Testing a Leaf for Starch
Testing a leaf for starch is the simplest photosynthesis lab. A positive test for starch in a leaf provides evidence that photosynthesis has occurred. The test for starch is a leaf is an is an extension of the Iodine Test for Starch. Glucose is the product of photosynthesis, and is rapidly converted to granules of starch - a polymer of glucose - for storage. Starch granules have been visualized in the stroma of the chloroplast as well as the cytoplasm. The test for starch in a leaf is also a part of the other photosynthesis experiments. So, take time to understand the test for starch in a leaf. Got Questions related to the Principle of the Iodine Test? See the IODINE TEST FOR STARCH (FOOD TEST) Apparatus/Materials
Procedure
The video above shows the steps in Testing a Leaf for the Presence of Starch. The Procedure outlined below is slightly different.
Observations/Results
Discussion Questions & Answers
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 products - glucose and oxygen. Most photosynthesis takes places in the green parts of plants and more specifically, in the leaves. Photosynthesis involves 2 stages - the Light-dependent Stage and the Light-independent stage. In the light-dependent 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. Glucose may be immediately used, translocated in the form of fructose or sucrose or stored as starch granules. The overall equation for photosynthesis is:
Why is starch tested for and not glucose?
Glucose is the product of photosynthesis, and is rapidly converted to granules of starch - a polymer of glucose - for storage. Starch granules have been visualized in the stroma of the chloroplast as well as the cytoplasm. It is advantageous to the plant to convert glucose to starch. Starch is insoluble in water and therefore not a solute in the solution component of the stroma. Glucose on the other hand is soluble in water and if it accumulated in the stroma, the stroma would become more concentrated - hypertonic - relative to the cytoplasm. Water would move from the cytoplasm ( higher water potential) via osmosis into the stroma of the chloroplasts (lower water potential), causing them to swell and burst. . What observations will you make if the test is positive? The iodine solution will turn from brown to blue - black. Explain the significance of boiling the leaf in water. Boiling the leaf in water
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. Explain the significance of boiling the leaf in ethanol Chlorophyll is a green pigment and so masks the colour change of the iodine test for starch. Chlorophyll needs to be removed from the leaf i.e. the leaf needs to be ' decolourized' for changes to be observed. A decolourized leaf is pale yellow or green. Ethanol is an organic solvent and so extracts chlorophyll from the leaf. Account for the texture of the leaf after being boiled in ethanol. The leaf was brittle as ethanol dehydrated (extracted water) from the leaf. 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. State whether starch is present Starch is present OR Starch is not present. Which products of photosynthesis may be present but not revealed by the iodine test? Glucose, fructose and sucrose are all carbohydrate products of photosynthesis not revealed by the iodine test for starch. 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. 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. |
Photosynthesis Study Guide
Iodine Test for Starch
Testing a Leaf for Starch Why Destarch a Plant Effect of Light Intensity |
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