Discovery of photosynthesis

Water is essential for plant growth

During 1600’s Jan Baptista van Helmont, a Belgian chemist, physiologist and physician performed an experiment with a willow tree. He planted the willow in a pot filled with soil and kept under controlled environment. He carefully and precisely watered the plant over a period of 5 years.
He then concluded that, the plant grew because of nutrient taken from the water, but not from the soil. The conclusion of Helmont was inaccurate, however, his experiment proved that water is essential for the growth of plants.

Green plants restore air

Joseph Priestley (1771) conducted a set of experiments with a mint plant and mouse. He had performed following experiment.

• Priestley kept a breathing mouse in close bell jar and observed, the mouse soon suffocated.
• He had kept a burning candle in a close bell jar and observed, that candle soon extinguished.
• He then kept a mint plant in the close bell jar with breathing mouse or burning candle and observed that, mouse was still alive and candle continued to burn.

On the basis of above experiments, he concluded that, burning candle or breathing mouse foul the air and mint plant restored the air whatever fouled by Burning candle or breathing mouse.
He published the experimental findings in 1774 in “Experiments and Observations of Different Kinds of Air, Volume I.”

Green parts of the plant release oxygen in the presence of sunlight

Jan Ingenhousz (1779) conducted the Priestley’s experiment by putting the setup once in the light and once in the dark and concluded that, for restoration of fouled air, sunlight is essential for plants.
He conducted another set of experiment with submerged aquatic plant and observed that, bubbles come out from the green parts of the plant. He later identified these bubbles to be oxygen. He then concluded that, light is essential to produce oxygen from the green parts of the plant.

First action spectra was discovered

During late 1800s T. W. Engelmann split the visible light into its spectral components and illuminated the green alga, Spirogyra kept in a suspension of aerobic bacteria. He observed that, oxygen-seeking aerobic bacteria accumulated in the region of blue and red light of split spectrum. It was because of the oxygen evolution taken place by the spiral chloroplast in the same region. Thus, first idea of action spectrum come into existence.

Empirical equation of photosynthesis was described

Meanwhile, Julius von Sachs proved that, glucose in synthesized in the green parts of the plant and stored in the form of starch. He observed that, chlorophyll is located in the special bodies (chloroplasts) of the cells. During the 19th century it has been established that, plants use light energy to make carbohydrate from CO₂ and H₂O. and empirical equation is written as:

CO₂ + H₂O → [CH₂O] + O₂

Where, [CH₂O] is the glucose.

Photosynthesis is a reduction-oxidation process

It was C. B. van Niel who during 1920s while working on green and purple photosynthetic bacteria proved that, photosynthesis is a light-dependent reaction where, hydrogen from a suitable oxidizable donor reduces CO₂ into carbohydrate. This is expressed as:

CO₂ + 2H₂A → [CH₂O] +2A + H₂O

In green plants, H₂O is the hydrogen donor and is oxidized to oxygen. He concluded that, O₂ evovled by the green plants comes from the H₂O, not by CO₂. This was later proved by using radioisotopic trace technique.
Thus, overall balanced reaction of photosynthesis in green plants is:

6CO₂ + 12H₂O → C₆H₁₂O₆ + 6H₂O + 6O₂