A student carries out a standard laboratory procedure to test for the presence of starch in a green leaf that has been exposed to sunlight. In which correct sequential order should the student perform the following experimental steps?
- 1Boil the leaf in water for several minutes to kill the cell protoplasm and stop biochemical reactions.
- 2Immerse the leaf in hot ethanol using a water bath to extract chlorophyll pigments.
- 3Dip the decolorized leaf into warm water to soften the tissue.
- 4Spread the leaf flat on a white tile and add a few drops of iodine solution to observe the color change.
Answer
The correct sequence for testing a leaf for starch is: first, boil the leaf in water to kill the cells; second, immerse the leaf in hot ethanol using a water bath to remove chlorophyll; third, rinse the leaf in warm water to soften it; and finally, spread the leaf on a white tile and apply iodine solution.
The leaf must first be boiled in water to kill the protoplasm and rupture cell membranes so reagents can enter. Next, heating the leaf in ethanol (in a water bath for safety) extracts chlorophyll, turning the leaf pale yellow/white so color changes are clearly visible. Decolorizing with ethanol leaves the leaf brittle, so it is dipped in warm water to soften it. Finally, spreading it flat and adding iodine solution allows clear observation of the blue-black color change indicating starch.
Step-by-Step Solution
Key Concept
Experimental Starch Test in Leaves