Testing a Leaf for Starch - GCSE Biology
Introduction to Photosynthesis
- Photosynthesis is the process by which plants make food using light energy.
- Plants absorb light using chlorophyll, converting carbon dioxide and water into glucose.
- Glucose is used for respiration, converted to cellulose, proteins, oils, or stored as starch.
Testing a Leaf for Starch
- Purpose: To determine if a plant has been photosynthesizing by checking for starch presence.
Starch Testing Procedure
- Boil the Leaf: Place the leaf in boiling water for 30 seconds to stop chemical reactions.
- Decolorize the Leaf: Place the leaf in boiling ethanol to remove chlorophyll. This makes the leaf white and easier to observe for color changes.
- Rehydrate Leaf: Wash leaf with water to soften it and lay it on a white tile.
- Iodine Test: Add iodine solution. If the leaf turns blue/black, starch is present.
Variegated Leaves
- Variegated leaves have green (chlorophyll-containing) and white (non-chlorophyll) parts.
- Only green parts turn blue/black with iodine, indicating photosynthesis occurred only where chlorophyll is present.
Importance of Chlorophyll and Light
- Chlorophyll is essential for photosynthesis; only the sections of leaves with chlorophyll will show a positive starch test.
- Light is crucial for photosynthesis; without light, starch does not form.
Experimental Controls and Safety
- Ethanol Hazard: Ethanol is flammable and should not be heated with a naked flame.
- Control Measure: Use a hot water bath instead of a Bunsen burner to boil ethanol.
Conclusion
- The starch test for photosynthesis demonstrates the necessity of chlorophyll and light in the process.
- This experiment helps to visually understand the areas of a plant that are actively photosynthesizing.
Related Topics
- Factors limiting photosynthesis
- Carbon dioxide use in photosynthesis
- Cells, organ systems and ecosystems
For more detailed information and additional resources, students can explore related sections and external links provided in the BBC Bitesize guide.