Biology lab AIM: To test a leaf for starch APPARATUS/MATERIALS : * bunsen burner, tripod stand, wire gauze(OR 90oC electric water bath OR hot plate) * 250 cm3 beaker * boiling tube * anti- bumping granules * forceps * test tube holder * white tile * leaf to be tested (hibiscus leaves are excellent) * 90% ethanol * iodine/potassium iodide solution PROCEDURE: The video above shows the steps in Testing a Leaf for the Presence of Starch. The Procedure outlined below is slightly different. 1. Remove a green leaf fom a plant that has been exposed to sunlight for a few hours 2.
Half-fill a 250cm3 beaker with water. Heat the water until it boils. Keep the water at boiling point. 3. Use the forceps to place the leaf in the boiling water. Boil for 2 minutes. 4. Turn off the Bunsen Burner. (If you are using a heat source without a naked flame - electric water bath or hot plate - this step is uneccesary. ) 5. Place the boiled leaf in a boiling tube containing 90% ethanol. 6. Place the boiling tube in hot water and boil for 10 minutes or until the leaf decolourizes. ( It may be necessary to replace the ethanol) 7. Gently remove the leaf and wash with a fine trickle of cold tap water. . Spead the leaf evenly on a white tile. 9. Add a few frops of iodne/potassium iodide solution to the leaf and note any observations. OBSERVATIONS/RESULTS: * The leaf was flaccid (soft) after being boiled in water * The ethanol changed from colourless to green * The leaf was brittle after being boiled in ethanol * The leaf become flaccid once more after being rinsed in cold water | OBSERVATION| INTERPRETATION| POSITIVE TEST| Iodine/Potassium Iodide solution changed from brown to blue- black| Starch is present| NEGATIVE TEST| No colour change| Starch is not present|
DISCUSSION QUESTIONS AND ANSWERS 1. Describe Photosynthesis. Use a symbol equation. Photosynthesis is the process by which light energy is harvested by chlorophyll and used to convert inorganic raw materials - carbon dioxide and water to the products glucose and oxygen. Most photosynthesis takes places in the green parts of plants, in particular the leaves. Photosynthesis involves 2 stages - the Light-dependent Stage and the Light-independent stage. In the light-depedent stage, water is photolyzed by sunlight into Hydrogen and oxgen.
Hydrogen enters the second stage, oxgen is gaseous by-product. In the light - independent stage enzymes catalyse the reduction of carbon dioxide by hydrogen. The carbohydrate, glucose is first formed. Glucoe may be immediately used, translocated in the form of fructose or sucrose or stored as starch granules. The overall equation for photosynthesis is: 2. Which organic substance does iodine/potassium iodide solution detect the presence of? What observations will you make if the test is positive? Starch. The iodine solution will turn from brown to blue - black. 3.
Explain the significance of boiling the leaf in water. Boiling the leaf in water: * Removes the waxy cuticle which prevents entry of iodine/potassium iodide solution. * Ruptures cell membranes to make starch granules in cytoplasm and chloroplasts accessible to iodine/potassium iodide solutiion. Cell membranes are selectively permeable and do not readily allow the penetration of iodine. * Denatures enzymes, particularly those which convert starch to glucose e. g. diastase. Boiling arrests all chemical reactions, since enzymes which catalyse the reactions are denatured.
Denatured enzymes have altered or destroyed active sites due to heat, pH, ionic concentration 4. Account for the texture of the leaf after being boiled in water. The leaf was flaccid (soft) as cell membranes were ruptured and turgor pressure was lost. 5. Explain the significance of boiling the leaf in ethanol Chlorophyll is a green pigement and so masks the colour change of the iodine test for starch. Chlorophyll needs to be removed from the leaf i. e. theleaf needs to be ' decolourized' for chabges to be observed. A decolourized leaf is pale yellow or green.
Ethanol is an organic solvent and so extracts chlorophyll from the leaf. 6. Account for the texture of the leaf after being boiled in ethanol. The leaf was brittle as ethanol dehydrated (extracted water) from the leaf. 7. Explain the significance of rinsing the leaf in water. The leaf was rinsed in water to rehydrate it. Iodine solution is an aqueous solution of iodine/potassium iodie - potassium tri-iodide; water is needed inside the leaf to enable penetration by diffusion. 8. State whether starch is present. 9. Which products of photosynthesis may be present but not revelead by the iodine test?
Glucose, fructose and sucrose are all carbohydrate products of photosynthesis not revealed by the iodine test for starch. 10. Do your results indicate that starch is the first product of photosynthesis? The results do not indicate whether starch is the initial, intermediate or final product of photosynthesis. 11. How can this experiment be extended to demonstrate conclusively that photosynthesis has taken place? We do not know whether starch has accumulated in the leaf as a result of photosynthesis or whether the starch is permanently present in the leaf. To improve this experiment, the plant needs to be de-starched, and the leaves tested for the presence of starch. * Once it is demonstrated that all starch has been utilized or removed, photosynthesis is allowed to take place by exposing the plant to sunlight. * The leaves of the plant are now tested for the presence of starch. * If starch is present, then it has been synthesized by photosynthesis. We can then conclude that the presence of starch indicates that photosynthesis has taken place www. thebrilliantbiologystudent. com