The Importance of Carbon Dioxide in Photosynthesis
Photosynthesis is the process by which green plants and some other organisms use sunlight to synthesize food from carbon dioxide and water. Carbon dioxide is needed for photosynthesis.
To show that carbon dioxide is necessary for photosynthesis.
Potted plant, sodium hydroxide*, ethanol*, iodine solution*, heat source, cooking pot, water, test tube*, white tile*, dropper*, cotton wool, empty water bottle or clear plastic bag, and rubber bands.
Sodium hydroxide is corrosive to skin and wood. Even when dilute it can blind if it gets into the eyes. Neutralise spills with a weak acid.
Ethanol is very ammable! Make sure that student cover their test tubes to avoid excess release of ethanol vapour. If a test tube catches re, instruct student to cover the tube with a non- ammable object to extinguish the ame.
1. Put a potted plant in a dark place for 24 hours to de-starch its leaves.
2. Enclose one leaf in a clear plastic bag or empty plastic water bottle containing approximately one teaspoon of sodium hydroxide.
3. Seal the plastic container so that no air can enter. The aim of this is to prevent the leaf from coming into contact with carbon dioxide
4. Allow the plant to sit in direct sunlight for at least 6 hours.
Figure 6.4: Experiment to show the importance of CO2 in photosynthesis
1. Choose one of the leaves that has been deprived of carbon dioxide.
2. Submerge this leaf in boiling water for about 3 minutes.
3. Remove the leaf from the water and insert it into a test tube containing ethanol and plug the test tube with a piece of cotton wool. Note: the test tube should be less than half full of ethanol.
4. Submerge the test tube in the boiling water and leave it to boil until the leaf loses all of its colour.
5. Once the leaf has lost its colour, remove it from the ethanol solution and dip it brie y into the boiling water to remove the ethanol and soften it.
6. Spread the decolourized leaf on a white tile and add iodine solution drop wise until the whole leaf is covered.
7. Record your observations and draw a picture showing the colour pattern of the leaf. The leaves should test negative for starch.
After adding iodine solution to the leaf, it retains the colour of iodine; it should not form the blue-black colour of an iodine-starch complex. The
iodine colour implies that the leaf has no starch, which means that photo-synthesis did not occur. This proves that carbon dioxide is necessary for photosynthesis.
Collect all the used materials, cleaning and storing items that will be used later. No special waste disposal is required.
It is important that the leaf does not contain a thick waxy coating or else the iodine will not penetrate the leaf. Ethanol boils at a lower temperature than water, thus it can be boiled in a water bath. Sodium hydroxide is used in the bag with the leaf to absorb any CO2 that might be present. The plant must be kept in darkness prior to this experiment to ensure that all starch from prior photosynthesis is consumed.
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