Early Experiments on Photosynthesis Notes | Study Biology Class 11 - NEET

Introduction

Photosynthesis is a photochemical process in which the light energy absorbed by the plants is converted into chemical energy to produce oxygen. However the process existed for ages, but it was discovered only in 1800. A number of scientists conducted several experiments to prove the existence of photosynthesis.
Photosynthesis

Photosynthesis Discovery: Early Experiments
Photosynthesis requires raw materials like water, carbon dioxide and cellular component like plastids. Plants utilize these raw materials to synthesize carbohydrates only in the presence of light. These key features of photosynthesis were revealed during the mid-nineteenth century.
Below mentioned are the experiments that were conducted by the early scientists in support of photosynthesis.
1. Experiment to Prove Carbon dioxide is essential for Photosynthesis
Materials required: A healthy potted plant, a wide-mouthed glass bottle with a split cork, potassium hydroxide solution (KOH), and starch solution.
Experiment:

• Select a healthy potted plant and place it in the darkroom for two to three days to ensure leaves are free from starch.
• In a wide-mouthed glass bottle add 10-15 ml of potassium hydroxide solution and split the cork vertically.
• Now carefully insert half part of a leaf into a glass bottle through the split cork and the other half exposed to air.
• Place the complete unit undisturbed in sunlight for about 3 – 4 hours.
• After 4 hours detach the leaf from the plant and slowly remove it out from the bottle and test it with the starch solution.
• 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 turned its colour to dark brown indicating the presence of starch in it. 

Conclusion: In this experiment, we can conclude that carbon dioxide is essential for photosynthesis. Both the portion 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 amongst the first to carry out these experiments.

2. Experiment by Joseph Priestley
In 1770, after a series of experiments, Joseph Priestley came to a conclusion regarding the essentiality of air for photosynthesis and also for the growth of plants.
Materials required: A bell jar, candle, rat, and a plant.
Experiment:

• Priestley kept a burning candle and a rat together in the single bell jar.
• After some time, candle extinguished and the rat died.
• For the second time, he kept a burning candle, rat, and a green plant together in the bell jar.
• He observed that neither the candle got extinguished, nor did the rat die.

Conclusion: Based on his observations, Priestley concluded that in the first case, the air in the bell jar got polluted by the candle and rat. However, in the second case, the plant reinstated the air that was spoiled by the candle and the rat.
But it took another few years to reveal what was exactly released by the plant to keep the rat alive and the candle burning.
3. Other Experiments
Followed by Priestley, Jan Ingenhousz conducted experiments using the same set-up and showed that sunlight is essential for plants to reinstate the air that was spoiled by the candle or rat.
Jean Senebier demonstrated that plants absorb carbon dioxide and release oxygen during photosynthesis.
Julius Robert Mayer proposed that plants convert light energy into chemical energy.
Later, the experiments by 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.
By the mid-nineteenth century, scientists outlined the process of photosynthesis. 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.