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7. Bioenergetics eLearn.Punjab

Practical Work:

Investigation for the presence of starch
We know that in the process of photosynthesis, plants synthesize glucose by reducing CO2 and for
this reduction hydrogen is obtained from water. In many plants the synthesized glucose is converted
into starch. So presence of starch in a leaf confirms that the plant has carried out photosynthesis.
The presence of starch is investigated through starch test.
Problem:
How would it be proved that starch is present in a leaf?
Hypothesis:
A fresh leaf has carried out photosynthesis and has accumulated starch in its cell.
Deduction:
If the experimental leaf is processed for starch test, it would give positive results for starch.
Material required:
Fresh leaves, a 500 ml beaker, a forceps, a test tube, ethanol, dilute iodine solution, dropper, petri
dish.
Background information:
• When a leaf is dipped in boiling water for sometime, it is killed and becomes soft.
• A soft and decolourized leaf can be tested in starch test. When soft leaf is boiled in ethanol it

loses chlorophyll but retains starch in it.
• Starch turns blue when treated with dilute iodine solution (Fig 7.8).
Procedure:
1. Dip a leaf in a beaker of boiling water for about ten seconds.
2. Take the leaf out of boiling water and put it in a test tube of ethanol.
3. Put the test tube in a beaker of hot water for ten minutes. The ethanol starts boiling and the

leaf in it gets decolourized.
4. Wash the leaf by moving it up and down in water in a beaker and put the washed leaf in a petri

dish.
5. Perform starch test with the leaf. It is done by putting drops of iodine solution on the leaf.

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