Early Experiments on Photosynthesis

The daily performance of living things depends on the energy which comes from food.  Life on earth is driven by solar energy.  Plants as producers perform the photosynthesis in the presence of sunlight to meet their own needs as well as to provide energy for other living beings in the process.  Hence, they are called as autotrophs. This physicochemical process is the basis of existence on earth. Let’s recall photosynthesis and early experiments conducted by various scientists to understand the process.



Photosynthesis is the light-driven process of preparing organic compounds by plants. Plants trap light energy and convert it into chemical energy (carbohydrates) in the presence of water and carbon dioxide. The whole process takes place in an organelle called plastids.  The essential cellular components needed for the photosynthesis are plastids and chlorophyll. Photosynthesis proceeds in two reactions- light reaction and dark reaction.  The rate of photosynthesis depends on many factors such as the intensity of light, temperature, availability of water and minerals. The overall reaction is:

Reaction for photosynthesis

The organic compounds synthesized during photosynthesis are the primary source of food. Moreover, it produces air which we breathe in. Hence, photosynthesis is crucial for life existing on the planet earth.

Photosynthesis Process: Early Experiments

We studied that photosynthesis requires raw materials like water, carbon dioxide and cellular component, plastids. Also, 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.

So far, we have studied the experiments to detect the presence of starch. Let us learn more about the experiments 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 a starch solution.


  1. Select a healthy potted plant and place 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 carefully 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. After 4 hours detach the leaf 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 color change but the other half part exposed to surroundings turned its color 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.

Experiments to prove carbon dioxide is essential for photosynthesis.

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.

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.


  1. Priestley kept a burning candle and a rat together in the single bell jar.
  2. After some time, candle extinguished and the rat died.
  3. For the second time, he kept a burning candle, rat, and a green plant together in the bell jar.
  4. 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.

Priestley experiment -Experiments to prove air is essential for photosynthesis.

But it took another few years to reveal what was exactly released by the plant to keep the rat alive and the candle burning. 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. Later, the experiments by Julius von Sachs revealed that glucose was produced by plants and later 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.

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.

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