Starch in Plant Leaves and the Requirement of Light for Photosynthesis

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During the photosynthesis, plant convert CO2 into sugar which can be used up by cell respiration for energy for the plant cells or the sugar can be converted to starch and stored. This experiment will allow you to determine where in the plant is the starch stored and what happens to the starch when the plant is deprived of light.

Purpose/Objectives:

To define the term photosynthesis and practice iodine test for the presence of starch in leaves. Compare the results of iodine test of two leaves one that has been exposed to sunlight and one that wrapped or unexposed.

Methods:

Parts of the geranium plant leaves have been covered by aluminum wrap for 48 hours.  Take off a leaf with its petiole (stem) that has an aluminum shade on part of it. Make a drawing of the leaf exactly as you see it with the shades.

Remove the shade and place the leaf in boiling alcohol for about 10 minutes until the leaf is bleached white(the alcohol removes the chlorophyll).

Place the leaf carefully in to a dish of distilled water for about 2 minutes. Then place the leaf carefully in the Petri dish and add several drops of iodine solution.

Draw a picture of the leaf in the Petri dish at the end of the 2 minutes. Compare it to the 1st drawing. Your result will be these two pictures. Make sure that you label the pictures.

Results and Data:

The leaf that was not wrapped turns a dark color, but the leaf that was wrapped remains the same. The fact that the wrapped leaf did not change in color shows that it did not change color shows that it did not have any starch. The change in color in the unwrapped leaf indicates the presence of starch. The difference between the leaves is the exposure to sunlight.

Conclusion/Answers to Questions:

What conclusions can you make by looking at the leaf? Is starch stored in the leaf of the plant?

The results prove that sunlight is necessary for the manufacture of plant food. This process in which plants use sunlight and other natural resources to make their food is called photosynthesis. The green color or pigment is called chlorophyll. The light is absorbed by the chlorophyll which also transfers the energy thus derived and in some brings about the photosynthetic reactions. This experiment also shows that starch is present in the leaves of plants.

What Happened and how did you show that light is required for photosynthesis?

The energy from sunlight is responsible for the manufacture of food for plants; it is represented on the wrapped and unwrapped leaves.

Questions:

1) Why were some of the leaves of the plants covered with aluminum shades?

To avoid sunlight some of the leaves of the plants were covered with aluminum shades.

2) What happened in the areas covered with the aluminum shades?

It slightly turns to yellow.

3)  Explain the reaction of iodine, i.e. why did it turn the light exposed leaf a dark color?

The reaction of iodine on the leaves unwrapped indicates the presence of starch that is why the exposed leaf turns dark, because iodine reacts with starch that produces dark color.

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