried to different environmental conditions without destroying the plant. A living plant uprooted and kept in the dark will wither before 48 hours.
Experiment to show the Carbon (IV) oxide is necessary for photosynthesis
Aim: To show that carbon (IV) oxide is necessary for photosynthesis.
Method: Two potted plants well watered are kept in darkness for 24 hours or more to render them starch free. The leaves of both plants are tested for starch until the leaves are starch free. Each of the potted plants is placed on vaselined glass plate (to prevent air containing carbon (1V) oxide from leaking into the bell jar) and covered with a large bell jar.
A dish containing water is placed in bell jar “A” with air containing carbon (IV) oxide as a control.
The soil surface together with the body of the pot is covered with a polythene bag to prevent the output of respiration by soil organisms. This is the control experiment.
In bell jar “B” is placed a dish containing lime water to absorb carbon (IV) oxide.
The two bell jars are exposed to sunlight for about 4 hours. Some leaves are taken from each plant at the end of the experiment and tested for starch.
Result: The leaves of the plant in bell jar”A” turns blue black while the leaves of the plant from jar “B” remains brown in colour showing absence of starch.
Conclusion: Starch is not formed in the absence of carbon (IV) oxide. Therefore carbon (IV) oxide is necessary for photosynthesis.
Experiment to show that sunlight it important for photosynthesis.
Aim: To show that light is necessary for photosynthesis.
Method: Two potted plants well watered, are kept in darkness for 24 hours or more. The leaves of both plants are tested for starch until they are starch free. One of the potted plants is exposed to sunlight for 4 hours. The other potted plant remains in darkness for 4 hours. The leaves from the two potted plants are then tested for starch.
Result: The leaves from the potted plant exposed to light turns blue black while the leaves of the plant kept in darkness remains brown showing absence of starch.
Conclusion: Without light starch is not formed, therefore light is necessary for photosynthesis.
Experiment to show that chlorophyll is necessary for photosynthesis
Aim: To demonstrate that chlorophyll is necessary for photosynthesis.
Variegated leaves in which chlorophyll is distributed in parches are required for this experiment. Some leaves of ice plant, Acalypha, Croton and Calladium are good examples for this experiment. The areas with chlorophyll are green while the other areas without chlorophyll are coloured red, blue, yellow or white.
Method: The plants with variegated leaves are allowed to receive sunlight for 4-6 hours. Some of the leaves are taken and mapped. The leaves are then boiled in water to kill the protoplasm. They are decolourized by placing them in boiling alcohol. The leaves are then washed in water to soften the tissues.
The decolourized leaves are spread on white tiles. Iodine solution is spread over them.
Result: Some areas of the leaf turn blue-black while the other areas turn yellowish brown. Match the leaves with their maps. By comparing the tested leaf in diagram “B” with the first drawing of diagram “A” the blue-black areas correspond to the areas with chlorophyll while the yellowish brown areas correspond to the areas without chlorophyll.
Conclusion: Since starch is formed only in the areas with green pigments (chlorophyll) chlorophyll is therefore necessary for photosynthesis.
Experiment to show that oxygen is a by product of photosynthesis
Aim: To show that oxygen is released during photosynthesis.
Method: Green aquatic plants such as Ceratophyllum or Spirogyra filaments are collected in a beaker together with some water in which the plants are growing. A glass funnel is inverted over the plants. A test tube filled with water is then inverted over the stem of the glass funnel. The apparatus is then placed in sunlight. A little quantity of sodium tri-oxo carbon (IV) acid is dissolved in the water to increase the quantity of carbon (IV) oxide needed for photosynthesis. A control experiment is set in a similar way, but kept in a dark room to stop photosynthesis. As photosynthesis occurs, some gas bubbles are collected on the stem, which eventually break off and collect in the test tube. The gas displaces the water in the test tube. When sufficient quantity has been collected, the test tube is removed while closing the mouth with the thumb. The gas is tested with a glowing splint.
Result: The gas rekindles a glowing splint. No gas collects in the control experiment.
Conclusion: Since oxygen is the only colourless gas that rekindles a glowing splint, the gas released is oxygen.
Water culture experiment
Aim: To demonstrate the effects of nitrogen, phosphorus, sulphur, potassium, magnesium, calcium and iron deficiencies in flowering plants.
Method: Eight clean and sterilized jars are obtained. They are labelled A to H. A culture solution is prepared by adding the right proportions of nitorgen, phosphorus, sulphur, potassium, magnesium, calcium and iron. The eight bottles are then filled as follows:
Jar A: Culture solution containing nitrogen, phosphorus, sulphur, potassium, magnesium, calcium and iron. This is the control.
Jar B: Culture solution without nitrogen.
Jar C: Culture solution without phosphorus.
Jar D: Culture solution without sulphur.
Jar E: Culture solution without potassium.
Jar F: Culture solution without magnesium.
Jar G: Culture solution without calcium.
Jar H: Culture solution without iron.
Eight healthy bean seedlings are obtained. Each seedling is then placed in each jar. Each jar is then wrapped with black paper to prevent penetration of light thereby preventing the growth of algae.
Each jar is kept in a warm place that receives sunlight for about three weeks. Observe daily throughout the period of experiment.
Result: The seedling in jar A is luxuriant in growth, healthy with green leaves and well developed roots. Seedling of jar B shows stunted growth and yellow leaves. Seedling of jar C shows stunted growth, particularly the roots. Seedling of jar D shows stunted growth and yellow leaves. Seedling of jar E shows yellow and brown leaf margin with stunted growth. Seedling of jar F shows poor growth and yellow leaves. Seedling of jar G shows stunted growth, death of terminal buds and poor root development. Seedling of jar H shows yellow leaves and poor growth.
Conclusion: Deficiency of the essential minerals can affect the normal growth of plants.
In carrying out this experiment, the following precautions are necessary for successful results.
- Air must be bubbled to the solution daily for the healthy growth and aeration of the roots.
- The seedling must be healthy, and the same size and age.
- All the jars must be kept under the same conditions.
- The culture solution must be renewed weekly to prevent depletion of some nutrients.
- All the jars used must be covered with black paper or cloth to prevent light from entering thereby preventing the growth of algae in the culture solution. If algae grow in the culture solution, they will compete with the seedling for nutrients.
- The cork and cotton wool must be kept dry so as to prevent the stem from rotting. Cotton wool is used to hold the stem to avoid damage to the young and delicate stem.
- The jars are sterilized to prevent the growth of bacteria which may cause diseases.