Testing Starch in a (Variegated) Leaf - Lab Report
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Aim: To test various leaves for the presence of starch Hypothesis: After the iodine reagent is added, the green areas on the leaf, where starch is supposedly present, will turn to a blue-black colour - the positive test for starch. Method: 1. Obtain your leaf and cut a small sample of your leaf; 2. Immerse the leaf samples in hot water for 1-2 minutes (depending on the size of the leaf samples) 3. Remove the leaf samples from the hot water and keep them carefully to one side. 4. Half fill a boiling tube with alcohol and place the tube in hot water until bubbles start to form in the alcohol. 5. Place the leaf samples in the hot alcohol for about 5 minutes. 6.
Repeat experiment 11. Write observations 12. Draw conclusions Safety Procedures: * Handle the hot water bath with care * The alcohol is highly flammable; handle the test tube with care * Keep all flammable items away from the experiment. * Small samples were taken of the experiment, so that the experiment could be replicated (repeated). * The leaf was placed in hot water to remove the waxy covering (the cuticle), and to break the cell membranes, and stopping all chemical processes. * Since chlorophyll dissolves in alcohol (solvent), the alcohol is first heated as it works best at higher temperatures. * The leaf samples were placed in the hot alcohol so they could be decolourised.
Conclusion: From the experiment, we can now conclude that the starch and chlorophyll was present in the same place. When the chlorophyll uses sunlight to produce glucose (as well as other products), the glucose is converted into starch for storage of sugars. This starch was present in the places of the chlorophyll, the green pigment. Also we can conclude that starch is only present when chlorophyll is present. Evaluation: The leaf was boiled to break the cell membranes, and to stop chemical processes. Although there may not have been sufficient time for this to happen, and thus the cell membranes may have just been partially broken. All of the chlorophyll from the leaf sample may not have dissolved in the alcohol, the solvent. A small amount may have remained in the leaf sample. - Rohan M
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Here's what a teacher thought of this essay
This report on the starch test on leaves adequately describes the method and likely outcome of the test for green leaves grown in the light.
GCSE exam questions very rarely ask for this method but focus on investigations where the plant has been denied one factor required for photosynthesis. The writer does mention that a positive result was achieved only for the green areas of the leaf and the essay would have benefited from a discussion of variegation in plants where leaves have both green and white areas.
The writer's method is a little vague in places and would be difficult to repeat since exact details of size and temperature are omitted.
Marked by teacher Ross Robertson 26/04/2013
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