Biology Photosynthesis Lab

925 words 4 pages
Biology lab
AIM: To test a leaf for starch
* 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

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
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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


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