Activity & Practical on Light Is Necessary For Photosynthesis Experiment | Class-10th Level

Activity & Practical on Light Is Necessary For Photosynthesis Experiment | Class-10th Level | Edugrown

Photosynthesis is a process by which plants prepare food. During this reaction, carbon dioxide and water are converted into glucose by chlorophyll in the presence of light energy.

Aim

To show experimentally that light is necessary for photosynthesis.

MATERIALS REQUIRED

A healthy potted plant, beaker, a pair of forceps, tripod stand, wire gauze, bunsen burner, black paper, paper clips, iodine solution, alcohol, water bath etc.

THEORY

Plants prepare their food by the process called photosynthesis. To make food, plants need CO₂ water, chlorophyll and light/sunlight. In absence of any of these plants cannot prepare their food.
Plants can prepare their food in blue light.
The rate of photosynthesis depends on all three factors i.e.—light, temperature, availability of components, i.e.,— CO₂ and H₂0.
If the intensity of light increases the rate of photosynthesis also increases.
When light falls on plants they show light reaction. In this light reaction the water in leaves undergo photolysis
i.e.,—the water splits to form oxygen and hydrogen due to photons of light. The oxygen gas is released out in the atmosphere but hydrogen is kept by the plant. It is this hydrogen that combines with CO₂ to form carbohydrate (reduction reaction). Hence; photosynthesis is an oxidation-reduction reaction.
Photosynthesis reaction:

PROCEDURE

Take a healthy potted plant and keep it in a dark room for 48 hours so that all the starch gets used up.
Now cover one leaf of a plant with a black paper using paper clip.
Keep this plant in sunlight for about six hours.
Pluck two leaves from the plant, one that is covered and the other one that is uncovered.
Dip the leaves in boiling water for a few minutes.
Now immerse the leaves in a beaker containing alcohol.
Carefully place this beaker in water bath and heat it till the alcohol begins to boil.
Observe the colour of the leaves and solution.
Wash the leaves with lot of fresh water.
Now dip the leaves in iodine solution for a few minutes.
Now observe the colour of leaves and compare them.

OBSERVATIONS

When leaves are boiled in alcohol, the alcohol solution becomes green and the leaves become colourless.
When iodine solution is added on the leaves
(a) a leaf covered with black paper showed no colour changes with iodine solution.
(b) another leaf which was not covered with black paper when dipped in dilute iodine solution, the colour of leaf changed to blue-black.

Inference

During photosynthesis plants prepare starch.
When the leaf is covered, it is not allowed to take sunlight and hence, no starch was prepared in the leaf.
Iodine solution becomes blue-black in presence of starch. On adding iodine solution to covered leaf no colour change was observed. It indicates that no starch was made by this leaf.
Whereas the uncovered leaf got sunlight for 6 hours and when iodine solution was added to it, the colour changed to blue-black.
This proves that sunlight is required for photosynthesis.

Procedure II

Select a potted plant, keep it in dark room for 48 hours.
Select a healthy leaf and clip a portion of it with dark colour paper using clips.
Keep this plant in sunlight for 6 hours.
Then do the same steps (4-11) as in procedure 1 on previous page.

Precautions

Select a small healthy, herbaceous potted plant.
Do not destarch the plant for more than 48 hours.
Choose a leaf and clip it carefully so that it does not break or crack from the stem.
Alcohol is highly inflammable, be careful while boiling leaf in alcohol using water bath.
Wash the alcohol from the leaves and then do the iodine test.
Satisfactory results will not be obtained if the plant is not completely de-starched.

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