Photosynthesis: Experiments & Limiting Factors
Core: Starch Test, Rate Investigations, Hydrogencarbonate Indicator | Supplement: Limiting Factor Analysis
The Starch Test Protocol
This test proves that chlorophyll, light, and carbon dioxide are needed for photosynthesis by detecting the presence of starch (the storage product of glucose).
Testing Chlorophyll
Use a variegated leaf (green & white parts). Only green parts contain chlorophyll and will turn blue-black.
Testing Light
Cover part of the leaf with black paper. Covered area gets no light and won't make starch.
Testing CO₂
Place plant in a sealed container with soda lime (absorbs CO₂). No CO₂ = no starch.
Control
Normal leaf in light with CO₂ should turn blue-black (positive result).
Virtual Starch Test
Step 1 of 4Step 1: Boil the Leaf in Water
Place the leaf in boiling water for 30 seconds to KILL the cells and stop all enzyme activity.
Examiner Trap
Don't say "the leaf turns blue-black." The STARCH reacts with iodine to produce the blue-black color. Also, removing chlorophyll is essential—without it, you can't see the color change clearly.
Paper 6 Focus
You must know how to plan investigations, identify variables (independent, dependent, control), describe safety precautions (especially for ethanol), and interpret experimental results. Practice drawing graphs that show limiting factors—identify the slope (factor is limiting) versus the plateau (something else is limiting).
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