The steps involved in testing a green leaf for the presence of starch are a series of careful procedures designed to isolate and identify the carbohydrate within the plant tissue.
Testing a Green Leaf for Starch: A Step-by-Step Guide
Testing a green leaf for starch is a common experiment used to demonstrate photosynthesis, as starch is a primary product of this process. The procedure ensures that chlorophyll, which would obscure the results, is removed, and the cell structure is prepared for the starch-iodine reaction.
Here are the precise steps:
-
Boiling the Leaf in Water
- Action: Dip the leaf in boiling water for a few minutes.
- Purpose: This crucial initial step kills the leaf cells, denatures enzymes that might break down starch, and softens the leaf tissue, making it more permeable. It also breaks down cell membranes, allowing for easier penetration of subsequent solutions into the cell.
-
Decolorizing the Leaf in Alcohol (Using a Water Bath)
- Action: Carefully place the above beaker in a water-bath and heat till the alcohol begins to boil.
- Purpose: The leaf is then transferred to a test tube containing alcohol (ethanol), which is heated using a water bath. Heating alcohol directly over a flame is extremely dangerous due to its flammability. The alcohol extracts and removes the green chlorophyll pigment from the leaf. This decolorization is essential because the green color would otherwise mask the blue-black color change that indicates the presence of starch, making the test result unclear. The leaf will turn brittle and pale after this step.
-
Washing and Applying Iodine Solution
- Action: Now dip the leaf in a dilute solution of iodine for a few minutes.
- Purpose: After decolorization, the brittle leaf should be carefully rinsed in water to remove any residual alcohol. Following this, the leaf is dipped into a dilute iodine solution (iodine-potassium iodide solution) for several minutes. Iodine acts as a chemical indicator that specifically reacts with starch.
-
Observing the Result
- Action: Blue - black colouration shows the presence of starch.
- Interpretation: Observe the color of the leaf. If starch is present, the leaf will turn a distinct blue-black colouration. This reaction occurs because iodine molecules get trapped within the coiled structure of amylose, a component of starch, forming a complex that strongly absorbs light, appearing blue-black. If starch is absent, the leaf will remain the yellowish-brown color of the iodine solution or the pale color it had after decolorization.
This systematic process ensures accurate identification of starch within the leaf, providing clear visual evidence of its presence.
