Prior knowledge of the photosynthesis and early experiments

Some important events related to photosynthetic process are described below.

(1) Stephan Hales was the first to prove that gaseous constituents of the air and light also contribute to the building up of the plant body.

(2) Joseph Priestley concluded that plants purify the impure air released by animals. Joseph Priestley (1733-1804) in 1770 performed a series of experiments that revealed the essential role of air in the growth of green plants. Priestley discovered oxygen in 1774.

Figure: Priestley's experiment

Priestley observed that a candle burning in a closed space -a bell jar, soon gets extinguished (Figure a, b, c, d). Similarly, a mouse would soon suffocate in a closed space. He concluded that a burning candle or an animal that breathe the air, both somehow, damage the air. But when he placed a mint plant in the small bell jar, he found that the mouse stayed alive and the candle continued to burn. Priestley hypothesised as follows: Plants restore to the air whatever breathing animals and burnning candles remove.

(3) Jan Ingenhousz mentioned, that in sunlight green leaves give out dephlogisticated air (air rich in oxygen) and in dark these green parts give out phlogisticated air (air rich in CO2) and make the air impure.

(4) Jean Senebier discovered that green plants utilize carbon dioxide. It was thus a great land mark in the discovery of photosynthesis.

(5) N.T. de Saussure proved that the volume of carbon dioxide consumed is equal to the volume of oxygen liberated. He also showed that water is essential requirement in photosynthesis.

(6) Pelletier and Caventou named the green coloured substance in the leaves as chlorophyll.

(7)  F.F. Blackman observed that the process of photosynthesis has two steps : a photochemical reaction which proceeds only in the presence of light and a dark reaction for which light is not necessary. Besides this, he also proposed the law of limiting factor.

(8) Warburg performed Flashing experiment on Chlorella.

(9) Emerson and Arnold, with the help of experiments determined that the light reaction of photosynthesis has two distinct photochemical processes.

(10) Robert Hill's experiment on Stellaria media demonstrated that in the presence of sunlight, water and a suitable hydrogen acceptor, isolated chloroplasts release oxygen, even if carbon dioxide is absent. This experiment is considered to be equivalent to light reaction. Hill's oxidants are hydrogen acceptors. The common ones are ferricyanide, benzoquinone and dichlorophenol indophenol(DCPIP), while NADP+ is natural H+ acceptor in photosynthesis.

(11) Van Niel, based on his studies of purple and green sulphur bacteria demonstrated that hydrogen released from a suitable oxidisable compounds reduces CO2 to carbohydrates and proposed that water is the source of O2.

(12) Ruben, Hassid and Kamen using radioactive oxygen (O18) confirmed that the source of oxygen in photosynthesis is water.

(13) Julius von Sachs (1854) found that the green parts in plants is where glucose is made and glucose is usually stored as starch.

(14) T.W. Engelmann experimented on Cladophora and Spirogyra. He used a prism to split light into its components and illuminated the green alga placed in a suspension of aerobic bacteria. He observed the accumulation of bacteria in the region of blue and red light of the split spectrum. Thus, he described the first action spectrum of photosynthesis.

(15)  Melvin Calvin and his coworkers discovered various reactions involved in the conversion of carbon dioxide into carbohydrates using C14O2. These reactions are collectively known as C3 cycle or Calvin cycle. He was awarded Nobel Prize for this work in 1961.

(16) M.D. Hatch and C.R. Slack discovered a new series of reactions involved in photosynthetic CO2 assimilation in many tropical plants. In these plants the first stable compound in photosynthetic process is a 4-carbon compound, hence, this series of reactions is known as C4 cycle.

(17) Hill and Bendall proposed the scheme of light reaction.

(18) Huber et. al. studied three dimensional structure of reaction center of bacteria Rhodopseudomonas viridis and got Nobel prize.