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When testing for the presence of starch in a leaf, the reason for dipping the decolorized leaf in hot water is to…
A. detect the starch
B. kill the leaf
C. soften the leaf
D. remove the chlorophyll
Care must be taken to understand and correctly answer the question. When testing for the presence of starch in a leaf sample, it is noteworthy to state that the leaf often gets dipped into hot water twice (not once). The first being to kill the leaf, this is before de-colorization. After the removal of chlorophyll (decolorized) of the leaf in a boiling ethanol, it is again dipped into hot water to soften.
Now for the right answer to the above question:
- To detect the starch, we apply iodine to the decolorized leaf and not hot water. so, A is quite incorrect.
- The leaf is at first, before de-colorization, dipped in hot water to kill it. Observe that the leaf here is not already decolorized. So, B is also incorrect.
- After boiling in ethanol to decolorize and remove chlorophyll, the leaf is dipped into hot water this second time to soften the leaf. So, C is correct.
- D is incorrect; to remove the chlorophyll, we boil the leaf in ethanol.
You may please note that the whole steps for the experiment is listed below:
- Put green leaf in hot boiling water (to kill and break the leaf cells).
- Put killed leaf into boiling alcohol (to remove chlorophyll and decolorize).
- Put decolorized leaf in hot water (to soften decolorized leaf).
- Apply drops of iodine solution to softened leaf (to test for presence of starch).
We offer closely analyzed answers to questions of past exams, and resolve them by analyzing each option for their merits and demerits.
Use the questions and answers session to deal further on this topic…
/ culled from 2017 JAMB-UTME biology question 10 /