This is a process where certain bacteria which are autotrophs manufacture their own food from simple inorganic substance such as carbon (iv) oxide, water, hydrogen sulphide by using the chemical energy released during the process as their source of energy. The bacteria have enzyme system capable of trapping chemical energy.

For example, the Sulphur bacteria in the soil can oxidize hydrogen sulphide around it to chemical energy.

2H2S + O  ——–> S + 2H2O + Chemical energy

Experiments to test for photosynthesis

Experiment 1

Aim: To test for the presence of starch in leaf

Material Required: Fresh leaves from outdoor plant, beaker, boiling tubes, dropping tube, white       tile and iodine solution.

Method: First boil the leaf in water for 4-6 minutes so as to

(i)         Kill the cell

(ii)        To inactivate the enzyme

(iii)       To burst starch grain present

Then dip the leaf into a test tube containing 90% alcohol to decolourise the leaf. After that the leaf is dipped into water to soften it later pour few drops of iodine solution on the leaf in the control experiment a leaf from a plant kept in the dark cupboard is plucked and tested for starch.

Observation: The leaf that was plucked from 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 has turned blue-black with iodine solution, it shows that photosynthesis has taken place or starch is present in the leaf.

Experiment II

Aim: To show that sunlight is necessary for photosynthesis

Material Required: A potted plant, strip of black, paper, clips, cupboard

Method: The potted plant is first de-starched by putting it in a dark cupboard for 1-2days. This is to remove all the traces of starch formed in the leaves. After this, the middle of o ne of the leaves is covered by a strip of black paper, both at the front and back with the acid of clip. The whole plant is then placed in sunlight.

After about 3-5hours, the paper is removed, the leaf is then tested for starch.

Observation: Only the exposed parts 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 showing that starch is absent

Conclusion: The experiment shows that sunlight is necessary for photosynthesis

Experiment III

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

Material 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 conical flask contains caustic soda solution. The solution will absorbs any traces of carbon (iv) oxide inside the flask. The flask’s 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 from the flask) and the other outside the flask (control experiment) are plucked and tested for starch.

Observation: After testing 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 has a positive test

Experiment IV

Aim: To show that chlorophyll is necessary for photosynthesis

Material Required: A variegated plant like croton, coleus or acalpha plant. A variegated plant has green and white patches on the leaf.

Method: The potted variegated plant is exposed to sunlight for about 3-5hours after which a variegated leaf is plucked fresh from the plant during the day time when there is sunlight. The variegated leaf is tested for starch

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

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

Experiment V

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

Material Required: A water plant e.g. Elodea, glass funnel, beaker, water, test-tube, glowing splinter

Method: An elodea plant is kept in a beaker filled with water. 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 accumulate at the top of the test tube. The gas is tested with a glowing splinter

Observation: It will be seen that the gas formed at the top of the tube rekindles glowing splinter showing the presence of oxygen.

Conclusion: The experiment shows that oxygen is given off a by-product during photosynthesis



1a.      Define Photosynthesis?

  1. Describe the light reaction stage
  2. Explain how photosynthesis occur in green plant
  3. List five conditions necessary for photosynthesis

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