Question

Hydrogen donor in photosynthesis of purple/green sulphur bacteria is
(a) ${ H }_{ 2 }$
(b) ${ H }_{ 2 }{ O }$
(c) $N{ H }_{ 3 }$
(d) ${ H }_{ 2 }S{ O }_{ 3 }$

Answer 1

Hint: This is not a type of oxygenic photosynthesis that occurs in higher plants while
This is an anoxygenic photosynthesis. This occurs in photo lithoautotrophic bacteria. The pigments also
differ from the green photosynthetic plants.

Complete step by step answer:

${ H }_{ 2 }$ is a hydrogen donor in photosynthesis of purple/green sulphur bacteria. In this type of photosynthesis instead of oxygen, sulphur or other oxidised product is formed.

Additional information:
- This photolithotrophic bacteria are capable of entrapping solar energy and utilizing it for the synthesis of complex food material. This process occurs in the presence of pigments and they are bacteriochlorophyll and bacterioviridin.
- Purple sulphur bacteria and green sulphur bacteria contain bacteriochlorophyll (bacterio purpurin) and bacterioviridin pigment respectively.
- Bacterial photosynthesis differs from the photosynthesis of higher plants in not liberating oxygen which is termed as anoxygenic type of photosynthesis.
- In bacterial photosynthesis, water is not a source of electrons because water acts as a reducing power which converts Carbon dioxide into glucose.
- The bacteria obtain reducing power from various compounds such as hydrogen sulphide, thiosulphate or even some organic compounds.
- No oxygen is evolved as it does not involve splitting of water. Equation for anoxygenic photosynthesis may be written as follows:
$C{ O }_{ 2 }+{ H }_{ 2 }S + [ Solar \ energy ] \xrightarrow{enzymes} = Sugar+\left( Sulphur \ or \ other \ oxidised \ compound \right) + { H }_{ 2 }O$
Examples: Purple sulphur bacteria: Thiospirillum, Chromatium.
                   Green sulphur bacteria: Chlorobium.
So, the correct answer is ‘${ H }_{ 2 }$’.

Note:
- The entrapping of solar energy and utilizing it for the synthesis of complexes is done by the pigments like bacteriochlorophyll (bacterio purpurin) and bacterioviridin.
- The hydrogen released by ${ H }_{ 2 }$ is picked up by ${ NAD }^{ + } which gets reduced to ${ NAD }{ H }_{ 2 }$ acting as reducing power.