Classwork Exercise and Series (Biology-SS1): Evidence Of Photosynthesis In Plant

Evidence of Photosynthesis in Plant

Experiment to Test For Starch in A Fresh Green Leaf

Aim: to test for the presence of starch in the leaf.

Materials required: fresh green leaves from outdoor plants, beakers, burner, boiling tubes, dropping tube, white tiles and iodine solution.

Method: Firstly, boil the leaf in water for 4-6 minutes so as to kill the cells, inactivate the enzymes and burst starch grains present. Then dip the test tube containing 70% alcohol to decolourize the leaf. After that, the decolourized leaf is dipped into a beaker containing hot water to soften it. Finally, place the leaf in a white tile and pour few drops of iodine solution on the leaf. In a control experiment, a leaf from plant kept in the dark cupboard is plucked and tested for starch.

 

Observation: It is seen that the leaf that was plucked from the potted plant outside turned blue-black with iodine solution while the other leaf (control experiment) remained colourless.

Conclusion: Since the leaf in the real experiment turned blue-black with iodine solution, it shows that photosynthesis has taken place or starch is formed by the leaf.

Experiment to Show That Light Is Necessary For Photosynthesis

Aim: To show that sunlight is necessary for photosynthesis

Materials: A potted plant, strip of black paper, clips, cupboard.

Method: The potted plant is first de-starched by putting it in dark cupboard for 1 – 2 days. This is to remove all traces of starch formed in the leaves. After this, the middle of one of the leaves is covered by a strip of black paper, both at the front and back with the aid of clips. The whole plant is then placed in sunlight. After about 3 – 5 hours, the paper is removed. The leaf is then tested for starch.

Observation: Only the exposed parts, i.e., the top and bottom of the leaf turned blue-black with the iodine solution which shows the presence of starch while the area that was covered with black paper will remain colourless, indicating that starch is absent.

Conclusion: The experiment shows that sunlight is necessary for photosynthesis.

Experiment to Show That Carbon Dioxide Is Necessary For Photosynthesis

Aim: To show that carbon (IV) oxide is necessary for photosynthesis

Materials required: A potted plant, Vaseline, conical flask, split cork, retort stand and caustic soda (sodium hydroxide) solution.

Method: Use a leaf attached to a potted plant. The leaf is enclosed in the flask containing caustic soda solution. This solution absorbs the carbon (IV) oxide inside the flask. The flask mouth is corked and smeared with Vaseline at the neck to make it air tight. The whole experiment is now exposed to sunlight for several hours. Two leaves (one inside the flask) and the other outside the flask (control experiment) are plucked and tested for starch.

 

Observation: At the end of the test for starch, the leaf inside the flask did not show blue-black colour, indicating absence of starch formation because of lack of carbon (IV) oxide inside the flask while the leaf outside the flask shows blue-black colour, indicating the presence of starch.

Conclusion: This experiment shows that carbon (IV) oxide is necessary for photosynthesis to take place.

Experiment to Show That Chlorophyll Is Necessary For Photosynthesis

Aim: To show that chlorophyll is necessary for photosynthesis

Materials required: A variegated plant is needed. A variegated plant has green and white patches on the leaves.

Method: the potted variegated plant is exposed to sunlight for about 3-5 hours, after which a variegated leaf is plucked fresh from the plant during the day time when there is sunlight. Make a drawing of the leaf and map out the green and white patches. The variegated leaf is then tested for starch.

 

Observation: It will be noticed that the green parts of the variegated leaf are stained blue-black with iodine solution while the white part remain colourless.

Conclusion: This experiment shows that chlorophyll is necessary for photosynthesis to take place.

Experiment to Show That Oxygen Is Given Off During Photosynthesis

Aim: To show that oxygen is given out as a by-product during photosynthesis

Materials required: A water plants e.g. Elodea, glass funnel, beaker, water, test tube, and splinter.

Method: Fill up the beaker with water. Then place the water plant (e.g. Elodea) at the bottom of the beaker. This is followed by the filling of the test tube with water and then inverts it over the stem of the funnel. The whole set-up is then placed in the sunlight for several hours. Tiny bubbles of gas will start to appear on the surface of the leaves and these break off and accumulate at the top of the test tube.

Observation: It is seen that the gas formed at the top of the test tube rekindles a glowing splinter showing the presence of oxygen.

Conclusion: This experiment shows that oxygen is given off as a by-product during photosynthesis.