The Early Experiments

It was believed that

  1. CO2 splits into carbon and O2.
  2. Carbon combines with H20 molecule to form glucose
  3. O2 is evolved as a by- product.

C. Van Neil (1930), First demonstrated that

  1. Photosynthesis is essentially a light dependent reaction in which hydrogen from a suitable oxidisable compound reduces CO2 to carbohydrates.
  2. He observed that purple and green sulphur bacteria use H2S and not water in photosynthesis. Sulphur is set free.
  3. From the above observation, he suggested that the O2 evolved by CO2 green plants comes from H2O and not from CO2.

 

Hill's Reaction

Robert Hill (1937) performed an experiment in which

  1. Isolated chloroplasts from spinach leaves are suspended in water which was without CO2.
  2. Ferric salts are added as hydrogen acceptor.
  3. The suspension was exposed to sunlight.
  4. It was observed that Ferric salts are reduced to ferrous and O2 bubbles evolve.

This be clearly proved that the water splits. It is called Photoysis of water or Hill's Reaction.

 

'A' is unknown hydrogen acceptor present in the chloroplasts.

 

Ruben and Kamen (1941) confirmed and directly proved that the source of O2 evolved during photosynthesis is water with use of heavy isotope of Oxygen, 18O2.

Dr. Arnon (1954) discovered that the substance present in the Chloroplast that acts as H-acceptor is Co-enzyme. Nicotinamide Adenine Dinucleotide Phosphate [NADP].

 

Photosynthesis is a REDOX reaction, where H2O is oxidized and CO2 is reduced.