question_answer 28)

  The process of nitrogen fixation by bacteria does not take place in the presence of (a) molecular form of hydrogen                                (b) elemental form of oxygen (c) water                                                              (d) elemental form of nitrogen

Answer:

  (b) Certain bacteria such as Azotobacter (occur freely in the soil) and Fihizobium (occur in root nodules of leguminous plants as pea, grom, bean, etc), convert the atmospheric nitrogen into water soluble nitrates. The process of bio fixation of nitrogen is called as nitrogen fixation.                                                      Enzymes required tor biofixation of nitrogen are nitrogenases. There nitrogenases are very suseptible to destruction by oxygen. Many bacteria cease the production of enzymes in the presence of oxygen. Many nitrogen fixing bacteria grow in anaerobic conditions or respire to bring down oxygen levels or binding oxygen with the proteins like leg haemoglobin.