Practical: Investigating Photosynthesis
Step 1: Boiling Water: The leaf is placed in boiling water for approximately 30 seconds. This process kills the leaf tissue and denatures enzymes, stopping all chemical reactions and making the membranes permeable.
Step 2: Ethanol Bath: The leaf is then boiled in ethanol (using a water bath for safety) to remove chlorophyll. Removing the green pigment is essential so that the subsequent color change of the iodine can be clearly observed.
Step 3: Softening: After the ethanol bath, the leaf becomes brittle. It is dipped in warm water to rehydrate and soften the tissue, making it easier to spread out on a white tile.
Step 4: Iodine Application: Iodine solution is added to the leaf. A color change from yellow-brown to blue-black indicates the presence of starch and, by extension, that photosynthesis has occurred.
Light Requirement: To prove light is necessary, a portion of a de-starched leaf is covered with aluminium foil. After exposure to light, only the uncovered areas will test positive for starch, while the covered area remains yellow-brown.
Carbon Dioxide Requirement: A plant is placed in an enclosed environment with potassium hydroxide (KOH), which absorbs from the air. Without , the leaf cannot produce starch, even in the presence of light.
Chlorophyll Requirement: Using a variegated leaf (which has naturally white and green areas) allows for the testing of chlorophyll. Only the green areas containing chlorophyll will produce starch and turn blue-black with iodine.
| Factor Tested | Experimental Variable | Control/Negative Result |
|---|---|---|
| Light | Uncovered leaf area | Covered area (Yellow-brown) |
| Normal air exposure | KOH environment (Yellow-brown) | |
| Chlorophyll | Green leaf area | White leaf area (Yellow-brown) |
Oxygen vs. Starch: Counting bubbles measures the rate of photosynthesis in real-time, whereas the starch test is a qualitative measure of whether photosynthesis has occurred over a period of time.
Mechanical vs. Chemical: Boiling the leaf is a mechanical preparation to break cell walls, while the iodine test is a chemical reaction to detect specific carbohydrate structures.
Safety Protocol: Ethanol is highly flammable and must never be heated over an open Bunsen flame. Always use an electric water bath or a beaker of pre-boiled water to heat ethanol safely.
Verification of Results: When interpreting results, always check if the plant was de-starched. If a student forgets to de-starch, a positive starch test proves nothing about the conditions during the experiment.
Quantitative Accuracy: When counting bubbles, ensure the light source is at a fixed distance or use a gas syringe for more precise volume measurements to avoid errors caused by varying bubble sizes.