Mechanism of Photosynthesis

Source of Oxygen released during Photosynthesis:

Van Neil was the first person who pointed out the role of light in Photosynthesis. He studied photosynthesis in sulfur bacteria (green and purple). These bacteria are autotophs, as they contain bacteriochlorophyll (simpler in structure than ordinary chlorophyll). These bacteria live in swamps and bonds where Hydrogen Sulfide is abundant. Hydrogen Sulfide is the source of Hydrogen used by these bacteria to reduce Carbon dioxide into carbohydrates, Sulfur is released.

Van Neil assumed that light decomposes Hydrogen Sulfide into Hydrogen and Sulfur. Hydrogen is then used afterwards in a certain dark reactions to reduce Carbon dioxide into carbohydrates. As in the following equation:

Sun Light energy

6 CO2 + 12 H2S C6H12O6 + 6 H2O + 12 S

Bacterio-chlorophyll

On this basis, Van Neil assumed that light reactions in green plants are similar to that happening in sulfur bacteria, except that in green plants it is water that is decomposed by light into Hydrogen and Oxygen. Hydrogen is used afterwards to reduce Carbon dioxide in a series of reactions that don’t require light into carbohydrates, and Oxygen releases. So, Van Neil proposed that the release of Oxygen comes from water, a case which is similar to Sulfur that is released from Hydrogen Sulfide. So the general chemical equation that represents Photosynthesis in green plants:

Sun Light energy

6 CO2 + 12 H2O C6H12O6 + 6 H2O + 6 O2

Chlorophyll

Verifying the theory of Van neil experimentally:

A group of scientists in California University used the green alga Chlorella in an experiment to prove the correctness of Van Neil theory experimentally. They provided the alga with all conditions favorable for photosynthesis.

The water used in the experiment contained the Oxygen isotope 18 (O18) instead of the ordinary Oxygen 16 (O16).

They discovered that the Oxygen which evolved during photosynthesis of the alga was the isotope Oxygen 18 (O18), and not Oxygen 16 (O16). This proves that the source of Oxygen evolved is water and not Carbon dioxide.

In order to confirm this result, they repeated the experiment using ordinary water containing the ordinary Oxygen 16 (O16), and Carbon dioxide containing the isotope Oxygen 18 (O18). The Oxygen evolved was ordinary Oxygen 16. The ordinary Oxygen 16 (O16) comes from the ordinary water.

The 1^st. experiment:

Sun Light energy

6 CO2(16) + 12 H2O(18) C6H12O6(16) + 6 H2O(16) + 6 O2(18)

Chlorophyll

The 2^nd. experiment:

Sun Light energy

6 CO2(18) + 12 H2O(16) C6H12O6(18) + 6 H2O(18) + 6 O2 (16)

Chlorophyll