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Difficulty: EasyPlant Nutrition and Photosynthesis

A student performed an experiment to test for the presence of starch in a green leaf exposed to sunlight. What is the correct sequence of steps for carrying out this procedure from start to finish?

  1. 1Boil the leaf in water for a few minutes.
  2. 2Boil the leaf in ethanol using a water bath.
  3. 3Rinse the decolourized leaf in warm water.
  4. 4Spread the leaf on a white tile and add iodine solution.

Answer

The correct sequence of steps for testing a leaf for starch is: boiling the leaf in water, boiling the leaf in ethanol, rinsing the leaf in warm water, and adding iodine solution.
The leaf starch test follows a specific functional sequence: boiling in water kills cells and makes membranes permeable, boiling in ethanol extracts green chlorophyll, rinsing in warm water softens the stiffened leaf, and applying iodine solution confirms starch presence via a blue-black color change.

Step-by-Step Solution

1
Determine the initial step to stop biological activity in the leaf.
The leaf is boiled in water.
Boiling kills the cell protoplasm and breaks cell membranes, making them permeable to testing reagents.
2
Determine the step required to remove leaf pigments.
The leaf is boiled in ethanol using a water bath.
Chlorophyll must be extracted so its green color does not mask the blue-black reaction with iodine.
3
Determine the step required to restore leaf flexibility.
The leaf is rinsed in warm water.
Alcohol dehydrates the leaf and makes it stiff; warm water restores its softness.
4
Determine the final step to test for starch.
Iodine solution is added to the leaf.
Iodine solution reacts with starch produced during photosynthesis to yield a characteristic blue-black color.

Key Concept

Experimental procedure for leaf starch testing as evidence of photosynthetic activity
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Topics

Plant Nutrition and Photosynthesis

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