(i) Give an account of biological nitrogen fixation.
(ii) Give characteristic role of magnesium and molybdenum in plants.

Explanation

(i) Biological Nitrogen Fixation : The fixation of nitrogen into ammonia by living organism is called biological nitrogen fixation. It is carried out by several groups of bacteria, which is able to absorb elemental nitrogen from the atmosphere and combine it into compounds that serve as nutrients for plants.
The nitrogen fixing micro-organisms are of following types :
1. Free living (Non-symbiotic) : Free living nitrogen fixers are bacteria such as Rhodospirillum, Axotobacter and Cyanobacteria. e.g. Nostoc, Anabaena etc.
2. Symbiotic : Symbiotic nitrogen bacteria such as Rhizobia and Frankia have symbiotic association with roots of higher plants. e.g. the association between legume and bacteria of the genus Rhizobium.
Symbiotic Biological Nitrogen Fixation : The symbiotic bacterium Rhizobium is found in a association with root nodules of leguminous plants. The bacteria thrive well in these nodules and fixes atmospheric nitrogen for the plant with the help of enzyme nitrogenase present in the root nodule and in turn gets water and nutrient from the plant. The nitrogenase enzyme catalyses the conversion of atmospheric `N_(2)` into `NH_(3)` as follows :
`N_(2) + 8e^(-) + 8H^(+) + "16 ATP" + 16 H_(2)O overset("Nitrogenase")(rarr) 2NH_(3) + H_(2) + "16 ADP" + 16 Pi`
Ammonia formed as a result of nitrogen fixation is used by plants for synthesis of amino acid from which the proteins and other organic compounds are synthesized in plants.
(ii) (a) Role of Magnesium in plants :
It forms parts of chlorophyll molecule.
Maintains structure of ribosome.
It is required for the synthesis of DNA and RNA.
It activates the enzymes required for the phosphate metabolism in photosynthesis and respiration.
Its deficiency in plants results in premature fall of leaves and flower buds, delay in seed germination and purple spots on leaves.
(b) Role of Molybdenum in plants :
It is required for nitrogen fixation.
It is the component of many enzymes like nitrate reductase. Causes nitrogen deficiency in plants, interveinal chlorosis and necrosis in leaves and retardation of growth.