Early Experiments to Understand Photosynthesis: Have you heard the term photosynthesis? Have you ever wondered how we came to know that plants require sunlight or water for photosynthesis? Let us understand all these facts. Only photosynthesis is the means by which certain organisms can make their own organic food from inorganic raw material with the help of solar energy. The organisms performing photosynthesis are therefore called autotrophs. The study on photosynthesis started around 300 years ago. Simple experiments have shown that chlorophyll (green pigment of the leaf), light, and CO₂ are required for photosynthesis to occur.

Photosynthesis

Photosynthesis is the process of the plant by which they utilize sunlight, water (H₂O), carbon dioxide (CO₂), and chlorophyll molecules to form oxygen and glucose (C₆H₁₂O₆) molecules. It converts light energy into chemical energy. Let us study the various historical experiments of photosynthesis.

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Historical Account of Experiments on Photosynthesis

There have been several simple experiments done that led to a gradual development in our understanding of photosynthesis

i. Joseph Priestley (1733-1804):  In 1770, Priestly revealed the essential role of air in the growth of green plants through several experiments. He discovered oxygen in 1774. In an experiment done, Priestley observed that a candle is burning in a closed space, i.e., a bell jar, soon gets extinguished. Similarly, a mouse would die of suffocation in a closed space due to the unavailability of oxygen. Through his experiments, he concluded that both the burning candle and the mouse contaminates the air they use. But, when a mint plant was placed in the same bell jar, the mouse stayed alive, and the candle continued to burn. As a result, Priestley concluded that plants add to the air what a breathing mouse and burning candle remove or use.

ii. Jan Ingenhousz (1730-1799): Ingenhousz, through his experiments, showed that sunlight is essential for the plant process that helps to somehow purify the air fouled by the breathing of mice and the burning candle. In another experiment with an aquatic plant (Hydrilla), he showed that small bubbles were formed around the green parts of the plant in bright sunlight. While in the dark, no bubbles were formed. He identified those bubbles to be oxygen. Therefore,  he showed that in the presence of sunlight, only the green parts of the plants could release oxygen.

iii. Julius von Sachs (1854): He found that the green parts in plants are the place where glucose is made, and glucose is usually stored as starch. Later, he showed that the green substance in plants (now called chlorophyll) is located in special bodies (now called chloroplasts) within the plant cells. 

iv. T.W. Engelmann (1843-1909): He experimented on Cladophora using a prism; he split light into its spectral components and then illuminated a green alga kept with aerobic bacteria. The bacteria were used to detect the sites of oxygen evolution. He found that the bacteria accumulated mainly in the blue and red light regions of the split spectrum. And thus, the first action spectrum of photosynthesis was described. The empirical equation representing the total process of photosynthesis for organisms evolving oxygen was understood as

v. Cornelius van Niel (1897-1985): Van Neil, based on his studies of purple and green sulphur bacteria, demonstrated that during photosynthesis, oxygen evolved by the green plants comes from water and not from carbon dioxide. The hypothesis was later proved by using radioisotopic techniques.

Where H₂A is the oxidisable compound (H₂O or H₂S).

The correct equation to represent the overall process of photosynthesis could thus be summed as

Where C₆H₁₂O₆ is glucose and O₂ is released from water.

vi. Ruben, Kamen, and Hassid used a heavy, but non-radioactive, stable isotope of oxygen ¹⁸O to prove that O₂ evolved during light reaction comes from H₂O and not from CO₂.

Some Important Experiments Related to Photosynthesis

Let us now study certain experiments to study the need for light, chlorophyll and carbon dioxide for photosynthesis.

Experiment to Demonstrate that Light and Chlorophyll is Necessary for Photosynthesis: 

Aim: To demonstrate the use of light and chlorophyll in photosynthesis.

Materials Required: Destarched leaf, Black strip, Iodine solution.

Procedure: 

1. Take a destarched potted plant having variegated leaves and cover 2-3 leaves with black paper. 
2. Expose the potted plant to sunlight for 1-2 hours. 
3. Pluck one covered leaf and one exposed leaf.
4. Both the leaves are then dipped in iodine solution. 

Observation: The leaf which was covered does not pass the starch test proving that in the absence of light, photosynthesis cannot occur. The exposed leaf to sunlight shows blue and black spots wherever chlorophyll is present to show a positive starch test.

Conclusion: Green parts of the leaf contain chlorophyll. Hence they carry out photosynthesis and produce starch which turns blue-black when tested with iodine. This experiment proves that sunlight and chloroplast are important for photosynthesis.

Experiment to Demonstrate that Carbon Dioxide is Necessary for Photosynthesis (Moll’s Half leaf experiment): 

Aim: To demonstrate the use of carbon dioxide in photosynthesis.

Materials Required: Potted plant, beaker, KOH solution.

Procedure: 

1. Take a potted plant and enclose a part of one leaf in a test tube.
2. Fill this test tube with some KOH soaked cotton (which absorbs CO₂) while the other half of the leaf is exposed to air.
3. Pluck the half-covered leaf after a few hours.
4. Dip the leaf in iodine solution.

Conclusion: When the two halves of leaf were tested for starch, it was found that only the exposed part of the leaf tested positive for starch. This showed us that CO₂ is required for photosynthesis.

Summary

Photosynthesis is the process used by green plants to form glucose molecules by using water, carbon dioxide and chlorophyll molecules. Various scientists performed several experiments to prove the usage of light and carbon dioxide in the formation of food in plants. Scientists like Priestley and Ingenhousz proved that the plants in the presence of light release pure air (oxygen), which is later taken up by other organisms for the purpose of respiration. Later on, Engelmann performed several experiments to give the action spectrum of light and proved that the maximum photosynthesis occurs in the blue and red light of the visible spectrum.

Frequently Asked Questions (FAQs) on Early Experiments to Understand Photosynthesis

Q.1. Which plant was used by Ingenhousz?
Ans: Ingenhousz used an aquatic plant Hydrilla to prove plants produce pure air.

Q.2. Which process involves the release of oxygen in plants?
Ans: Oxygen is released in plants by the process of the photolysis of water.

Q.3. Which chemical solution was used in Moll’s half leaf experiment?
Ans: KOH was used in Moll’s half leaf experiment to prove that carbon dioxide is necessary for photosynthesis purposes.

Q.4. Where does photosynthesis take place?
Ans: Photosynthesis takes place in the green part of the plant, which contains chloroplast.

Q.5. Which plant was used by Engelmann to show the action spectrum of the light?
Ans: Engelmann used Cladophora to show the action spectrum of light.

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