The most common and effective method to prove that starch is produced during photosynthesis is through a simple, yet demonstrative, iodine test on plant leaves. This test relies on iodine's ability to act as an indicator, changing color in the presence of starch.
The Starch Test: A Practical Demonstration of Photosynthesis
To empirically show that starch is a product of photosynthesis, a controlled experiment comparing a leaf exposed to light versus one kept in darkness is typically performed. This experiment highlights the essential role of light in starch production.
Understanding the Principle
Iodine solution serves as a clear indicator in this experiment. Iodine is an indicator that turns blue-black in the presence of starch. This chemical reaction provides a visual confirmation of starch presence.
Experimental Setup
To ensure accurate results, the plant used for the experiment needs to be prepared carefully:
- Destarching: Place the plant in a dark room for at least 24-48 hours. This process, known as destarching, ensures that any pre-existing starch stored in the leaves is used up. This way, any starch detected after the experiment can be confidently attributed to photosynthesis during the experimental period.
- Differential Light Exposure:
- Select two healthy leaves on the destarched plant.
- Cover a part of one leaf with opaque material (e.g., aluminum foil or a black paper clip) to block light from reaching it, while leaving the rest of the leaf exposed.
- Alternatively, you can use two separate leaves, ensuring one is fully exposed to light and the other is completely covered or kept in darkness.
- Photosynthesis Period: Expose the plant to bright sunlight for several hours (4-6 hours is usually sufficient) to allow photosynthesis to occur in the exposed parts of the leaves.
Procedure Steps
After the photosynthesis period, follow these steps to test for starch:
- Harvest Leaves: Carefully detach the selected leaves from the plant.
- Boil in Water: Immerse the leaves in boiling water for a few minutes. This kills the leaf cells, breaks down cell membranes, and allows for easier penetration of iodine later.
- Decolorize with Alcohol: Transfer the boiled leaves into a test tube containing ethanol (alcohol). Place the test tube in a hot water bath (do not heat alcohol directly over a flame, as it is highly flammable). The alcohol will dissolve the chlorophyll, turning the leaves pale or white. This step is crucial because chlorophyll's green color would otherwise mask the iodine color change.
- Rinse: Rinse the decolorized leaves in warm water to rehydrate them and remove any residual alcohol.
- Apply Iodine Solution: Lay the decolorized leaves flat on a white tile or petri dish. Apply a few drops of iodine solution evenly over the surface of each leaf.
Interpreting the Results
Observe the color changes in the different parts of the leaves:
| Leaf/Leaf Section | Condition | Expected Color Change (with Iodine) | Indication |
|---|---|---|---|
| Exposed to Light | Performed Photosynthesis | Turns Blue-Black | Starch is present. |
| Covered/In Darkness | No Photosynthesis (or limited) | Remains Reddish-Brown | Starch is absent or present in very small amounts. |
As per the reference, "When you put iodine on the leaves, one of them will turn blue-black and the other will be a reddish-brown." Specifically, "The leaf that was in the light turns blue-black, which demonstrates that the leaf has been performing photosynthesis and producing starch." Conversely, the part or leaf that was covered or kept in darkness will not turn blue-black, indicating the absence of starch due to the lack of light for photosynthesis.
This clear difference in color confirms that starch is indeed produced in the parts of the leaf that received light, thus proving its formation during the process of photosynthesis.
