[Biology Class Notes] on Early Experiments on Photosynthesis Pdf

Photosynthesis is a process by which plants produce their food. It is a photochemical process in which the light energy is absorbed by the plants and is converted into chemical energy to produce oxygen. This process was followed by the plants for ages. But it’s discovery and identification were done only in 1800 and several scientists conducted many different types of experiments to prove the existence of photosynthesis. 

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Photosynthesis Discovery – Early Experiments

The process of photosynthesis is carried by some of the required raw materials like water, carbon dioxide, and cellular components like plastids. Plants make use of these raw materials to synthesize carbohydrates in the presence of sunlight. These key features of photosynthesis were revealed during the mid-nineteenth century.

Some of the experiments that were conducted by the early scientists to explore photosynthesis in a better way are – 

Experiment to Prove the Importance of Carbon Dioxide in Photosynthesis

Materials Required: A healthy potted plant, a wide-mouthed glass bottle with a split cork, potassium hydroxide solution (KOH), and starch solution.

Experiment:

1. Take a healthy potted plant and keep it in the darkroom for two to three days to ensure leaves are free from starch.

2. In a wide-mouthed glass bottle add 10-15 ml of potassium hydroxide solution and split the cork vertically.

3. Now minutely, insert half part of a leaf into a glass bottle through the split cork and the other half exposed to air.

4. Place the complete unit undisturbed in sunlight for about 3 – 4 hours.

5. Remove the leaf after 4 hours from the plant and slowly remove it out from the bottle and test it with the starch solution.

6. We can observe that the half part leaf which was inside the glass bottle (KOH solution) did not show any colour change but the other half part exposed to surroundings became dark brown indicating the presence of starch in it.

Conclusion: In this experiment, we can conclude that carbon dioxide is essential for photosynthesis. Both portions of leaf received the same amount of water, chloroplasts, and sunlight but the half part which was inside the glass bottle did not receive carbon dioxide. 

Later, many improvised experiments were conducted by scientists to analyze the essential components for photosynthesis. Joseph Priestley (1733-1804) was the first scientist amongst others to carry out these experiments.

Experiment by Joseph Priestley

After conducting a series of experiments in 1770, Joseph Priestley concluded that the essentiality of air for photosynthesis and also for the growth of plants.

Materials Required: A candle, rat, a bell jar, and a plant.

Experiment:

1. Firstly, a burning candle and a rat were kept together in the single bell jar.

2. After some time, the candle extinguished and the rat died.

3. For the second time, he kept a burning candle, rat, and a green plant all together in the bell jar.

4. He observed that neither the candle got extinguished, nor did the rat die.

Conclusion: Based on his observations, the scientist Priestley concluded that in the first case, the air in the bell jar got polluted by the candle and the existence of the rat. However, in the second case, the plant restored the air that was spoiled by the candle and the rat. But this function of the plants was not revealed quite soon by scientists. 

Other Experiments

Scientist Jan Ingenhousz also conducted experiments using the same set-up but the twist was the presence of sunlight that was highlighted as being an essential product for plants to refresh the air that was polluted by the candle or rat.

Jean Senebier came to a conclusion which said that plants absorb carbon dioxide and release oxygen during photosynthesis.

Julius Robert Mayer demonstrated that plants convert light energy into chemical energy.

Later, Julius von Sachs revealed that glucose was produced by plants.

T.W Engelmann discovered the role of chlorophylls and Cornelius van Niel uncovered that the release of oxygen by plants is from water (H2O), not from carbon dioxide. He also gave the general photosynthesis equation. 

An outline was drawn for the process of photosynthesis by scientists. They concluded that light is essential for photosynthesis, and plants use carbon dioxide and water for the preparation of glucose (carbohydrate), where water molecules are the hydrogen donors and oxygen (O2) is the by-product of this biological process.