With the help of a labelled diagram show
(a) Nitrogen cycle in nature
(b) Describe briefly any two processes involved in the cycling of `N_(2)` in the environment.
OR
With the help of a labelled diagram show the cycling of carbon in nature. What are the two ways in which carbon dioxide is fixed in the environment.

Explanation

Draw fig (nitrogen cycle) and label it fully.
(b) Four major steps of nitrogen cycle are nitrogen fixation, nitrogen assimilation, decomposition and denitrification.
(i) Nitrogen fixation. It is conversion of inert nitrogen gas into biologically acceptatble forms. Nitrogen fixation is carried out by a number of free living (i.e. Azotobacter) and symbiotic bacteria (e.g., Rhizobium inlegume roots) and blue-green algae (cyanobacteria). They absorb atmospheric nitrogen and reduce it to ammonia state. Ammonia combines with organic acids to produce amino acids. The latter form proteins. After their death, proteins and other nitrogen compounds are decomposed to form ammonia and nitrates.
(b) Nitrogen asimilation: It is carried out by the plants. They absorbg nitrates and ammonium ions from the soil. Nitrate is first chznged into ammonium form. Ammonium combines with organic acids to form amino acids and other nitrogenous compounds. Amino acids give rise to proteins. Nucleotides produce nucleic acids (DNA, RNA). Enzymes are formed from proteins. Animals obtain organic nitrogen from plants dirctly or through food chain
OR
Draw fig. (Carbon cycle) and label it fully.
Two of `CO_(2)` fixation (i) Photosynthesis. All producers pick up `CO_(2)` from their environment and change it into glucose and other organic compounds in the process of photosynthesis.
(ii) Formation of shells and skeleton. Aquatic animals absorb carbonates from water and use the same in building their shells and skeletons.