1. Old concept :
(1) According to old concept, the source of oxygen evolved in photosynthesis is ` CO_(2)`
(2) It is believed that during the process of photosynthesis , ` CO_(2)` splits into C and ` O_(2)`. The carbon combines with` H_(2)O` molecule to form glucose and oxygen is released as a by-product as shown under :
` CO_(2) + H_(2)O overset ("Sunlight") underset("Chlorophyll")to (CH_(2)O) + O_(2)`
2. Work of Van Niel :
(1) Van Niel showed that photosynthesis is a light dependent reaction. Hydrogen from suitable oxidisable compound reduces `CO_(2)` to carbohydrates.
(2) He obseved that ` H_(2)S` is used by purple and green sulphur bacteria. These bacteria do not use water in photosynthesis .
(3) He emphasised that `O_(2)` evolved by green plants does not come from ` CO_(2)` but is liberated from water
3. Work of Robert Hill :
(1) Robert Hill (1937) proved that the source of oxygen evolved in photosythesis is water and not carbon dioxide.
(2) Hill discovered that when suspensions of chloroplasts were illuminted in complex absence of `CO_(2)` in the presence of a suitable hydrogen acceptor (Ferric oxalate), the water molecule split into hydrogen and oxygen.
(3) The hydrogen released was accepted by ferric oxalate (hydrogen acceptor) which in turn was reduced to ferrous oxalate. This reaction is called Hill reaction as it was discovered by Robert Hill.
(4) Hill reaction can be represented by the following equation :
`2 H_(2)O + 2A overset(" Light") underset("Chlorophyll")to 2AH_(2) + O_(2)`
where A is hydrogen acceptor.
4. Work of Ruben and Kamen :
(1) Ruben and kamen (1941) proved that the souce of oxygen evolved during photosynthesis is water .
(2) With the use of radioactive oxygen ` .^(18)O_(2)` . they confimed that the source of ` O_(2)` evolved in photosynthesis is water.
5. Work of Dr Arnon :
(1) Dr Arnon and his co-workers in 1954 discovered the presence of a coenzyme in the chloroplasts called NADP (Nicotinamide Adenine Dinucleotide Phosphate) which acts as hydrogen acceptor.
(2) Photosynthesis is now considered as redox reaction as there is oxidation of `H_(2)O` and reduction of `CO_(2)`
