Question

explain how photosynthesis can occur without the production of oxygen, and how respiration can occur without requiring oxygen

Answer 1

Photosynthesis that occur without the production of oxygen is termed as anoxygenic photosynthesis and is carried out by certain bacteria. In this case instead of water hydrogen sulphide is used and so instead of oxygen, the byproduct formed is sulphur.

The type II reaction centre where this anoxygenic photosynthesis occurs is similar to photosystem II structurally and sequentially except it does not have a region for the oxygen-evolving complex.

Light is absorbed by reaction centre bacteriochlorophyllP870 and is excited. This excited P870 donates electron to bacteriopheophytin, which is then passed onto carriers down the transport chain. In this process, an electrochemical gradient is formed that helps in ATP production. when anoxygenic bacteria grow on Co2 as carbon source along with ATP, NADPH also needs to be made to reduce CO2. the source of the reducing power, in this case, is not H2O but H2S and thus instead of oxygen evolution they evolve sulphur when P870 is reduced to be available again for a photon.

In anaerobic respiration, oxygen is not required, instead other organic or inorganic molecules act as final electron acceptors. In this case, glucose is converted to pyruvate by the process of glycolysis and following that the pyruvate undergoes a process of fermentation to produce lactic acid and regenerate the NAD+ from NADH that was produced in glycolysis. This in turn produces ATP as the NAD+ can now be used again in glycolysis. Less energy is produced in this process than aerobic respiration.