Question

Oxygen is not produced during photosynthesis by
A. Cycas
B. Green sulphur bacteria
C. Chara
D. Nostoc

Answer 1

Hint: Some microorganisms can perform photosynthesis without the production of oxygen because in them the proton and electron donor is not water. Only one photosystem is present in these microorganisms. Some of these microorganisms are barophilic and are usually found in habitats with extremely high-pressure conditions.

Complete answer:
Non-oxygenic photosynthesis or anoxygenic photosynthesis is seen in green sulphur bacteria. In these bacterias only PS-I is present and therefore only cyclic photophosphorylation takes place. The reaction center in these bacterias contains B890 pigment which responds to ultraviolet red light. Oxygen is not evolved during photosynthesis as water is not the H+ donor.

Additional Information:
- Many other types of bacterias also show this type of photosynthesis, like purple bacterias. They are of two types of purple sulphur bacteria and purple non sulphur bacteria.
- C.B Van Niel proposed that sulfur bacterias used $H_2S$ as hydrogen ion and electron donor, and instead of $O_2$, S was released.
- Several experiments have been conducted on photosynthesis, some of the named ones are:
- Moll’s half leaf experiment- he proved the importance of $CO_2$ in photosynthesis.
- Wilmot’s bubbler experiment can be performed to figure out the rate of photosynthesis.
- Ganong’s light screen confirmed that sunlight is important for the photosynthetic process.
- Rober Hill showed that in photosynthesis photochemical splitting of water occurs.
- Three scientists named Ruben, Kamen and Hashid used radioisotopic methods to prove that oxygen in photosynthesis comes from water.

So, the correct answer is, “Oxygen is not produced during photosynthesis by green sulphur bacterias.”

Note: Splitting of water takes place in PS-II, which is absent in the sulphur bacterias, and hence splitting of water is also absent. The main proton and electron donor in plants is water whereas in bacterias it is dihydrogen sulfide or $H_2S$. Also in anoxygenic type of photosynthetic bacteria, the main pigment is known as bacteriochlorophyll. The light-harvesting unit in these anoxygenic bacterias is known as chlorosome.