Question

Assertion: Nitrogen-fixing bacteria in legume root nodules survive in oxygen-depleted cells of nodules.
Reason: Leghaemoglobin completely removes oxygen from the cells of the nodules
A. Both assertion and reason are correct and the reason is the correct explanation for the assertion
B. Both assertion and reason are correct but the reason is not the correct explanation for the assertion
C. Assertion is correct but the reason is incorrect
D. Both assertion and reason are incorrect

Answer 1

Hint: There are several types of nitrogen-fixing symbiotic relationships found but the most prominent among them is the relationship between the legume and bacteria. The species like Rhizobium which is rod-shaped bacteria have such a relationship with the roots of the various legumes like alfalfa, sweet clover, sweet pea etc.

Complete answer:
The leguminous plants are in the symbiotic association with the nitrogen-fixing bacteria. The most common association on roots is nodules. The nodules are the small outgrowths on the roots. The formation of nodules includes a sequence of various interactions between the Rhizobium and the roots of the host plants. The Rhizobium bacteria multiply and colonize the surroundings of the roots and get attached to the cells of the epidermal and the root hairs. The root hairs become curled and the bacteria capture the root hair. The formation of nodules started in the cortex of the root. The bacteria then go into the cells which leads to the differentiation of specialised nitrogen-fixing cells. The nodules are formed which make a direct connection with the host for the exchange of nutrients.
The nodules contain all the biochemical components like the enzyme nitrogenase and leghaemoglobin. The leghemoglobin is a red pigment which is present in the root nodules of the leguminous plants. The leghemoglobin is an oxygen-carrier molecule which helps in the protection of nitrogen-fixing enzyme nitrogenase from the effect of the oxygen because the enzyme is very sensitive to oxygen and it requires anaerobic condition. The nodules are adapted in such a way that it makes sure that the enzyme is protected from oxygen. The enzyme nitrogenase is a Mo-Fe protein and catalyses the atmospheric nitrogen conversion into the ammonia, the first stable product of the nitrogen fixation.
Hence, the correct answer is ‘C. Assertion is correct but the reason is incorrect’.

Note: The microbes such as Frankia also help in the production of nitrogen-fixing nodules on the roots of non-leguminous plants like Alnus. Both the Rhizobium and Frankia are free-living in soil but they are symbionts which can fix atmospheric nitrogen. The microbes live as aerobes but at the time of the nitrogen-fixing process, they become anaerobic.