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Experiments conducted to prove the formation of starch in plants

On adding iod...

Question

On adding iodine to a leaf boiled in alcohol, a blue-black colour appears. This means that

A

Starch is produced in the leaf

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B

Starch is not produced in the leaf

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C

Glucose is produced in the leaf

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D

Glucose is not produced in the leaf

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Solution

The correct option is A Starch is produced in the leaf
The iodine test is used to test for the presence of starch.
When treated with iodine solution, iodine forms a complex with starch that produces an intense blue-black colour. This indicates that starch was produced in the leaves. If there has been no starch produced, then the leaves would appear bleached or colourless.
Iodine does not react with glucose or other molecules producing coloured products. Therefore presence of other substances cannot detected by iodine test.

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Q. (a) why is the leaf boiled in alcohol for a short time?
(b) why do we dip the leaf in water before adding the iodine solution?

Q. Statement 1: When iodine solution is added to the leaves, a change in colour to blue indicates the presence of starch.

Statement 2: Chlorophyll is removed by boiling the leaf in water or alcohol.

Q. In an experiment, a destarched, long and narrow green leaf (still attached to a potted plant) is introduced into a wide-mouthed glass bottle through an airtight cork. The glass bottle contains some potassium hydroxide solution in its lower part (but the leaf remains above this solution). The whole set-up is placed in sunlight for 2-3 days. The leaf is then taken out. After removing chlorophyll, the leaf is treated with iodine solution. No blue-black colour is produced in the leaf.
(a) Which substance could not be synthesised in the green leaf?
(b) What is the function of potassium hydroxide solution inside the glass bottle?
(c) Which condition necessary for photosynthesis is not fulfilled in this experiment?

Q. In an experiment to test the presence of starch in a leaf, the leaf is boiled in alcohol for a few minutes using a water bath. This is an essential step in the experiment because alcohol:
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(4) dissolves chlorophyll from leaf

Q. A leaf is boiled in alcohol before using iodine for a starch test in order to

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Experiments conducted to prove the formation of starch in plants

Standard VIII Biology

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